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  Physics is the branch of science concerned with the laws that govern the structure of the universe, and the properties of matter and energy and their interactions. For convenience, physics is often divided into branches such as atomic physics, nuclear physics, particle physics, solid-state physics, molecular physics, electricity and magnetism, optics, acoustics, heat, thermodynamics, quantum theory, and relativity. Before the 20th century, physics was known as natural philosophy.  
  The Atom  
  An atom is the smallest unit of matter that can take part in a chemical reaction, and which cannot be broken down chemically into anything simpler. An atom is made up of protons and neutrons in a central nucleus surrounded by electrons. The atoms of the various elements differ in atomic number, atomic weight, and chemical behavior.  
  Thomson on the Number of Corpuscles in an Atom
  J. J. Thomson's paper on the number of particles, or as he called them corpuscles, in an atom. The Web site is a reproduction of Thomson's publication in Philosophical Magazine vol 11, in June 1906.  
  nucleus The core of the atom is the nucleus, a dense body only one ten-thousandth the diameter of the atom itself. The simplest nucleus, that of hydrogen, comprises a single stable positively charged particle, the proton. Nuclei of other elements contain more protons, and additional particles called neutrons, of about the same mass as the proton but with no electrical charge. Each element has its own characteristic nucleus with a unique number of protons—the atomic number. The number of neutrons may vary. Atoms of a single element with different numbers of neutrons are called isotopes. Although some isotopes tend to be unstable and exhibit radioactivity, they all have identical chemical properties.  
  electron The nucleus is surrounded by a number of moving electrons, each of which has a negative charge equal to the positive charge on a proton, but which weighs only c0129-01.gif times as much. In a neutral atom, the nucleus is surrounded by the same number of electrons as it contains protons. According to quantum theory, the position of an electron is uncertain; it may be found at any point. It is more likely, however, to be found in some places than others. The region of space in which an electron is most likely to be found is called an orbital. The chemical properties of an element are determined by the ease with which its atoms can gain or lose electrons from its outer orbitals.  
  To remember the order of atomic orbitals:  
  Spin pairs don't form—go higher.  
  The Pauli selection rule states that two electrons of like spin may not be present in any orbital if they have the same set of quantum numbers. Since the electronic orbitals in atoms are listed by the letters s, p, d, f, g, h, in order of ascending energy, this mnemonic acts as a useful reminder.  


  proton A proton is a positively charged subatomic particle, a constituent of the nucleus of all atoms. It belongs to the baryon subclass of the c0016-01.gifhadrons. A proton is extremely long-lived, with a life span of at least 1032 years. It carries a unit positive charge equal to the negative charge of an electron. Its mass is almost 1,836 times that of an electron, or 1.67 x 10–27 kg. Protons are composed of two up quarks and one down quark held together by gluons (see section of quarks below). The number of protons in the atom of an element is equal to the atomic number of that element.  




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  neutron The neutron is a composite particle, being made up of three quarks, and therefore belongs to the baryon group of the c0016-01.gifhadrons. Neutrons have about the same mass as protons but no electric charge, and occur in the nuclei of all atoms except hydrogen. They contribute to the mass of atoms but do not affect their chemistry.  
  An isotope is one of two or more atoms that have the same atomic number (same number of protons), but which contain a different number of neutrons, thus differing in their atomic mass. They may be stable or radioactive (see radioisotope below), naturally occurring or synthesized. Hydrogen, for example, has the isotopes 2H (deuterium) and 3H (tritium). Elements at the lower end of the periodic table have atoms with roughly the same number of protons as neutrons. These elements are called stable isotopes. The stable isotopes of oxygen include 16O, 17O, and 18O.  
  Fajans on the Concept of Isotopes
  Extract from Kasimir Fajans paper of 1913 titled ''Radioactive Transformations and the Periodic System of The Elements." The paper describes Fajans's discovery of isotopes, and goes on to offer examples of radioactive transformations leading to their production.  
  cloud chamber The cloud chamber devised by C. T. R.
Wilson was the first instrument to detect the tracks of
atomic particles. It consisted originally of a cylindrical
glass chamber fitted with a hollow piston, which was
connected, via a valve, to a large evacuated flask. The
piston falls rapidly when the valve is opened, and water
vapor condenses along the tracks of any particles in
the chamber.
  A radioisotope or radioactive isotope is a naturally occurring or synthetic radioactive form of an element. Elements with high atomic mass numbers have many more neutrons than protons and are therefore less stable. It is these isotopes that are more prone to radioactive decay. One example is 238U, uranium-238. Most natural isotopes of atomic weight below 208 are not radioactive. Those from 210 and up are all radioactive. Most radioisotopes are made by bombarding a stable element with neutrons in the core of a nuclear reactor (see fission below). The radiations given off by radioisotopes are easy to detect (hence their use as tracers), and in some instances can penetrate substantial thicknesses of materials, and can have profound effects (such as genetic mutation) on living matter.  
  Splitting the Atom  
  An accelerator is a device to bring charged particles (such as protons and electrons) up to high speeds and energies, at which they can be of use in industry, medicine, and pure physics. At low energies, accelerated particles can be used to produce the image on a television screen and generate X-rays (by means of a c0016-01.gifcathode-ray tube), destroy tumor cells, or kill bacteria. When high-energy particles collide with other particles, the fragments formed reveal the nature of the fundamental forces (described below).  
  Early accelerators directed the particle beam onto a stationary target; large modern accelerators usually collide beams of particles that are traveling in opposite directions. This arrangement doubles the effective energy of the collision. The world's most powerful accelerator is the 2-km/1.25-mi diameter machine at Fermilab near Batavia, Illinois, United States. This machine, the Tevatron, accelerates protons and antiprotons and then collides them at energies up to a thousand billion electron volts (or 1 TeV, hence the name of the machine). The largest accelerator is the Large Electron Positron Collider at CERN near Geneva, which has a circumference of 27 km/16.8 mi around which electrons and positrons are accelerated before being allowed to collide.  
  ALEPH Experiment
  Home page of one of the four high energy particle physics experiments which uses the LEP at CERN. This Web site displays images of the components of ALEPH, including a cutaway schematic diagram of the huge detector assembly.  




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  linear accelerator The linear accelerator, or linac, consists of a line of metal tubes, called drift tubes, through which the particles travel. The particles are accelerated by electric fields in the gaps between the drift tubes. The world's longest linac is also a colliding beam machine: the Stanford Linear Collider, in California, in which electrons and positrons are accelerated along a straight track, 3.2 km/2 mi long, and then steered to a head-on collision with other particles, such as protons and neutrons  
  cyclotron In a cyclotron an electric field is used to bend the path of a particle into a circle so that it passes repeatedly through the same electric field. A cyclotron consists of an electromagnet with two hollow, metal semicircular structures, called dees, supported between the poles of an electromagnet. Particles such as protons are introduced at the center of the machine and travel outward in a spiral path, being accelerated by an oscillating electric field each time they pass through the gap between the dees. Cyclotrons can accelerate particles up to energies of 25 MeV (25 million electron volts). To produce higher energies, new techniques are needed.  
  synchrotron In the synchrotron, particles travel in a circular path of constant radius, guided by electromagnets. The strengths of the electromagnets are varied to keep the particles on an accurate path. Electric fields at points around the path accelerate the particles.  
  Accelerator Physics Page
  Virtual library dedicated to accelerator physics, with pages on design and components, as well as direct links to laboratories throughout the world.  
  fission Fission is the splitting of a heavy atomic nucleus into two or more major fragments. It is accompanied by the emission of two or three neutrons and the release of large amounts of nuclear energy. Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits. The neutrons released spontaneously by the fission of uranium nuclei may therefore be used in turn to induce further fissions, setting up a c0016-01.gifchain reaction that must be controlled if it is not to result in a nuclear explosion. The minimum amount of fissile material that can undergo a continuous chain reaction is referred to as the critical mass.  
  In nuclear fusion, the nuclei of light elements, such as hydrogen, combine to form the bigger nucleus of a heavier element, such as helium. The resultant loss in their combined mass is converted into energy. Stars and thermonuclear weapons are powered by nuclear fusion. Very high temperatures and pressures are thought to be required in order for fusion to take place. Under these conditions the atomic nuclei can approach each other at high speeds and overcome the mutual repulsion of their positive charges. At very close range another force, the strong nuclear force, comes into play, fusing the particles together to form a larger nucleus.  
  All-text site, but packed with information about fusion research and its applications. It is quite well organized and it includes a glossary of commonly used terms to aid the uninitiated.  
  As fusion is accompanied by the release of large amounts of energy, the process might one day be harnessed to form the basis of commercial energy production. So far no successful fusion reactor—one able to produce the required conditions and contain the reaction—has been built. An important step was taken in 1991, however, when, in an experiment that lasted 2 seconds, a 1.7 megawatt pulse of power was produced by the Joint European Torus (JET) at Culham, Oxfordshire, UK. This was the first time that a substantial amount of fusion power had been produced in a controlled experiment, as opposed to a bomb. In 1997 JET produced a record 21 megajoule of fusion power, and tested the first large-scale plant of the type needed to supply and process tritium in a future fusion power station.  
  Subatomic Particles  
  Subatomic particles are particles that are smaller than an atom. Such particles may be indivisible elementary particles, such as the electron and quark, or they may be composites, such as the proton, neutron, and alpha particle.  
  elementary particle An elementary particle is a subatomic particle that is not made up of smaller particles, and so can be considered one of the fundamental units of matter. There are three groups of elementary particles: quarks, leptons, and gauge bosons.  
  antiparticle An antiparticle is a particle corresponding in mass and properties to a given elementary  




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Principal Subatomic Particles


  particle but with the opposite electrical charge, magnetic properties, or coupling to other fundamental forces. For example, an electron carries a negative charge whereas its antiparticle, the positron, carries a positive one. When a particle and its antiparticle collide, they destroy each other, in the process called  




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  "annihilation," their total energy being converted to lighter particles and/or photons. A substance consisting entirely of antiparticles is known as antimatter.  
  There are six types or "flavors" of quarks (up, down, charm, strange, top, and bottom), each of which has three varieties or "colors": red, green, and blue (visual color is not meant, although the analogy is useful in many ways). To each quark there is an antiparticle, called an antiquark. Quarks combine in groups of three to produce heavy particles called baryons, and in groups of two to produce particles with masses between those of electrons and protons, called mesons. Baryons and mesons together are known as hadrons, and they and their composite particles are influenced by the strong nuclear force. Quarks have electric charges that are fractions of the electronic charge (+c0133-01.gif or –c0039-01.gif of the electronic charge).  
  Leaping Leptoquarks
  Part of a larger site maintained by Scientific American, this page follows the trail of two German physicists in search of the elusive leptoquark. Find out about events that led them to believe they were witnessing a new phenomenon: a particle that combined aspects of the two elementary particles that make up atoms, leptons and quarks.  
  baryon A baryon is a heavy subatomic particle made up of three quarks. The baryons form a subclass of the hadrons and comprise the nucleons (protons and neutrons) and hyperons. Baryons have half-integral spins.  
  meson A meson is a group of unstable subatomic particles made up of two indivisible elementary particles. It has a mass intermediate between that of the electron and that of the proton, is found in cosmic radiation, and is emitted by nuclei under bombardment by very high-energy particles. There are believed to be 15 ordinary types. The last of these to be found was identified by physicists at Fermilab, United States in 1998. Mesons have whole-number or zero spins.  
  Leptons are light particles. There are six types: the electron, muon, and tau; and their neutrinos, the electron neutrino, muon neutrino, and tau neutrino. Each also has a corresponding antiparticle. These particles are influenced by the weak nuclear force but not the strong force, and do not interact strongly with other particles or nuclei.  
  electron The electron is a stable, negatively charged elementary particle and is a constituent of all atoms. The electrons in each atom surround the nucleus in groupings called shells; in a neutral atom the number of electrons is equal to the number of protons in the nucleus. This electron structure is responsible for the chemical properties of the atom.  
  positron The positron is the antiparticle of the electron and has the same mass as an electron but an equal and opposite charge. The positron was discovered in 1932 by the U.S. physicist Carl Anderson at California Institute of Technology, United States, its existence having been predicted by the British physicist Paul Dirac in 1928. This was the first example of antimatter.  
  muon The muon is an elementary particle (found by the US physicist Carl Anderson in cosmic radiation in 1937) similar to the electron except for its mass which is 207 times greater than that of the electron. It has a half-life of 2 millionths of a second, decaying into electrons and neutrinos. The muon produces the muon neutrino when it decays. The muon was originally thought to be a meson and is thus sometimes called a mu meson, although current opinion is that it is a lepton.  
  tau The tau is an elementary particle with the same electric charge as the electron but a mass nearly double that of a proton. It has a lifetime of around 3 x 10–13 seconds. The tau produces the tau neutrino when it decays.  
  neutrino Neutrinos are any of three uncharged elementary particles (and their antiparticles) of the lepton class, having a mass too close to zero to be measured. The most familiar type, the antiparticle of the electron neutrino, is emitted in the beta decay of a nucleus. The other two are the muon and tau neutrinos. The existence of the tau neutrino has never been conclusively confirmed but it is believed to cause a less stable particle, tau, to be emitted from the nucleus of an atom when the atom is struck by a tau neutrino. The tau decays almost instantaneously. Researchers at Fermilab believe they found traces of the tau released during an experiment in 1997.  
  gauge bosons  
  Gauge bosons carry the four fundamental forces between other particles. There are four types: gluon, photon, weakon, and graviton. The gluon carries the strong nuclear force, the photon the electromagnetic force, the weakons the weak nuclear force, and the graviton the force of gravity (see fundamental forces below). Gravitons have yet to be discovered.  
  gluon A gluon is a gauge boson that carries the strong nuclear force, responsible for binding quarks together  




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  European Laboratory for Particle Physics
  Information about CERN, the world class physics laboratory in Geneva. As well as presenting committees, groups, and associations hosted by the Laboratory, this official site offers important scientific material and visual evidence on several activities and projects currently undertaken by the various groups.  
  to form the strongly interacting subatomic particles known as c0016-01.gifhadrons. There are eight kinds of gluon. Gluons cannot exist in isolation; they are believed to exist in balls ("glueballs") that behave as single particles. Glueballs may have been detected at CERN in 1995 but further research is required to confirm their existence.  
  photon The photon is the elementary particle or "package" (quantum) of energy in which light and other forms of electromagnetic radiation are emitted. The photon has both particle and wave properties; it has no charge, is considered massless but possesses momentum and energy. It is the carrier of the electromagnetic force. According to quantum theory the energy of a photon is given by the formula E = hf, where h is Planck's constant and f is the frequency of the radiation emitted.  
  Beam Me Up: Photons and Teleportation
  Part of a larger site maintained by Scientific American, this page reports on the amazing research conducted by physicists at the University of Innsbruck who have turned science fiction into reality by teleporting the properties of one photon particle to another.  
  weakon The weakon, or intermediate vector boson, carries the weak nuclear force, one of the fundamental forces of nature. There are three types of weakon, the positive and negative W particle and the neutral Z particle.  
  Radioactivity is the spontaneous alteration of the nuclei of radioactive atoms, accompanied by the emission of radiation. It is the property exhibited by the radioactive isotopes of stable elements and all isotopes of radioactive elements, and can be either natural or induced. There are three types of radioactive radiation: alpha particles, beta particles, and gamma rays.  
  When alpha, beta, and gamma radiation pass through matter they tend to knock electrons out of atoms, ionizing them. They are therefore called ionizing radiation. Alpha particles are the most ionizing, being heavy, slow moving, and carrying two positive charges. Gamma rays are weakly ionizing as they carry no charge. Beta particles fall between alpha and gamma radiation in ionizing potential. Alpha, beta, and gamma radiation are dangerous to body tissues because of their ionizing properties, especially if a radioactive substance is ingested or inhaled.  
  radioactive decay Radioactive decay occurs when the unstable nuclei of radioactive elements, such as radium and various isotopes of uranium and the transuranic elements, disintegrate to become more stable. This changes the element's atomic number, thus transmuting one element into another. The energy given out by disintegrating atoms is called atomic radiation and consists either of alpha, beta, or gamma rays. Alpha and beta decay are the most common forms. Certain lighter, artificially created, isotopes also undergo radioactive decay. Radioactive decay takes place at a constant rate expressed as a specific half-life, which is the time taken for half of any mass of that particular isotope to decay completely.  
  Radioactive decay can take place either as a one-step decay, or through a series of steps that transmute one element into another. This is called a decay series or chain, and sometimes produces an element more radioactive than its predecessor. For example, uranium 238 decays by alpha emission to thorium 234; thorium 234 is a beta emitter and decays to give protactinium 234. This emits a beta particle to form uranium 234, which in turn undergoes alpha decay to form thorium 230. A further alpha decay yields the isotope radium 226.  
  half-life During radioactive decay, the time in which the strength of a radioactive source decays to half its original value is known as its half-life. In theory, the decay process is never complete and there is always some residual radioactivity. For this reason, the half-life  
  (least stable)  
  4.4 × 10–22 sec  
  4.2 × 10–6 sec  
  36 min  
  3.3 hours  
  4.551 × 109 years  
  1.39 × 1010 years  
(most stable)
  1.5 × 1024 years  





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  of a radioactive isotope is measured, rather than the total decay time. It may vary from millionths of a second to billions of years. Radioactive substances decay exponentially; thus the time taken for the first 50% of the isotope to decay will be the same as the time taken by the next 25%, and by the 12.5% after that, and so on. For example, carbon-14 takes about 5,730 years for half the material to decay; another 5,730 for half of the remaining half to decay; then 5,730 years for half of that remaining half to decay, and so on. Plutonium-239, one of the most toxic of all radioactive substances, has a half-life of about 24,000 years. The final product in all modes of decay is a stable element.  
  Rutherford's Discovery of Half-Life
  Transcript of Ernest Rutherford's paper describing his discovery of the half-life of radioactive materials.  
  alpha decay In alpha decay an alpha particle (two protons and two neutrons) is emitted from a nucleus and the atomic number decreases by two to form a new nucleus. For example, an atom of uranium isotope of mass 238, on emitting an alpha particle, becomes an atom of thorium, mass 234.  
  beta decay Beta decay is the disintegration of the nucleus of an atom to produce a beta particle, or high-speed electron, and an electron-antineutrino. During beta decay a neutron in the nucleus changes into a proton, thereby increasing the atomic number by one while the mass number stays the same. For example, the decay of the carbon 314 isotope results in the formation of an atom of nitrogen (mass 14, atomic number 7) and the emission of an electron. The mass lost in the change is converted into kinetic (movement) energy of the beta particle. Beta decay is caused by the weak nuclear force, one of the fundamental forces of nature operating inside the nucleus.  
  Rutherford on the Discovery of Alpha and Beta Radiation
  Transcript of Rutherford's paper describes the nature of the two types of radiation he discovered to be emitted from uranium as it decays.  
  alpha particle Alpha particles are positively charged, high-energy particles emitted from the nucleus of a radioactive atom. They consist of two neutrons and two protons and are thus identical to the nucleus of a helium atom and are one of the products of the spontaneous disintegration of radioactive elements such as radium and thorium. The process of emission, alpha decay, transforms one element into another, decreasing the atomic (or proton) number by two and the atomic mass (or nucleon number) by four. Because of their large mass, alpha particles have a short range of only a few centimeters in air, and can be stopped by a sheet of paper. They have a strongly ionizing effect on the molecules that they strike, and are therefore capable of damaging living cells. Alpha particles traveling in a vacuum are deflected slightly by magnetic and electric fields.  
  beta particle The beta particle is an electron emitted at high velocity from a radioactive atom that is undergoing spontaneous disintegration. Beta particles do not exist in the nucleus but are created on disintegration—beta decay—when a neutron converts to a proton to emit an electron. Beta particles are more penetrating than alpha particles, but less so than gamma radiation; they can travel several meters in air, but are stopped by 2–3 mm of aluminum. They are less strongly ionizing than alpha particles and, like cathode rays, are easily deflected by magnetic and electric fields.  
  gamma radiation Gamma rays comprise very high-frequency electromagnetic radiation, similar in nature to X-rays but of shorter wavelength (wavelengths of less than 10–10) emitted by the nuclei of radioactive substances during decay or by the interactions of high-energy electrons with matter. Gamma rays are stopped only by direct collision with an atom and are therefore very penetrating; they can, however, be stopped by about 4 cm/1.5 in of lead or by a very thick concrete shield. Gamma emission usually occurs as part of alpha or beta emission. They are less ionizing in their effect than alpha and beta particles, but are dangerous nevertheless because they can penetrate deeply into body tissues such as bone marrow. They are not deflected by either magnetic or electric fields. Gamma radiation is used to kill bacteria and other microorganisms, sterilize medical devices, and change the molecular structure of plastics to modify their properties (for example, to improve their resistance to heat and abrasion). Cosmic gamma rays have been identified as coming from pulsars, radio galaxies, and quasars, although they cannot penetrate the earth's atmosphere.  
  Electricity and Magnetism  
  Electricity is all phenomena caused by electric charge, whether static or in motion.  




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  electric charge Electric charge is the property of some bodies that causes them to exert forces on each other and is caused by an excess or deficit of electrons in the charged substance and is therefore either positive or negative. Objects with a like charge always repel one another while objects with an unlike charge attract each other. In atoms, electrons possess a negative charge, and protons an equal positive charge. Atoms have no charge but can sometimes gain electrons to become negative ions or lose them to become positive ions. A coulomb (C), named for the French scientist Charles Augustin de Coulomb (1736–1806), is the unit of charge, and is defined as the charge passing a point in a wire each second when the current is exactly 1 amp.  
  static electricity Static electricity is an electric charge that is stationary, usually acquired by a body by means of electrostatic induction or friction. Rubbing different materials can produce static electricity or cause them to be electrically charged so that it they have an excess or deficit of electrons, as seen in the sparks produced on combing one's hair or removing a nylon shirt. This charge on the object exerts an electric field in the space around itself that can attract or repel other objects. In some processes static electricity is useful, as in paint spraying where the parts to be sprayed are charged with electricity of opposite polarity to that on the paint droplets, and in xerography.  
  potential divider A potential divider is a resistor or a
chain of resistors connected in series in an electrical
circuit. It is used to obtain a known fraction of the total
voltage across the whole resistor or chain. When a
variable resistor, or potentiometer, is used as a potential
divider, the output voltage can be varied continuously
by sliding a contact along the resistor. Devices like
this are used in electronic equipment to to vary volume,
tone, and brightness control.
  electric current An electric current is the movement of electrically charged particles through a conducting material. For charge to flow in a circuit there must be a c0016-01.gifpotential difference (pd) applied across the circuit. Conventionally, current is regarded as a movement of positive electricity from points at high potential to points at a lower potential. Potential difference is often supplied in the form of a battery that has a positive terminal and a negative terminal. Under the influence of the potential difference, the electrons are repelled from the negative terminal side of the circuit and attracted to the positive terminal of the battery. A steady flow of electrons around the circuit is produced. Current flowing through a circuit can be measured using an ammeter and is measured in c0016-01.gifamperes (or amps). Direct current (D.C.) flows continuously in one direction; alternating current (A.C.) flows alternately in each direction. The flow of current is measured in amperes (symbol A).  
  induced current Movement of a magnet in a coil of wire induces a current.  
  In a circuit the battery provides energy to make charge flow through the circuit. The amount of energy supplied to each unit of charge is called the electromotive force (emf). The unit of emf is the volt (V). A battery has an emf of 1 volt when it supplies 1 joule  




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  moving-coil meter A simple moving-coil meter.
Direct electric current (D.C.) flowing through
the wire coil combined with the presence of a
magnetic field causes the coil to rotate; this
in turn moves a pointer across a calibrated
scale so that the degree of rotation can be
related to the magnitude of the current.
  To remember whether current leads voltage or lags it in reactive circuits:  
  Think of "Eli the Ice man." In inductive ("L") circuits, voltage ("E") leads current ("I''), hence "E L I." In capacitive ("C") circuits, it is the other way, so "I C E"  


  of energy to each coulomb of charge flowing through it. The energy carried by flowing charges can be used to do work, for example to light a bulb, to cause current to flow through a resistor, to emit radiation, or to produce heat. When the energy carried by a current is made to do work in this way, a potential difference can be measured across the circuit component concerned by a voltmeter or a cathode-ray oscilloscope. The potential difference is also measured in volts. Power, measured in watts, is the product of current and voltage. Although potential difference and current measure different things, they are related to one another. This relationship was discovered by the German physicist Georg Ohm, and is expressed by Ohm's law: the current through a wire is proportional to the potential difference across its ends. The potential difference divided by the current is a constant for a given piece of wire. This constant for a given material is called the resistance. When current flows in a component possessing resistance, electrical energy is converted into heat energy.  
  To remember the order of items in an AC circuit:  
  The voltage (V) across a capacitor (C) lags the current (I) by 90 degrees. The voltage across the inductor (L) leads the current by 90 degrees. This is given by the word CIVIL, when split into CIV and VIL.  


Ohm's law
  To remember one expression of this:  
  Vampires are rare.  
  (volts = amps × resistance)  


  conductors Electrical conduction is the flow of charged particles through a material giving rise to electric current. Electrical conductors are substances, such as metals, that allow the passage of electricity through them readily. In metals and other conducting materials, the charge is carried by negatively charged free electrons that are not bound tightly to the atoms and are thus able to move through the material.  
  Conduction in many liquids involves a flow not merely of electrons, but of atoms or groups of atoms as well. When a salt such as sodium chloride is dissolved in water, the chlorine atoms each gain an electron and become negatively charged, while the sodium atoms each lose one and become positively charged. These charged atoms, or ions, can move through the liquid and transport electricity. Current flows by the movement of charged ions through a solution or molten salt (the electrolyte), resulting in the migration of ions to the electrodes: positive ions (cations) to the negative electrode (cathode) and negative ions (anions) to the positive electrode (anode). This process is called electrolysis and represents bi-directional flow of charge as opposite charges move to oppositely charged electrodes. In metals, charges are only carried by free electrons and therefore move in only one direction. Gases are, under normal circumstances, almost completely nonconducting. They may be ionized by irradiation with X-rays or by radioactive radiations. They are more readily maintained in a conducting state at high temperatures, as in the electric arc, or at low pressures, as in electric discharge lamps.  
  A magnetic field is created around all conductors that carry a current. When a current-bearing conductor is made into a coil it forms an electromagnet with a magnetic field that is similar to that of a bar magnet, but which disappears as soon as the current is switched off. The strength of the magnetic field is  




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  Maxwell's screw rule Maxwell's screw
rule, named for the physicist James Maxwell,
predicts the direction of the magnetic field
produced around a wire carrying electric
current. If a right-handed screw is turned so
that it moves forward in the same direction
as the current, its direction of rotation will
give the direction of the magnetic field.
  directly proportional to the current in the conductor—a property that allows a small electromagnet to be used to produce a pattern of magnetism on recording tape or disk that accurately represents the sound or data to be stored. The direction of the field created around a conducting wire may be predicted by using Maxwell's screw rule.  
  Fleming's rules Fleming's rules give the direction
of the magnetic field, motion, and current in electrical
machines. The left hand is used for motors, and the
right hand for generators and dynamos.
  Electrical Decomposition by Michael Faraday
  Transcript of Faraday's paper in Philosophical Transactions of the Royal Society, 1834 in which Faraday describes for the first time the phenomena of electrolysis.  
  A conductor carrying current in a magnetic field experiences a force, and is impelled to move in a direction perpendicular to both the direction of the current and the direction of the magnetic field. The direction of motion may be predicted by Fleming's left-hand rule. The magnitude of the force experienced depends on the length of the conductor and on the strengths of the current and the magnetic field, and is greatest when the conductor is at right angles to the field. A conductor wound into a coil that can rotate between the poles of a magnet forms the basis of an electric motor.  
  loudspeaker A moving-coil loudspeaker.
Electrical signals flowing through the
wire coil turn it into an electromagnet,
which moves as the signals vary. The
attached cone vibrates, producing
sound waves.
  insulators Insulators, such as rubber, are extremely poor conductors in which the electrons are more tightly bound to the atoms and conduction is less.  
  semiconductors Semiconductors are substances with electrical conductivity intermediate between metals and insulators and used in a wide range of electronic devices. Certain crystalline materials, most notably silicon and germanium, have a small number of free electrons that have escaped from the bonds between the  




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  parallel circuit In a parallel circuit, the
components are connected side by side,
so that the current is split between two
or more parallel paths or conductors.
  atoms. The atoms from which they have escaped possess vacancies, called holes, which are similarly able to move from atom to atom and can be regarded as positive charges. Current can be carried by both electrons (negative carriers) and holes (positive carriers). Such materials are known as intrinsic semiconductors.  
  Conductivity of a semiconductor can be enhanced by doping the material with small numbers of impurity atoms which either release free electrons (making an n-type semiconductor with more electrons than holes) or capture them (a p-type semiconductor with more holes than electrons). When p-type and n-type materials are brought together to form a p–n junction, an electrical barrier is formed which conducts current more readily in one direction than the other. This is the basis of the semiconductor diode, used for rectification, and numerous other devices including c0016-01.giftransistors, rectifiers, and c0016-01.gifintegrated circuits (silicon chips). The conductivity of semiconductors can also be improved by the addition of heat or light. Increase of temperature frees more electrons, so the conductivity of nonmetals increases with rising temperature.  
  series circuit In a series circuit, the components
of the circuit are connected end to end, so that the
current passes through each component one after the
other, without division or branching into parallel circuits.
  superconductivity Superconductivity is the increase in electrical conductivity at low temperatures. The resistance of some metals and metallic compounds decreases uniformly with decreasing temperature until at a critical temperature (the superconducting point), within a few degrees of absolute zero (0K/ –273.15°C/–459.67°F), the resistance suddenly falls to zero. In the superconducting state, an electric current will continue indefinitely once started, provided that the material remains below the superconducting point  
  In 1986 IBM researchers achieved superconductivity with some ceramics at –243°C/ –405°F, opening up the possibility of "high-temperature" superconductivity; Paul Chu at the University of Houston, Texas, achieved superconductivity at –179°C/ –290°F, a temperature that can be sustained using liquid nitrogen. In 1993 Swiss researchers produced an alloy of mercury, barium, and copper which becomes superconducting at 133 K (–140°C/–220°F). A high-temperature semiconductor material, called bismuth ceramic, which is superconducting at 100 K, became commercially available in 1997.  
  Some metals, such as platinum and copper, do not become superconductive; as the temperature decreases, their resistance decreases to a certain point but then rises again. Superconductivity can be nullified by the application of a large magnetic field.  
  Introduction to High Temperature Superconductivity
  Texas Center for Superconductivity at the University of Houston, Texas, United States, offers a beginners guide to the phenomenon of high temperature superconductivity. The site, run by one of the world centers for superconductor research, includes some of the classic images of superconductors at work, and describes in reasonably understandable terms the history and theory of the materials and their bizarre properties.  
  electromagnetism Magnetic fields are produced either by current-carrying conductors or by permanent magnets. In current-carrying wires, the magnetic field lines are concentric circles around the wire. Their direction depends on the direction of the current and their strength on the size of the current. If a conducting wire is moved within a magnetic field, the magnetic field acts on the free electrons within the conductor, displacing them and causing a current to flow. The force acting on the electrons and causing them to move is  




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  greatest when the wire is perpendicular to the magnetic field lines. The direction of the current is given by the left-hand rule. The generation of a current by the relative movement of a conductor in a magnetic field is called electromagnetic induction. This is the basis of how a dynamo works.  
A magnet is any object that forms a magnetic field (displays magnetism), either permanently or temporarily through induction, causing it to attract materials such as iron, cobalt, nickel, and alloys of these. It always has two magnetic poles, called north and south. The world's most powerful magnet was built in 1997 at the Lawrence Berkeley National Laboratory, California, United States. It produces a field 250,000 times stronger than the earth's magnetic field (13.5 teslas). The coil magnet is made of an alloy of niobium and tin.
  magnetism and magnetic fields Magnetism refers to phenomena associated with magnetic fields. A magnetic field is the region around a permanent magnet, or around a conductor carrying an electric current, in which a force acts on a moving charge or on a magnet placed in the field. Magnetic fields are produced by moving charged particles: in electromagnets, electrons flow through a coil of wire connected to a battery; in permanent magnets, spinning electrons within the atoms generate the field. The field can be represented by lines of force, which by convention link north and south poles and are parallel to the directions of a small compass needle placed on them. A magnetic field's magnitude and direction are given by the magnetic flux density, expressed in teslas.  
  All substances are magnetic to a greater or lesser degree, and their magnetic properties, however feeble, may be observed when they are placed in an intense magnetic field. Materials that can be strongly magnetized, such as iron, cobalt, and nickel, are said to be ferromagnetic; this is due to the formation of areas called domains in which atoms, weakly magnetic because of their spinning electrons, align to form areas of strong magnetism. Magnetic materials lose their magnetism if heated to the c0016-01.gifCurie temperature. Furthermore, if the magnetizing force is increased, a stage is reached when the magnet becomes saturated, that is, its pole strength reaches a maximum value. Most substances are paramagnetic, being only weakly pulled toward a strong magnet. This is because their atoms have a low level of magnetism and do not form c0016-01.gifdomains. Diamagnetic materials, notably bismuth, are weakly repelled by a magnet since electrons within their atoms act as electromagnets and oppose the applied magnetic force. Antiferromagnetic materials have a very low susceptibility that increases with temperature.  
  molecular magnets When a magnet is broken in two, we do not obtain two halves, one with a north pole, the other with a south pole. Two new poles appear at the point of fracture. However often this process is repeated the same result is obtained: every magnet has two poles.  
  The German physicist Wilhelm Weber suggested that every magnet was really composed of magnetic particles or magnetized domains that are now believed to be of molecular dimensions. The Scottish physicist J. Alfred Ewing (1855–1935) developed his theory and suggested that, since the act of magnetization did not change the chemical character nor the weight of the specimen, but simply endowed it with magnetic properties, magnetizable substances consisted of molecular magnets. According to this theory, an ordinary piece of iron is made up of molecular magnets arranged in haphazard fashion, so that they neutralize each other's effects on external bodies. This disorder disappears when the iron is placed in a magnetic field and the molecular magnets are set with their axes parallel to the field: free poles appear at the ends of the magnet, while the central portions exhibit only feeble magnetic powers because equal and opposite poles neutralize each other's effects. This theory accounts for the appearance of new poles wherever the magnet is broken, and the state of saturation is reached when all the molecular magnets have been arranged in order. Subsequent loss of magnetism is explained by the partial return to disordered array.  
  magneton theory Early in the 20th century, Pierre Weiss suggested the existence of the magneton or elementary magnet, an analog of the electron, the elementary charge of electricity. An electric current flowing around a circular coil has a magnetic field similar to that of a magnet whose axis coincides with that of the coil: the electrical theory of matter attempts to ascribe the magnetic properties of bodies to the orbital motions of the electrons in the atom. The c0016-01.gifquantum theory of the atom developed by Niels Bohr supported the magneton theory, and subsequently direct experimental evidence of the existence of the magnetic moment associated with electron orbits was obtained by Otto Stern and Walther Gerlach in 1921.  




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  Matter is anything that has mass. All matter is made up of atoms, which in turn are made up of elementary particles; it ordinarily exists in one of three physical states: solid, liquid, or gas. The state it exists in depends on its temperature and the pressure on it. c0016-01.gifKinetic theory describes how the state of a material depends on the movement and arrangement of its atoms or molecules. In a solid, the atoms or molecules vibrate in a fixed position. In a liquid, they do not occupy fixed positions as in a solid, and yet neither do they have the freedom of random movement that occurs within a gas, so the atoms or molecules within a liquid will always follow the shape of their container. The transition between states takes place at definite temperatures, called melting point and boiling point. In chemical reactions matter is conserved, so no matter is lost or gained and the sum of the mass of the reactants will always equal the sum of the end products.  
  Antimatter is a form of matter in which most of the attributes (such as electrical charge, magnetic moment, and spin) of elementary particles are reversed. Such particles (antiparticles) can be created in particle accelerators, such as those at CERN in Geneva, Switzerland, and at Fermilab in the United States. In 1996 physicists at CERN created the first atoms of antimatter: nine atoms of antihydrogen survived for 40 nanoseconds.  
  Mass is the quantity of matter in a body as measured by its inertia (tendency to remain in a state of rest or uniform motion until an external force is applied). Mass determines the acceleration produced in a body by a given force acting on it, the acceleration being inversely proportional to the mass of the body. The mass also determines the force exerted on a body by gravity on earth, although this attraction varies slightly from place to place. In the SI system, the base unit of mass is the kilogram. At a given place, equal masses experience equal gravitational forces, which are known as the weights of the bodies. Masses may, therefore, be compared by comparing the weights of bodies at the same place.  
mass and volume
  To remember that if an object floats, it displaces water equal to its mass, but if it sinks, it displaces water equal to its volume.  
  Think of a pebble, made of neutronium. It is small, but it weighs a lot. If it were to displace water equal to its mass, then when you threw this little pebble into a swimming pool, all the water would have to jump out of the swimming pool. So it must only displace water equal to its volume.  


  density Density is the measure of the compactness of a substance; it is equal to its mass per unit volume and is measured in kg per cubic meter/lb per cubic foot. Density is a scalar quantity. The average density D of a mass m occupying a volume V is given by the formula: D = m/V. Relative density is the ratio of the density of a substance to that of water at 4°C/32.2°F.  
  change of state The state (solid, liquid, or gas) of any substance
is not fixed but varies with changes in temperature and pressure.
  A solid is a state of matter that holds its own shape (as opposed to a liquid, which takes up the shape of  




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  manometer The manometer indicates gas
pressure by the rise of liquid in the tube.
  its container, or a gas, which totally fills its container). According to c0016-01.gifkinetic theory, the atoms or molecules in a solid are not free to move but merely vibrate about fixed positions, such as those in crystal lattices.  
  A liquid is a state of matter between a solid and a gas. A liquid forms a level surface and assumes the shape of its container. Its atoms do not occupy fixed positions as in a crystalline solid, nor do they have freedom of movement as in a gas. Unlike a gas, a liquid is difficult to compress since pressure applied at one point is equally transmitted throughout (Pascal's principle). Hydraulics makes use of this property.  
  Gas is a form of matter, such as air, in which the molecules move randomly in otherwise empty space, filling any size or shape of container into which the gas is put. A sugar-lump sized cube of air at room temperature contains 30 trillion molecules moving at an average speed of 500 meters per second (1,800 kph/1,200 mph). Plasma is an ionized gas produced at extremely high temperatures, as in the sun and other stars, which contains positive and negative charges in equal numbers. It is a good electrical conductor. In thermonuclear reactions the plasma produced is confined through the use of magnetic fields.  
  Internet Plasma Physics Education Experience
  Aimed at teenagers, this site introduces some physics concepts through interactive pages on the topics of matter, electricity and magnetism, fusion, and energy. The site also contains a virtual fusion reactor and a page where you can send questions to scientists in the field.  
  Mechanics is the branch of physics dealing with the motions of bodies and the forces causing these motions, and also with the forces acting on bodies in equilibrium. It is usually divided into dynamics, or kinetics, and statics. Dynamics is the mathematical and physical study of the behavior of bodies under the action of forces that produce changes of motion in them. Statics is concerned with the behavior of bodies at rest and forces or moving with constant velocity where the forces acting on the body cancel each other out; that is, the forces are in equilibrium.  
  As well as dealing with the direct action of forces on bodies, mechanics studies the nature and action of forces when they act on bodies by the agency of machinery. This gives the origin of the word "mechanics": in its early stages it was the science of making machines. A machine in mechanics means any contrivance in which a force applied at one point is made to raise weight or overcome a resisting force acting at another point. All machines can be resolved into three primary machines: the lever, the inclined plane, and the wheel and axle.  
mechanical advantage
  To remember the definition of mechanical advantage:  
  Men always like eating.  
  (MA—load over effort [I/e])  


  inertia Inertia is the tendency of an object to remain in a state of rest or uniform motion until an external force is applied, as described by Isaac Newton's first law of motion.  
  couple Two equal but opposite forces (F) will
produce a turning effect on a rigid body, provided
that they do not act through the same straight line.
The turning effect, or moment, is equal to the
magnitude of one of the turning forces multiplied
by the perpendicular distance (d) between those two forces.




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  Newton's laws of motion  
  Isaac Newton's three laws of motion form the basis of Newtonian mechanics. (1) Unless acted upon by an external force, a body at rest stays at rest, and a moving body continues moving at the same speed in the same straight line. (2) An external force applied to a body gives it an acceleration proportional to the force (and in the direction of the force) and inversely proportional to the mass of the body. (3) When a body A exerts a force on a body B, B exerts an equal and opposite force on A; that is, to every action there is an equal and opposite reaction.  
  Force is any influence that tends to change the state of rest or the uniform motion in a straight line of a body. The action of an unbalanced or resultant force results in the acceleration of a body in the direction of action of the force, or it may, if the body is unable to move freely, result in its deformation (see c0016-01.gifHooke's law). Force is a vector quantity, possessing both magnitude and direction; its SI unit is the newton.  
  resolution of forces In mechanics, the resolution of
forces is the division of a single force into two parts
that act at right angles to each other. In the diagram,
the weight W of an object on a slope, tilted at an angle
q, can be resolved into two parts or components: one
acting at a right angle to the slope, equal to Wcos
q, and
one acting parallel to and down the slope, equal to Wsin
  speed and distance  
  In order to understand movement and what causes it, we need to be able to describe it. Speed is a measure of how fast something is moving. Speed is measured by dividing the distance traveled by the time taken to travel that distance. Hence speed is distance moved in unit time. Speed is a scalar quantity in which the direction of travel is not important, only the rate of travel.  
  Velocity is the speed of an object in a given direction. Velocity is therefore a vector quantity, in which both magnitude and direction of movement must be taken into account. The velocity at any instant of a particle traveling in a curved path is in the direction of the tangent to the path at the instant considered. The velocity v of an object traveling in a fixed direction may be calculated by dividing the distance s it has travelled by the time t taken to do so, and may be expressed as: v = s/t.  
  Acceleration is the rate of change of velocity of a moving body with time. This is also a vector quantity. Acceleration is usually measured in meters per second per second (m s–2) or feet per second per second (ft s–2). Because velocity is a vector quantity (possessing both magnitude and direction), a body traveling at constant speed may be said to be accelerating if its direction of motion changes. According to Isaac Newton's law of gravitation, all objects fall to earth with the same acceleration, regardless of mass. According to Newton's second law of motion, a body will accelerate only if it is acted upon by an unbalanced, or resultant, force. Acceleration due to gravity is the acceleration of a body falling freely under the influence of the earth's gravitational field; it varies slightly at different latitudes and altitudes. The value adopted internationally for gravitational acceleration is 9.806 m s–2/32.174 ft s–2. The average acceleration a of an object traveling in a straight line over a period of time t may be calculated using the formula:  
  The maximum speed with which a falling raindrop can hit you is about 29 kmph/18 mph. In a vacuum, the further an object falls, the more speed it gains, byt in the real world, air resistance eventually balances or the accelerating effect of gravity.  





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Physical Constants
Physical constants, or fundamental constants, are standardized values whose parameters do not change.
Constant Symbol Value in SI units
acceleration of free fall g 9.80665 m s–2
Avogadro's constant NA 6.0221367 × 1023 mol–1
Boltzmann's constant k 1.380658 × 10–23 J K–1
elementary charge e 1.60217733 × 10–19 C
electronic rest mass me 9.1093897 × 10–31 kg
Faraday's constant F 9.6485309 × 104 C mol–1
gas constant R 8.314510 J K–1 mol–1
gravitational constant G 6.672 × 10–11 N m2 kg–2
Loschmidt's number NL 2.686763 × 1025 m–3
neutron rest mass mn 1.6749286 × 10–27 kg
Planck's constant h 6.6260755 × 10–34 J s
proton rest mass mp 1.6726231 × 10–27 kg
speed of light in a vacuum c 2.99792458 × 108 m s–1
standard atmosphere atm 1.01325 × 105 Pa
Stefan—Boltzmann constant s 5.67051 × 10–8 W m–2 K–4


  a = vc0144-01.gif, where u is its initial velocity and v its final velocity.  
  A negative answer shows that the object is slowing down (decelerating).  
  momentum Momentum is a function both of the mass of a body and of its velocity and is the product of the mass of a body and its velocity. If the mass of a body is m kilograms and its velocity is v m s–1 then its momentum is given by: momentum = mv. Its unit is the kilogram meter-per-second (kg ms s–1) or the newton second. The momentum of a body does not change unless a resultant or unbalanced force acts on that body. The law of conservation of momentum is one of the fundamental concepts of classical physics. It states that the total momentum of all bodies in a closed system is constant and unaffected by processes occurring within the system.  
  forces and motion Galileo discovered that a body moving on a perfectly smooth horizontal surface would neither speed up nor slow down. All moving bodies continue moving with the same velocity unless a force is applied to cause an acceleration. The reason we appear to have to push something to keep it moving with constant velocity is because of frictional forces acting on all moving objects on earth. Friction occurs when two solid surfaces rub on each other; for example, a car tire in contact with the ground. Friction opposes the relative motion of the two objects in contact and acts to slow the velocity of the moving object. A force is required to push the moving object and to cancel out the frictional force. If the forces combine to give a net force of zero, the object will not accelerate but will continue moving at constant velocity. A resultant force is a single force acting on a particle or body whose effect is equivalent to the combined effects of two or more separate forces.  
  quantum mechanics  
  Quantum mechanics, or quantum theory, superseded Newtonian mechanics in the interpretation of physical phenomena on the atomic scale and is the theory that energy (the capacity for doing work) does not have a continuous range of values, but is, instead, absorbed or radiated discontinuously, in multiples of definite, indivisible units called quanta. Just as earlier theory showed how light, generally seen as a wave motion, could also in some ways be seen as composed of discrete particles (photons), quantum theory shows how atomic particles such as electrons may also be seen as having wavelike properties. Quantum theory is the basis of particle physics, modern theoretical chemistry, and the solid-state physics that describes the behavior of the silicon chips used in computers.  
  Subatomic Logic
  Part of a larger site maintained by Scientific American, this page provides information on the recent progress of scientists who are attempting to harness quantum physics to run a lightning fast, super-charged "quantum computer." This article explains how a quantum computer would work and why it would be so much faster than silicon-based computer systems.  




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  Quantum Age Begins
  Web site run by St. Andrews University, Scotland chronicling the discovery of quantum theory.  
  Thermodynamics deals with the transformation of heat into and from other forms of energy. It is the basis of the study of the efficient working of engines, such as the steam and internal combustion engines. The three laws of thermodynamics are: (1) energy can be neither created nor destroyed, heat and mechanical work being mutually convertible; (2) it is impossible for an unaided self-acting machine to convey heat from one body to another at a higher temperature; and (3) it is impossible by any procedure, no matter how idealized, to reduce any system to the absolute zero of temperature (0K/–273°C/–459°F) in a finite number of operations. Put into mathematical form, these laws have widespread applications in physics and chemistry.  
  Schrödinger's Cation
  Part of a larger site maintained by Scientific American, this page features an explanation of the quantum mechanics paradox known as "Schrödinger's Cat," an experiment devised by Erwin Schrödinger to illustrate the difference between the quantum and macroscopic worlds.  
  Heat is a form of energy possessed by a substance by virtue of the vibrating movement (kinetic energy) of its molecules or atoms. Heat energy is transferred by conduction, convection, and radiation. It always flows from a region of higher temperature (heat intensity) to one of lower temperature. Its effect on a substance may be simply to raise its temperature, or to cause it to expand, melt (if a solid), vaporize (if a liquid), or increase its pressure (if a confined gas).  
  Quantities of heat are usually measured in units of energy, such as joules (J) or calories (cal). The specific heat of a substance is the ratio of the quantity of heat required to raise the temperature of a given mass of the substance through a given range of temperature to the heat required to raise the temperature of an equal mass of water through the same range. It is measured by a calorimeter.  
  Explanation of Temperature Related Theories
  Detailed explanatory site on the laws and theories of temperature. It explains what temperature actually is, what a thermometer is, and the development of both, complete with illustrations and links to pioneers in the field. There is a temperature conversion facility and explanations of associated topics such as kinetic theory and thermal radiation.  
  Conduction is flow of heat energy through a material without the movement of any part of the material itself (compare electrical conduction, described above)—for example, when the whole length of a metal rod is heated when one end is held in a fire. Heat energy is present in all materials in the form of the kinetic energy of their vibrating molecules, and may be conducted from one molecule to the next in the form of this mechanical vibration. In the case of metals, which are particularly good conductors of heat, the free electrons within the material carry heat around very quickly.  
  Convection is the transmission of heat through a fluid (liquid or gas) in currents—for example, when the air in a room is warmed by a fire or radiator.  
  Radiation is heat transfer by infrared rays. All objects radiate heat; hotter objects emit more energy than cooler objects. Infrared radiation can pass through a  
heat transfer
  To remember the principles of heat transfer:  
  Conduction–imagine a line of passengers on a bus being asked to move down by the conductor, each passenger causing the next to bustle along (analogy for the movement/vibration of atoms that is passed along, causing heat to be transferred)  
  Convection–consider vector, a disease-carrying insect, e.g. a mosquito, which travels in swarms (very much like the movement of convection currents)  
  Radiation–heat radiation is a form of radiation–(think of nuclear fallout or the sun's radiation) and thus travels in waves undetected until they fall upon another body  





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  vacuum, travels at the same speed as light, can be reflected and refracted, and does not affect the medium through which it passes. For example, heat reaches the earth from the sun by radiation.  
  Energy is the capacity for doing work. Energy can exist in many different forms. For example, potential energy (PE) is energy deriving from position; thus a stretched spring has elastic PE, and an object raised to a height above the earth's surface, or the water in an elevated reservoir, has gravitational PE. Moving bodies possess kinetic energy (KE). All atoms and molecules possess some amount of kinetic energy because they are all in some state of motion (see c0016-01.gifkinetic theory). Adding heat energy to a substance increases the mean kinetic energy and hence the mean speed of its constituent molecules—a change that is reflected as a rise in the temperature of that substance.  
  Brownian Motion
  Transcript of "Remarks on Active Molecules" by Robert Brown from Additional Remarks on Active Molecules (1829). The text describes Robert Brown's observations of the random motion of particles.  
  Energy can be converted from one form to another, but the total quantity in a system stays the same (in accordance with the conservation of energy principle). Energy cannot be created or destroyed. For example, as an apple falls it loses gravitational PE but gains KE. Although energy is never lost, after a number of conversions it tends to finish up as the kinetic energy of random motion of molecules (of the air, for example) at relatively low temperatures. This is "degraded" energy that is difficult to convert back to other forms. All forms of energy tend to be transformed into heat and can not then readily be converted into other, useful forms of energy.  
  A body with no energy can do no work. For example, a flat battery in a flashlight will not light the flashlight. If the battery is fully charged, it should contain enough chemical energy to do the work involved in illuminating the flashlight bulb. When one body A does work on another body B, A transfers energy to B. The energy transferred is equal to the work done by A on B. Energy is therefore measured in joules. The rate of doing work or consuming energy is called power and is measured in watts (joules per second).  
  It is now recognized that mass can be converted into energy under certain conditions, according to Einstein's theory of relativity. This conversion of mass into energy is the basis of atomic power. Einstein's special theory of relativity (1905; see below) correlates any gain, E, in energy with a gain, m, in mass, by the equation E = mc2, in which c is the speed of light. The conversion of mass into energy in accordance with this equation applies universally, although it is only for nuclear reactions that the percentage change in mass is large enough to detect.  
  Radiation is the emission of radiant energy as particles or waves—for example, heat, light, alpha particles, and beta particles (see under electromagnetic waves below and radioactivity above). Of the radiation given off by the sun, only a tiny fraction of it, called insolation, reaches the earth's surface; much of it is absorbed and scattered as it passes through the atmosphere. The radiation given off by the earth itself is called ground radiation.  
  Radiation Reassessed
  Part of the Why Files project, published by the National Institute for Science Education (NISE) and funded by the National Science Foundation, this page provides insight into the controversy concerning the health effects of ionizing radiation.  
  background radiation Background radiation is radiation that is always present in the environment. By far the greater proportion (87%) of it is emitted from natural sources. Alpha and beta particles and gamma radiation are radiated by the traces of radioactive minerals that occur naturally in the environment and even in the human body, and by radioactive gases such as radon and thoron, which are found in soil and may seep upward into buildings. Radiation from space (cosmic radiation) also contributes to the background level.  
  Radioactivity in Nature
  Detailed explanation of the different types of radiation found naturally on earth and in its atmosphere, as well as those produced by humans. It includes tables of the breakdown of nuclides commonly found in soil, the oceans, the air, and even the human body.  
  solar radiation Solar radiation is radiation given off by the sun and consists mainly of visible light, ultraviolet radiation, and infrared radiation although the  




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electromagnetic spectrum
  To remember the different categories of radiation, in order of increasing wavelength:  
  Cary Grant expects unanimous votes in movie reviews tonight  
  (Cosmic, gamma, X-rays, ultraviolet, visible, infrared, microwave, radio, television)  


  whole spectrum of electromagnetic waves is present, from radio waves to X-rays. High-energy charged particles, such as electrons, are also emitted, especially from solar flares. When these reach the earth, they cause magnetic storms (disruptions of the earth's magnetic field), which interfere with radio communications.  
  ultraviolet radiation Ultraviolet radiation is electromagnetic radiation near the short wavelength—high frequency end of the electromagnetic spectrum with wavelengths from about 400 to 4 nm (where the X-ray range begins). Physiologically, ultraviolet radiation is extremely powerful, producing sunburn and causing the formation of vitamin D in the skin. Ultraviolet rays are also strongly germicidal and may be produced artificially by mercury vapor and arc lamps for therapeutic use.  
  infrared radiation Infrared radiation is invisible electromagnetic radiation of wavelength between between 104 m and 7 x 10–7 m—that is, between the limit of the red end of the visible spectrum and the shortest microwaves. All bodies above the absolute zero of temperature (0K/–273.15°C/–459.67°F) absorb and radiate infrared radiation. Infrared absorption spectra are used in chemical analysis, particularly for organic compounds.  
  X-rays X-rays are a band of electromagnetic radiation in the wavelength range 10–11 to 10–9 m (between gamma rays and ultraviolet radiation. Applications of X-rays make use of their short wavelength (as in X-ray diffraction) or their penetrating power (as in medical X-rays of internal body tissues). X-rays are dangerous and can cause cancer. The X-rays used in radiotherapy have very short wavelengths that penetrate tissues deeply and destroy them.  
  Waves are oscillations that are propagated from a source. Mechanical waves require a medium through which to travel. Electromagnetic waves do not; they can travel through a vacuum. Waves carry energy but they do not transfer matter.  
  Introduction to Waves
  Interactive site that begins with the basics—explaining and allowing you to manipulate wavelength, amplitude, and phase shift of a simple wave. Further into the site there are more complex examinations of such things as Huygen's principle, interference, and wave propagation. You will need to have a Java-enabled browser to get the most out of this site.  
  amplitude The amplitude is the maximum displacement of an oscillation from the equilibrium position (the height of a crest or the depth of a trough). With a sound wave, for example, amplitude corresponds to the intensity (loudness) of the sound. If a mechanical system is made to vibrate by applying oscillations to it, the system vibrates. As the frequency of the oscillations is varied, the amplitude of the vibrations reaches a maximum at the natural frequency of the system. If a force with a frequency equal to the natural frequency is applied, the vibrations can become violent, a phenomenon known as resonance.  
  longitudinal wave In a longitudinal wave, such as a sound wave, the disturbance of the medium is parallel to the wave's direction of travel. A longitudinal  
  longitudinal wave The motion of a longitudinal wave. Sound, for example,
travels through air in longitudinal waves: the waves vibrate back and forth
in the direction of travel. In the compressions the particles are pushed
together, and in the rarefactions they are pulled apart.




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  wave consists of a series of compressions and rarefactions (states of maximum and minimum density and pressure, respectively). Such waves are always mechanical in nature and thus require a medium through which to travel.  
  transverse wave  
  In a transverse wave, such as an electromagnetic wave, the displacement of the medium is perpendicular to the direction in which the wave travels. The directions of the electric and magnetic fields in electromagnetic waves are perpendicular to the wave motion. The medium (for example the earth, for seismic waves) is not permanently displaced by the passage of a wave.  
  transverse wave The diagram illustrates the
motion of a transverse wave. Light waves are
examples of transverse waves: they undulate at
right angles to the direction of travel and are
characterized by alternating crests and troughs.
Simple water waves, such as the ripples produced
when a stone is dropped into a pond, are also
examples of transverse waves.
  polarization Transverse waves can exhibit polarization. If the oscillations of the wave take place in lots of different directions (all at right angles to the directions of the wave) the wave is unpolarized. If the oscillations occur in one plane only, the wave is polarized. Light, which consists of transverse waves, can be polarized.  
  wavelength Wavelength is the distance between successive crests of a wave. This is measured as the distance between successive crests (or successive troughs) of the wave. It is given the Greek symbol l. The frequency of a wave is the number of vibrations per second. The reciprocal of this is the wave period. This is the time taken for one complete cycle of the wave oscillation. The speed of the wave is measured by multiplying wave frequency by the wavelength. The wavelength of a light wave determines its color; red light has a wavelength of about 700 nanometers, for example. The complete range of wavelengths of electromagnetic waves is called the electromagnetic spectrum.  
  frequency Frequency refers to the number of periodic oscillations, vibrations, or waves occurring per unit of time. The SI unit of frequency is the hertz (Hz), one hertz being equivalent to one cycle per second. Frequency is related to wavelength and velocity by the relationship f = v/l, where f is frequency, v is velocity and l is wavelength.  
  refraction When a wave moves from one medium to another (for example a light wave moving from air to glass) it moves with a different speed in the second medium. This change in speed causes it to change direction. This property is called refraction. The amount of refraction depends on the densities of the media, the angle at which the wave strikes the surface of the second medium, and the amount of bending and change of velocity corresponding to the wave's frequency (dispersion). Refraction differs from reflection (see below), which involves no change in velocity. The refractive index of a material indicates by how much a wave is bent. It is found by dividing the velocity of  
  refraction Refraction is the bending
of a light beam when it passes from one
transparent medium to another. This
is why a spoon appears bent when
standing in a glass of water and pools
of water appear shallower than they
really are. The quantity sin i/sin r has
a constant value, for each material,
called the refractive index.




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  prism The volume of a prism is determined
by multiplying the area of the cross section
by the length of the prism.
  the wave in the first medium by the velocity of the wave in the second medium.  
  dispersion Dispersion is a particular property of refraction in which the angle and velocity of waves passing through a dispersive medium depends upon their frequency. In the case of visible light the frequency corresponds to color. The splitting of white light into a spectrum (see under electromagnetic waves below) when it passes through a prism occurs because each component frequency of light moves through at a slightly different angle and speed. A rainbow is formed when sunlight is dispersed by raindrops.  
  To remember the order of colors:  
  Rare old yaks guzzle butter in volume.  
  (red, orange, yellow, green, blue, indigo, violet)  


  reflection Whenever a wave hits a barrier, reflection, occurs. The wave is sent back, or reflected, into the medium at a different angle. The law of reflection states that the angle of incidence (the angle between the ray and a perpendicular line drawn to the surface) is equal to the angle of reflection (the angle between the reflected ray and a perpendicular to the surface).  
  When light passes from a denser medium to a less dense medium, such as from water to air, both refraction (see above) and reflection can occur. If the angle of incidence is small, the reflection will be relatively weak compared to the refraction. But as the angle of incidence increases the relative degree of reflection will increase. At some critical angle of incidence the angle of refraction is 90°. Since refraction cannot occur above 90°, the light is totally reflected at angles above this critical angle of incidence. This condition is known as total internal reflection. Total internal reflection is used in fiber optics to transmit data over long distances, without the need of amplification  
  reflection The law of reflection: the angle of
incidence of a light beam equals the angle
of reflection of the beam.
  diffraction Diffraction is the spreading out of waves when they pass through a small gap or around a small object, resulting in some change in their direction. The degree of diffraction depends on the relationship between the wavelength and the size of the object or gap through which the wave travels. In order for this effect to be observed the size of the object or gap must be comparable to, or smaller than, the wavelength of the waves. Diffraction occurs with all forms of progressive waves—electromagnetic, sound, and water waves—and explains such phenomena as why long-wave radio waves can bend round hills better than short-wave radio waves. Large objects cast shadows because the difference between their size and the wave-  




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  length is so large that light waves are not diffracted round the object.  
  The wavelength of light ranges from 4 x 10–7 to 7 x 10–7, a few orders of magnitude smaller than radio waves. The slight spreading of a light beam through a narrow slit causes the different wavelengths of light to interfere with each other to produce a pattern of light and dark bands. A diffraction grating is a plate of glass or metal ruled with close, equidistant parallel lines used for separating a wave train such as a beam of incident light into its component frequencies (white light results in a spectrum). The wavelength of sound is between 0.5 m/1.6 ft and 2.0 m/6.6 ft. When sound waves travel through doorways or between buildings they are diffracted significantly, so that the sound is heard round corners. The regular spacing of atoms in crystals are used to diffract X-rays, and in this way the structure of many substances has been elucidated, including that of proteins.  
  Moseley Articles
  Transcript of Henry Moseley's article The High Frequency Spectra Of The Elements. In the article, Moseley lays the foundations for X-ray spectroscopy, and forms a relationship between atomic number and the frequency of the emitted spectra.  
  interference When two or more waves meet at a point, they interact and combine to produce a resultant wave of larger or smaller amplitude (depending on whether the combining waves are in or out of phase with each other). This is known as interference.  
  Professor Bubbles' Official Bubble Home Page
  Lively information about how to blow the best bubbles, and answers to frequently asked questions about bubbles—such as ''why are bubbles always round?" and "why do bubbles have color?"  
  Interference of white light (multiwavelength) results in spectral colored fringes; for example, the iridescent colors of oil films seen on water or soap bubbles. Interference of sound waves of similar frequency produces the phenomenon of beats, often used by musicians when tuning an instrument. With monochromatic light (of a single wavelength), interference produces patterns of light and dark bands. This is the basis of holography, for example. Interferometry can also be applied to radio waves, and is a powerful tool in modern astronomy  
  sound wave Sound is the physiological sensation received by the ear, originating in a vibration that communicates itself as a pressure variation in the air and travels in every direction, spreading out as an expanding sphere. All sound waves in air travel with a speed dependent on the temperature; under ordinary conditions, this is about 330 m/1,070 ft per second. The pitch of the sound depends on the number of vibrations imposed on the air per second (frequency), but the speed is unaffected. The loudness of a sound is dependent primarily on the amplitude of the vibration of the air. Sound travels as a longitudinal wave, that is, its compressions and rarefactions are in the direction of propagation. Reflection of a sound wave is heard as an echo. Diffraction explains why sound can be heard round doorways. Sound travels faster in denser materials, such as solids and liquids.  
  Sound waves, unlike light, travel faster in denser materials, such as solids and liquids, than they travel in air. When sound waves enter a solid, their velocity and wavelength increase and they are bent away from the normal to the surface of the solid.  
  electromagnetic waves  
  Electromagnetic waves are oscillating electric and magnetic fields traveling together through space. All electromagnetic waves travel at the same speed, the speed of light—nearly 300,000 km/186,000 mi per second. The (limitless) range of possible wavelengths and frequencies of electromagnetic waves, which can be thought of as making up the electromagnetic spectrum, includes radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. The different wavelengths and frequencies lend specific properties to electromagnetic waves.  
speed of light
  To remember the speed of light:  
  The speed of light is 299,792,458 m/s, which can be remembered from the number of letters in each word of the following phrase:  
  We guarantee certainty, clearly referring to this light mnemonic  


  radio wave A radio wave is an electromagnetic wave possessing a long wavelength (ranging from about 10–3 to 104 m) and a low frequency (from about 105 to 1011 Hz). Included in the radio-wave part of the spectrum are microwaves, used for both communications and for cooking; ultra high- and very high-frequency waves, used for television and FM (frequency modulation) radio communications; and short, medium, and  




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  electromagnetic waves Radio waves have the lowest frequency. Infrared
radiation, visible light, ultraviolet radiation, X-rays, and gamma rays have
progressively higher frequencies.
  long waves, used for AM (amplitude modulation) radio communications. Radio waves that are used for communications have all been modulated to carry information. Certain astronomical objects emit radio waves, which may be detected and studied using radio telescopes.  
  light wave A light wave is an electromagnetic wave in the visible range, having a wavelength from about 400 nanometers in the extreme violet to about 770 nanometers in the extreme red. Light is considered to exhibit particle and wave properties, and the fundamental particle, or quantum, of light is called the photon. The speed of light (and of all electromagnetic radiation) in a vacuum is approximately 300,000 km/186,000 mi per second, and is a universal constant denoted by c.  
  For all practical purposes light rays travel in straight  




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  lines, although Einstein demonstrated that they may be "bent" by a gravitational field. On striking a surface they are reflected or refracted with some absorption of energy, and the study of this is known as geometrical optics.  
  Sources of light have a characteristic spectrum or range of wavelengths. Hot solid objects emit light with a broad range of wavelengths, the maximum intensity being at a wavelength which depends on the temperature. The hotter the object, the shorter the wavelengths emitted, as described by Wien's displacement law. Hot gases, such as the vapor of sodium street lights, emit light at discrete wavelengths. The pattern of wavelengths emitted is unique to each gas and can be used to identify the gas.  
  Introduction to Mass Spectrometry
  Good introduction to the mass spectrometer and how it works. Different mass analyzer designs are described, together with sections on ionization and ion detectors.  
  Fundamental Forces  
  Four fundamental interactions are believed to be at work in the physical universe. There are two longrange forces—gravity and the electromagnetic force—and two very short-range forces that operate only inside the atomic nucleus: the weak nuclear force and the strong nuclear force. The relative strengths of the four forces are: strong, 1; electromagnetic, 10–2; weak, 10–6; gravitational, 10–40.  
  Gravity is the force of attraction that arises between objects by virtue of their masses. On earth, gravity is the force of attraction between any object in the earth's gravitational field and the earth itself. It is the force which keeps the planets in orbit around the sun. The gravitational force is the weakest of the four forces, but it acts over great distances. The particle that is postulated as the carrier of the gravitational force is the graviton.  
  Astronauts in space cannot belch. It is gravity that causes bubbles to rise to the top of a liquid, so space shuttle crews were forced to request less gas in their fizzy drinks to avoid discomfort.  


  electromagnetic force The electromagnetic force stops solids from falling apart, and acts between all particles with electric charge. The elementary particle that is the carrier for the electromagnetic force is the photon.  
  weak nuclear force The weak nuclear force is responsible for the reactions that fuel the sun and for the emission of beta particles from certain nuclei. The weakon, or intermediate vector boson, is the elementary particle that carries the weak nuclear force.  
  strong nuclear force The strong nuclear force was first described by the Japanese physicist Hideki Yukawa in 1935. It is the strongest of all the forces, acts only over very small distances (within the nucleus of the atom), and is responsible for binding together quarks to form hadrons, and for binding together protons and neutrons in the atomic nucleus. The particle that is the carrier of the strong nuclear force is the gluon, of which there are eight kinds, each with zero mass and zero charge  
  unified field theory  
  Unified field theory is a sought-for theory that would explain the four fundamental forces (strong nuclear, weak nuclear, electromagnetic, and gravity) in terms of a single unified force. By 1971 a theory developed by the U.S. physicists Steven Weinberg and Sheldon Glashow, the Pakistani physicist Abdus Salam, and others, had demonstrated the link between the weak nuclear and electromagnetic forces. Called the electroweak force, experimental support came from observation at CERN in the 1980s. The next stage is to develop a theory (called the grand unified theory) that combines the strong nuclear force with the electroweak force. The final stage will be to incorporate gravity into the scheme.  
  superstring theory  
  Superstring theory is a mathematical theory developed in the 1980s to explain the properties of elementary particles and the forces between them (in particular, gravity and the nuclear forces) in a way that combines relativity and quantum theory. In string theory, the fundamental objects in the universe are not pointlike particles but extremely small stringlike objects. These objects exist in a universe of ten dimensions, although, for reasons not yet understood, only three space dimensions and one dimension of time are discernible. There are many unresolved difficulties with superstring theory, but some physicists think it may be the ultimate "theory of everything" that explains all aspects of the universe within one framework.  




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  Relativity is the theory of the relative rather than absolute character of motion and mass, and the interdependence of matter, time, and space, as developed by German-born U.S. physicist Albert Einstein in two phases.  
  In his special theory of relativity, developed in 1905, starting with the premises that (1) the laws of nature are the same for all observers in unaccelerated motion, and (2) that the speed of light is independent of the motion of its source, Einstein arrived at some rather unexpected consequences. Intuitively familiar concepts, like mass, length, and time, had to be modified. For example, an object moving rapidly past an observer will appear to be both shorter and heavier than when it is at rest (that is, at rest relative to the observer), and a clock moving rapidly past the observer will appear to be running slower than when it is at rest. These changes are quite negligible at speeds less than about 1,500 km s–1, and only become appreciable at speeds approaching the speed of light.  
  Einstein's general theory of relativity, developed in 1915, treats gravitation not as a force but as the curvature of space-time around a body. A planet's orbit around the sun (as observed in three-dimensional space) arises from its natural trajectory in modified space-time; there is no need to invoke, as Isaac Newton did, a force of gravity coming from the sun and acting on the planet. Einstein's general theory accounts for a peculiarity in the behavior of the motion of the perihelion of the orbit of the planet Mercury that cannot be explained in Newton's theory.  
  Relativity also predicts that light rays should bend when they pass by a massive object, and that light should shift toward the red in the spectra of the sun or star in a gravitational field; both have been observed. Another prediction of relativity is gravitational waves, which should be produced when massive bodies are violently disturbed. These waves are so weak that they have not yet been detected with certainty, although observations of a pulsar (which emits energy at regular intervals) in orbit around another star have shown that the stars are spiraling together at the rate that would be expected if they were losing energy in the form of gravitational waves.  
  Einstein showed that, for consistency with the above premises (1) and (2), the principles of dynamics as established by Newton needed modification; the most celebrated new result was the equation E = mc2, which expresses an equivalence between mass (m) and energy (E), c being the speed of light in a vacuum. In "relativistic mechanics," conservation of mass is replaced by the new concept of conservation of "mass-energy."  
  The manuscript for Einstein's special theory of relativity sold for $5 million in 1996.  


  General relativity is central to modern astrophysics and cosmology; it predicts, for example, the possibility of black holes. General relativity theory was inspired by the simple idea that it is impossible in a small region to distinguish between acceleration and gravitation effects (as in a lift one feels heavier when the lift accelerates upward), but the mathematical development of the idea is formidable. Such is not the case for the special theory, which a nonexpert can follow up to E = mc2 and beyond.  
  Usenet Relativity FAQ
  Concise answers to some of the most common questions about relativity. The speed of light and its relation to mass, dark matter, black holes, time travel, and the Big Bang are some of the things covered by this illuminating series of articles based both on Usenet discussions and good reference sources. The site also directs the visitors to appropriate discussion groups where they can pose more questions.  
  Space-time is the combination of space and time used in the theory of relativity. Einstein showed that time was in many respects like an extra dimension (or direction) to space. Space and time can thus be considered as entwined into a single entity, rather than two separate things.  
  Space-time is considered to have four dimensions: three of space and one of time. In relativity theory, events are described as occurring at points in space-time. The general theory of relativity describes how space-time is distorted by the presence of material bodies, an effect that we observe as gravity.  
  Einstein's Legacy
  Illustrated introduction to the man and his greatest legacy—relativity and the concept of space-time. There is a film and audio clip version of the page courtesy of a U.S. scientist and details about how current research is linked to Einstein's revolutionary ideas.  




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  Physics Chronology  
Physics Chronology
c. 435 B.C. Greek philosopher Leucippus is the first to propose the atomic theory. It is developed later by his pupil Democritus.
c. 420 B.C. Greek philosopher Democritus of Abdera develops Leucippus' atomic theory and states that space is a vacuum and that all things consist of eternal, invisible and indivisible atomon (atoms). He also posits necessary laws by which they interact.
c. 250 B.C. Greek mathematician and inventor Archimedes discovers the principle that bears his name—that submerged bodies are acted upon by an upward or buoyant force equal to the weight of the fluid displaced.
1039 The Muslim scientist Abu'Ali al-Hasan (Alhazen) writes treatise on optics explaining the function of lenses, curved mirrors, refraction, and other phenomena.
1590 Italian scientist Galileo publishes De Motul/On Motion, in which he discusses his ideas and discoveries about the motion of objects.
1600 The English physician William Gilbert writes De magnete/On Magnetism, a pioneering study of electricity and magnetism, which distinguishes between electrostatic and magnetic effects.
1604 Italian scientist Galileo Galilei discovers his law of falling bodies, proving that gravity acts with the same strength on all objects, independent of their mass. Traditionally, he is believed to have demonstrated this by dropping balls of the same size but different masses from the top of the Leaning Tower at Pisa.
1642 French mathematician Blaise Pascal puts forward the principles of hydraulics, the use of liquids to transmit force.
1647 Pascal demonstrates the pressure exerted by the atmosphere, using it to raise water and wine 12 m/40 ft up tubes fastened to a ship's mast.
1660 English physicist Robert Hooke discovers the law now named for him—that the extension of an elastic material such as a spring is in proportion to the force exerted on it.
1660 English mathematician Isaac Newton begins work on the calculus, a fundamental tool in physics for studying rates of change.
1662 Anglo-Irish chemist and physicist Robert Boyle describes the law that will bear his name, stating that, for a fixed mass of gas in a container, the volume occupied by the gas is inversely proportional to the pressure it exerts.
1663 German physicist Otto von Guericke makes a machine for generating static electricity by friction, consisting of a ball of sulfur isolated from earth, and turned by an axle and winch.
1664 Hooke suggests that planetary orbits may be maintained by the constant attractive force of gravity between two bodies.
1675 Netwon proposes a corpuscular theory of light.
March 1676 French physicist Edmé Mariotte discovers the relationship between volume and pressure in a fixed mass of gas, independently of Boyle.
1678 Dutch physicist and astronomer Christiaan Huygens records his discovery of the polarization of light, responsible for phenomena such as double refraction.
1679 Hooke proposes an inverse-square law of gravity, preempting Newton's law of gravitation.
1680 Newton calculates that an inverse-square law of gravitational attraction between the sun and planets would explain the elliptical orbits discovered by Kepler. He also puts forward a theory that the air resistance encountered by a body increases in proportion to the square of its speed.
1687 Newton publishes Philosophiae naturalis principia mathematica/The Mathematical Principles of Natural Philosophy, his most important work. It presents his theories of motion, gravity, and mechanics, which form the basis of much of modern physics.
1690 Dutch physicist Christiaan Huygens propounds a theory of light as a longitudinal wave with vibration in the direction of its travel.
1745 German scientist Ewald Georg von Kleist invents the Leyden jar, a simple capacitor that accumulates and preserves electricity. The following year Duch scientist Pieter von Musschenbroek makes the same discovery independently.
1750 Scandinavian physicist Martin Stromer modifies the temperature scale devised by his mentor, the Swedish astronomer Anders Celsius. He inverts it, setting freezing point as 0°C and boiling point as 100°C, creating the Celsius scale still used today.
1767 English physicist Joseph Priestley publishes his History and Present State of Electricity, which suggests that electrical forces follow an inverse-square law, as does gravity.
1785 French scientist Charles Augustin Coulomb makes the first precise measurements of the electric forces of attraction and repulsion between charged bodies.
1787 French physicist Jacques-Alexandre Charles demonstrates that different gases expand by the same amount for the same temperature rise. It later becomes known as Charles's law.
1792 Italian physicist Alessandro Volta demonstrates the electrochemical series.
1798 U.S.-born British physicist and inventor Benjamin Thompson, Count Rumford, demonstrates experimentally the theory that heat is the increased motion of particles.





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1800 English physicist Thomas Young proposes a wave theory of light.
1800 Volta invents the voltaic pile made of discs of silver and zinc—the first battery.
1801 English chemist and physicist John Dalton formulates the law of partial pressure in gases—Dalton's Law—which states that each component of a gas mixture produces the same pressure as if it occupied the container alone.
1801 Young discovers the interference of light when he observes that light passing through two closely spaced pinholes produces alternating bands of light and dark in the area of overlap. He thereby establishes the wave theory of light.
1802 French chemist and physicist Joseph-Louis Gay-Lussac demonstrates that all gases expand by the same fraction of their volume when subjected to the same temperature increase; it permits the establishment of a new temperature scale.
1807 Young enunciates "Young's modulus, " a measurement of the elasticity of a material defined as the stress divided by the strain.
1811 Italian physicist Amedeo Avogadro proposes Avogadro's law which states that equal volumes of different gases under the same temperature and pressure conditions will contain the same number of molecules.
July 25, 1814 German physicist Joseph von Fraunhofer plots more than 500 absorption lines (Fraunhofer lines) and discovers that the relative positions of the lines is constant for each element. His work forms the basis of modern spectroscopy.
1815 French physicist Augustin-Jean Fresnel shows that light has transverse waves—he thus explains the diffraction of light.
June 1819 Danish physicist Hans Christian Oersted discovers electromagnetism when he observes that a magnetized compass needle is deflected by an electric current.
1820 French physicist André Ampère formulates Ampère's law, which states the relationship between a magnetic field and the electric current that produces it.
1821 English physicist Michael Faraday builds an apparatus that transforms electrical energy into mechanical energy—the principle of the electric motor.
1821 German physicist Thomas Seebeck discovers thermoelectricity—the conversion of heat into electricity—when he generates a current by heating one end of a metal strip comprising two metals joined together.
1824 French scientist Sadi Carnot publishes a pioneering study of thermodynamics in which he explains that a steam engine's power results from the decrease in temperature from the boiler to the condenser. He also describes the "Carnot cycle" whereby heat is converted into mechanical motion and mechanical motion converted into heat—the basis of the second law of thermodynamics.
1825 Ampère publishes Electrodynamics, in which he formulates the mathematical laws governing electric currents and magnetic fields. It lays the foundation for electromagnetic theory.
1827 German physicist Georg Ohm formulates Ohm's Law, which states that the current flowing through an electric circuit is directly proportional to the voltage, and indirectly proportional to the resistance.
1828 Scottish botanist Robert Brown observes the continuous motion of tiny particles in a liquid solution, now known as Brownian motoin.
1829 French mathematician Gustave-Gaspard Coriolis is the first to use the term "kinetic energy."
1830 U.S. scientist Joseph Henry discovers electromagnetic induction—the production of an electric current by change in magnetic intensity—but does not publish his discovery.
Aug 29, 1831 Faraday discovers electromagnetic induction—the production of an electric current by change in magnetic intensity (and also the principle of the electric generator).
July 1832 Henry discovers the phenomenon of self-induction—the production of electric current when a conductor is disconnected from a battery.
1833 Faraday announces the basic laws of electrolysis: that the amount of a substance deposited on an electrode is proportional to the amount of electric current passed through the cell, and that the amounts of different elements are proportional to their atomic weights.
1834 French physicist Benoît-Pierre clapeyron develops the second law of thermodynamics: entropy always increases in a closed system.
1836 French physicist Edmund Becquerel discovers the photovoltaic effect when he observes the creation of a voltage between two electrodes, one of which is exposed to light.
1839 Faraday discovers that each element has a specific electrical inductive capacity.
1840 English physicist James Joule states his law that the amount of heat produced per second in any conductor by an electric current is proportional to the product of the square of the current and the resistance of the conductor.
1842 Austrian physicist Christian Doppler describes how the frequency of sound and light waves changes with the motion of their source relative to the observe—the "Doppler effect."
1843 Joule determines the value for the mechanical equivalent of heat (now known as the joule), that is the amount of work required to produce a unit of heat.
1848 Scottish physicist William Thomson (Lord Kelvin) devises the absolute temperature scale. He defines absolute zero as –273°C/–459.67°F, where the molecular energy of molecules is zero. He also defines the quantities currently used to describe magnetic forces: magnitude of magnetic flux, beta, and H the magnetizing force.





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1850 French physicist Jean Foucault establishes that light travels slower in what than in air. He also measures the velocity of light to within 1% of its true speed.
1851 Kelvin states that energy in a closed system tends to become unusable waste heat–the second law of thermodynamics.
1859 German chemists Robert Wilhelm von Bunsen and Gustav Kirchhoff discover that each element emits a characteristic wavelength of light. It initiates spectrum analysis, a valuable tool for both chemist and astronomer.
1864 Scottish physicist James Clerk Maxwell introduces mathematical equations that describe the electromagnetic field, and predict the existence of radio waves.
1868 Swedish physicist Anders Ångström expresses the wavelengths of Fraunhofer lines in units of 10–10 m, a unit now known as the angstrom.
1874 Austrian physicist Ludwig Boltzmann develops the basic principles of statistical mechanics when he demonstrates how the laws of mechanics and the theory of probability, when applied to the motions of atoms, can explain the second law of thermodynamics.
1874 Irish physicist George Johnstone Stoney names the electron and estimates the value of its charge.
1879 British-born U.S. electrical engineer Elihu Thomson shows how induction coils can be used to increase current and step down voltage–the basic principle of the transformer developed a few years later.
1880 French physicists Pierre and Paul-Jacques Curie discover that electricity is produced when pressure is placed on certain crystals including quartz–the "piezoelectric" effect.
1882 German-born U.S. physicist Albert Michelson determines the speed of light to be 299,853 kps/186,329 mps.
1882 Scottish physicist Balfour Stewart postulates the existence of an electrically conducting layer of the outer atmosphere (now known as the ionosphere) to account for the daily variation in the earth's magnetic field.
1883 Irish physicist George Francis FitzGerald suggests that electromagnetic waves (radio waves) can be created by oscillating an electric current. A later demonstration of such waves by the German physicist Heinrich Hertz leads to the development of wireless telegraphy.
1883 U.S. inventor Thomas Alva Edison observes the flow of current between a hot electrode and a cold electrode in one of his vacuum bulbs. Known as the "Edison effect," it results from the thermionic emission of electrons from the hot electrode, and is the principle behind the working of the electron tube, which is to form the basis of the electronics industry.
1885 U.S. Physicist Henry Augustus Rowland invents the concave diffraction grating, in which 20,000 lines to the inch are engraved on spherical concave mirrored surfaces. The grating revolutionizes spectrometry by dispersing light and permitting spectral lines to be focused.
1886 U.S. astronomer and physicist Samuel Pierpont Langley begins the first systematic aerodynamic research. He measures lift and drag on models of wings and other objects, which he attaches to a counterweighted beam, mounted on a pivot, that may be rotated at a speed of up to 112 kph/70 mph.
1887 Hertz discovers the photoelectric effect, in which a material gives off charged particles when it absorbs radiant energy, when he observes that ultraviolet light affects the voltage at which sparking between two metal plates takes place. Later work on this phenomenon leads to the conclusion that light is composed of particles called photons.
1887 U.S. physicist Albert Michelson and U.S. chemist Edward Williams Morley fail in an attempt to measure the velocity of the earth through the "ether" by measuring the speed of light in two directions. Their failure discredits the idea of the ether and leads to the conclusion that the speed of light is a universal constant, a fundamental premise of Einstein's theory of relativity.
1891 Serbian-born U.S. inventor Nikola Tesla invents the Tesla coil, which produces a high-frequency high-voltage current.
1893 British physicist Oliver Heaviside theorizes that as the velocity of an electric charge increases so does its mass. It presages Einstein's special theory of relativity.
1893 German physicist Wilhelm Wien states that the maximum wavelength emitted by a hot body is inversely proportional to the absolute temperature of the body.
Nov 8, 1895 German physicist Wilhelm Conrad Röntgen discovers X-rays. Named because of their unknown origin, they revolutionize medicine and usher in the age of modern physics.
1896 British physicist Ernest Rutherford discovers that magnetic fields can be used to detect electromagnetic or radio waves.
1896 Scottish physicist Charles Thomson Rees Wilson develops the first cloud chamber.
1897 English physicist John Joseph Thomson demonstrates the existence of the electron, the first known subatomic particle. It revolutionizes knowledge of atomic structure by indicating that the atom can be subdivided.
1898 German physicist Gerhard Carl Schmidt and French physicist Marie Curie demonstrate, independently, that thorium is radioactive; it stimulates interest in radioactivity.





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1898 German physicist Wilhelm Wien discover the proton.
1899 Rutherford discovers alpha and beta rays, produced by the radioactivity of uranium.
1899 Thomson measures the charge of the electron.
1900 Canadian-born U.S. scientist Reginald Aubrey Fessenden discovers the principle of amplitude modulation (AM) of radio waves.
1900 French physicist Antoine-Henri Becquerel demonstrates that the beta particle is the same thing as the electron.
1900 French physiologist Paul Ulrich Villard discovers gamma rays.
1900 German physicist Max Planck suggests that black bodies (perfect absorbers) radiate energy in packets or quanta, rather than continuously. He thus begins the science of quantum physics, which revolutionizes the understanding of atomic and subatomic processes.
1902 British physicist Oliver Heaviside and U.S. electrical engineer Arthur Kennelly independently predict the existence of a conducting layer in the atmosphere that reflects radio waves.
1903 Rutherford discovers that a beam of alpha particles is deflected by electric and magnetic fields. From the direction of deflection he is able to prove that they have a positive charge and from their velocity he determines the ratio of their charge to thier mass. He also names the high-frequency electromagnetic radiation escaping from the nuclei of atoms as gamma rays.
1903 Scottish chemist William Ramsay shows that helium is produced during the radioactive decay of radium–an important discovery for the understanding of nuclear reactions.
1904 English physicist Charles Glover Barkla demonstrates that each element can be made to emit X-rays of a characteristic frequency.
1904 Japanese physicist Hantaro Nagaoka proposes a model of the atom in which the electrons are located in an outer ring and orbit the positive charge which is located in a central nucleus. The model is ignored because it is thought the electrons would fall into the nucleus.
1905 Kelvin, proposes a model of the atom in which positive and negatively charged spheres alternate.
1906 English physicist Frederick Soddy discovers that ionium and radiothorium are chemically indistinguishable variants of thorium but have different radioactive properties. He later calls them isotopes.
1906 German physicist Walther Herman Nernst formulates the third law of thermodynamics, which states that matter tends toward random motion and that energy tends to dissipate at a temperature above absolute zero.
1908 German physicist Hans Geiger and Rutherford develop the Geiger counter, which counts individual alpha particles emitted by radioactive substances.
1908 U.S. physicist Percy Williams Bridgman invents equipment that can create atmospheric pressures of 100,000 atmospheres (later 400,000) creating a new field of investigation.
1909 German physicist Albert Einstein introduces his idea that light exhibits both wave and particle characteristics.
1910 Thomson discovers the proton.
1910 French chemists Marie Curie and A. Diebierne isolate radium.
1910 German physicist Wolfgang Gaede develops the molecular vacuum pump, which can generate a vacuum of 0.00001 mm of mercury.
1911 Austrian physicist Victor Francis Hess discovers cosmic radiation using crewed balloons.
1911 Dutch physicist Heike Kamerlingh-Onnes discovers superconductivity, the characteristic of a substance to display zero electrical resistance when cooled to just above absolute zero.
1911 German physicist Albert Einstein calculates the deflection of light caused by the sun's gravitational field.
1911 Rutherford proposes the concept of the nuclear atom, in which the mass of the atom is concentrated in a nucleus occupying one ten-thousandth of the space of the atom and which has a positive charge balanced by surrounding electrons.
1911 Rutherford and Soddy devise a scheme for the "transmutation" of the elements, producing a simpler atom from a complex one.
1911 U.S. physicist Robert Millikan measures the electric charge on a single electron in his oil-drop experiment, in which the upward force of the electric charge on an oil droplet precisely counters the known downward gravitational force acting on it.
1912 German physicist Max von Laue demonstrates that crystals are composed on regular, repeated arrays of atoms by studying the patterns in which they diffract X-rays. It is the beginning of X-ray crystallography.
1913 Danish physicist Niels Bohr proposes that electrons orbit the atomic nucleus in fixed orbits thus upholding Rutherford's model jproposed in 1911.
1913 Thomson develops a mass spectrometer, called a parabola spectrograph. A beam of charged ions is deflected by a magnetic field to produce parabolic curves on a photographic plate.
1913 Thomson discovers neon-22, an isotope of neon. It is the first isotope of a nonradioactive element to be discovered.
1913 English physicists William and Lawrence Bragg develop X-ray crystallography by establishing that the orderly arrangement of atoms in crystals display interference and diffraction patterns. They also demonstrate the wave nature of X-rays.
1913 Einstein formulates the law of photochemical equivalence, which states that for every





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  quantum of radiation absorbed by a substance one molecule reacts.
1914 German physicists James Franck and Gustav Hertz provide the firstexperimental evidence for the existence of discrete energy states in atoms and thus verify Bohr's atomic model.
1916 Einstein publishes The Foundation of the General Theory of Relativity, in which he postulates that space is a curved field modified locally by the existence of mass and that this can be demonstrated by observing the deflection of starlight around the sun during a total eclipse. This replaces previous Newtonian ideas which invoke a force of gravity. Einstein also derives the basic equations for the exchange of energy between matter and radiation.
May 29, 1919 English astrophysicist Arthur Eddington and others observe the total eclipse of the sun on Principle Island (West Africa), and discover that the sun's gravity bends the light from the stars beyond the edge of the eclipsed sun, thus confirming Einstein's theory of relativity.
1919 English physicist Francis Aston builds the first mass-spectrograph, which allows him to separate ions or isotopes of the same element.
1919 Rutherford splits the atom by bombarding a nitrogen nucleus with alpha particles, discovering that it ejects hydrogen nuclei (protons). It is the first artificial disintegration of an element and inaugurates the development of nuclear energy.
1920 Rutherford recognizes the hydrogen nucleus as the fundamental particle and names it the "proton."
1922 U.S. physicist Arthur Holly Compton discovers that X-rays scattered by an atom have a shift in frequency. He explains the phenomenon, known as the Compton effect, by treating the X-rays as a stream of particles, thus confirming the wave–particle idea of light.
1924 English physicist Edward Appleton discovers that radio emissions are reflected by an ionized layer of the atmosphere.
1924 French physicist Louis de Broglie argues that particles can also behave as waves, laying the foundations for wave mechanics. He demonstrates that a beam of electrons has a wave motion with a short wavelength. The discovery permits the development of the electron microscope.
1926 Austrian physicist Erwin Schrödinger develops wave mechanics.
1927 German physicist Werner Heisenberg propounds the "uncertainty principle" in quantum physics, which states that it is impossible to simultaneously determine the position and momentum of an atom. It explains why Newtonian mechanics is inapplicable at the atomic level.
1928 English physicist Paul Dirac describes the electron by four wave equations. The equations imply that the electron must spin on its axis and that negative states of matter must exist.
1928 Germany physicist Rolf Wideröe develops the resonance linear accelerator, which he uses to accelerate potassium and sodium to an energy of 710 keV to split the lithium atom.
1928 Russian physicist George Gamow shows that the atom can be split using low-energy ions. It stimulates the development of particle accelerators.
1929 Irish physicist Ernest Walton and English physicist Douglas Cockcroft develop the first particle accelerator.
1931 U.S. physicists Ernest Lawrence and M. Stanley Livingston build a cyclotron (particle accelerator).
1932 British physicist James Chadwick discovers the neutron, an important discovery in the development of nuclear reactors.
1932 Bristish physicist John D. Cockcroft and Ernest Walker develop a high-voltage particle accelerator, which they use to split lithium atoms.
1932 U.S. scientist Carl David Anderson, while analyzing cosmic rays, discovers positive electrons ("positrons"), the first form of antimatter to be discovered.
1933 German physicists Walter Meissner and R Ochensfeld discover that superconducting materials expel their magnetic fields when cooled to superconducting temperatures—the Meissner effect.
1934 French physicists Frédéric and Iréne Joliot-Curie bombard boron, aluminum, and magnesium with alpha particles and obtain radioactive isotopes of nitrogen, phosphorus, and aluminum—elements that are not normally radioactive. They are the first radioactive elements to be prepared artificially.
1934 Italian physicist Enrico Fermi suggests that neutrons and protons are the same fundamental particles in two different quantum states. He bombards uranium with neutrons and discovers the phenomenon of atomic fission, the basic principle of atomic bombs and nuclear power.
1935 Japanese physicist Hideki Yukawa proposes the existence of a new particle, the meson, to explain nuclear forces.
1936 Anderson discovers the muon, an electron-like particle over 200 times more massive than an electron.
1936 U.S. physicists George Gamow and Edward Teller develop the theory of beta decay—the nuclear process of electron emission.
1938 The Soviet physicist Pyotr Kapitza discovers that liquid helium exhibits superfluidity, the ability to flow over its containment vessel





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  without friction, when cooled below 2.18 K/–270.97°C.
1939 French physicists Frédéric Joliot and Iréne Curie-Joliot demonstrate the possibility of a chain reaction when they split uranium nuclei.
1940 U.S. physicist J. R. Dunning leads a research team that uses a gaseous diffusion technique to isolate uranium-235 from uranium-238. Because uranium-235 readily undergoes fission into two atoms, and in doing so releases large amounts of energy, it is used for fueling nuclear reactors.
1940 U.S. physicists Edwin McMillan and Philip Abelson synthesize the first transuranic element, neptunium, by bombarding uranium with neutrons at the cyclotron at Berkeley, California.
1945 U.S. physicist Edwin M. McMillan and Soviet physicist V. I. Veksler (1943) independently describe the principle of phase stability. By removing an apparent limitation on the energy of particle accelerators for protons, it makes possible the construction of magnetic-resonance accelerators, or synchrotrons. Synchrocyclotrons are soon built at the University of California and in England.
1946 U.K. physicist Edward Appleton discovers the "Appleton layer" in the ionosphere, which reflects radio waves; it makes long-range radio communication possible and also aids the development of radar.
1947 U.S. physicist Willard Libby develops carbon-14 dating.
1948 Hungarian-British physicist Dennis Gabor invents holography, the production of three-dimensional images.
1948 U.S. physicists Richard Feynman and Julian S. Schwinger, and Japanese physicist Shin'ichiro Tomonaga, independently develop quantum electrodynamics, the theory that accounts for the interactions between radiation, electrons, and positrons.
1951 U.S. physicist Edward Purcell discovers line radiation (radiation emitted at only one specific wavelength) at 21 cm/8 in emitted by hydrogen is space. It allows the distribution of hydrogen clouds in galaxies and the speed of the Milky Way's rotation to be determined.
1952 U.S. nuclear physicist Donald Glaser develops the bubble chamber to observe the behavior of subatomic particles. It uses a superheated liquid instead of a vapor to track particles.
1953 U.S. physicist Murray Gell-Mann introduces the concept of "strangeness," a property of subatomic particles, to explain their behavior.
1956 U.S. physicists b. cook, G. R. Lambertson, O. Piconi, and W. A. Wentzel discover the antineutron by passing an antiproton beam through matter.
1956 U.S. physicists Clyde Cowan and Fred Reines detect the existence of the neutrino, a particle with no electric charge and no mass, at the Los Alamos Laboratory.
1957 Japanese physicist Leo Esaki discovers tunneling, the ability of electrons to penetrate solids by acting as radiating waves.
1957 U.S. physicists John Bardeen, Leon Cooper, and John Schrieffer formulate the theory of superconductivity, the characteristic of a solid material to lose its resistance to electric current when cooled below a certain extremely low temperature.
1961 Gell-Mann and Israeli physicist Yuval Ne'eman independently propose a classification scheme for subatomic particles that comes to be known as the Eightfold Way.
1961 U.S. physicist Robert Hofstadter discovers that protons and neutrons have an internal structure.
1962 Welsh physicist Brian Josephson discovers the Josephson effect, the high-frequency oscillation of a current between two superconductors across an insulating layer.
1964 Gell-Mann and George Zweig independently suggest the existence of the quark, a subatomic particle and the building block of hadrons, a subatomic particle that experiences the strong nuclear force.
1967 U.S. nuclear physicists Sheldon Lee Glashow and Steven Weinberg and Pakistani nuclear physicist Abdus Salam separately develop the electroweak unification theory, which explains "electromagnetic" interactions and the "weak" nuclear force.
1970 U.S. physicist Sheldon Glashow and associates postulate the existence of a fourth quark, which they name "charm."
1971 English theoretical physicist Stephen Hawking suggests that after the Big Bang, mini black holes no bigger than a proton but containing more than a billion metric tons of mass were formed and that they were governed by both the laws of relativity and of quantum mechanics.
1973 Researchers at the European Centre for Particle Research (CERN) find so me confirmation for the electroweak force—one of the four fundamental forces—when they discover neutral currents in neutrino reactions.
1974 Hawking suggests that black holes emit subatomic particles until their energy is diminished to the point where they explode.
July 4, 1978 Scientists at the Princeton Large Torus test reactor achieve a temperature of 60 million degrees Fahrenheit, and maintain if for one-twentieth of a second. It is hailed as a breakthrough for nuclear fusion.
1979 Physicists in Hamburg at DESY (Deutsches Elektron Synchroton) observe gluons—particles that carry the strong nuclear force which holds quarks together.
1980 The Tevatron at Fermilab located at the Fermi National Accelerator Laboratory in Batavia, Illinois, United States, is completed; the most powerful proton synchrotron in the world, it is





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  designed to operate at 1,000 GeV or 1 TeV (teraelectron volt).
June 1983 The W and Z subatomic particles are detected in experiments at the European Centre for Nuclear Research (CERN), Switzerland, by Italian physicist Carlo Rubbia and Dutch physicist Somin van der Meer; the existence of these particles had been predicted as carriers of the weak nuclear force.
1984 A team of international physicists at CERN in Geneva, Switzerland, discovers the sixth (top) quark; its discovery completes the theoretical scheme of subatomic building blocks.
1986 German physicist Johnannes Bednorz and Swiss physicist Karl Alex Müller announce the discovery of a superconducting ceramic material in which superconductivity occurs at a much higher temperature (30 K) than hitherto known, increasing the potential for use of superconductivity for more energy-efficient motors and computers. They receive the Nobel Prize for Physics—in record time—for their discovery.
1986 Scientists use 10 laser beams, which deliver a total energy of 100 trillion watts during one-billionth of a second, to convert a small part of the hydrogen nuclei contained in a glass sphere to helium at the Lawrence Livermore National Laboratory in California; it is the first fusion reaction induced by a laser.
Feb 12, 1987 Chinese physicist Paul Ching-Wu Chu and associates at the University of Houston, Texas, make a material that is superconducting at the temperature of liquid nitrogen –77 K or –196°C/–321°F.
March 1989 U.S. physicist Stanley Pons and English physicist Martin Fleischmann announce that they have achieved nuclear fusion at room temperature (cold fusion); other scientists fail to replicate their experiment.
July 14, 1989 The LEP (Large Electron Positron Collider) is inaugurated at the CERN research center in Switzerland; the new accelerator has a circumference of 27 km/16.8 mi and is the largest scientific apparatus in the world.
Nov 9, 1991 The Joint European Torus (JET) at Culham, near Oxford, England, produces a 1.7 megawatt pulse of power in an experiment that lasts 2 seconds. It is the first time that a substantial amount of fusion power has been produced in a controlled experiment, as opposed to an atomic bomb.
1992 The Hadron Electron Ring Accelerator (HERA) particle accelerator is built under the streets of Hamburg, Germany. Occupying a tunnel 6.3 km/3.9 mi in length, it is the world's most powerful particle accelerator, accelerating protons to energies of 820 GeV (billion electron volts) and electrons to 30 GeV.
June 1995 U.S. physicists announce the discovery of a new form of matter, called a Bose–Einstein condensate (because its existence had been predicted by Einstein and Indian physicist Satyendra Bose), created by cooling rubidium atoms to just above absolute zero.
1995 The Omega lasers is developed at the University of Rochester, New York, United States. It generates 60 trillion watts of ultraviolet light in pulses that last for 0.65 billionths of a second, and is used in researching the civil applications of nuclear fusion.
1995 U.S. scientists at Fermilab, near Chicago, Illinois, United States announce the discovery of the top quark, an elementary particle almost as heavy as a gold atom.
Jan 4, 1996 A team of European physicists at the CERN research center in Switzerland create the first atoms of antimatter: nine atoms of antihydrogen survive for 40 nanoseconds.
July 11, 1998 Researchers at the Fermi National Accelerator laboratory, announce the discovery of the tau neutrino.


  Ampère, Andre Marie (1775–1836) French physicist and mathematician who made many discoveries in electromagnetism and electrodynamics. He followed up the work of Hans Oersted on the interaction between magnets and electric currents, developing a rule for determining the direction of the magnetic field associated with an electric current. The unit of electric current, the ampere, is named for him.  
  Ampère's law is an equation that relates the magnetic force produced by two parallel current-carrying conductors to the product of their currents and the distance between the conductors. Today Ampère's law is usually stated in the form of calculus: the line integral of the magnetic field around an arbitrarily chosen path is proportional to the net electric current enclosed by the path.  
  Ångström, Anders Jonas (1814–1874) Swedish astrophysicist who worked in spectroscopy and solar physics. In 1861 he identified the presence of hydrogen in the sun. His outstanding Recherches sur le spectre solaire (1868) presented an atlas of the solar spectrum with measurements of 1,000 spectral lines expressed in units of one-ten-millionth of a millimeter, the unit which later became the angstrom. He also investigated the conduction of heat and devised a method of determining thermal conductivity in 1863. His ''Optical investigations" (1853) contains his principle of spectrum analysis, demonstrating that a hot gas emits light at the same frequency as it absorbs it when it is cooled. In 1867 he investigated the spectrum of the aurora borealis, the first person to do so.  




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  Avogadro, Amedeo, Conte di Quaregna (1776–1856) Italian physicist, one of the founders of physical chemistry, who proposed Avogadro's hypothesis on gases in 1811. His work enabled scientists to calculate Avogadro's number, or constant, and still has relevance for atomic studies. Avogadro made it clear that the gas particles need not be individual atoms but might consist of molecules., the term he introduced to describe combinations of atoms. No previous scientist had made this fundamental distinction between the atoms of a substance and its molecules.  
  Bardeen, John (1908–1991) U.S. physicist. He became the first double winner of a Nobel prize in the same subject (with Leon Cooper (1930– ) and Robert Schrieffer (1931– )) in 1972 for his theory of superconductivity, which states that superconductivity arises when electrons traveling through a metal interact with the vibrating atoms of the metal.  
  Becquerel, (Antoine) Henri (1852–1908) French physicist. The discovery of X-rays in 1896 prompted Becquerel to investigate fluorescent crystals for the emission of X-rays, and in so doing he accidentally discovered radioactivity in uranium salts. He subsequently investigated the radioactivity of radium, and in 1900 showed that it consists of a stream of electrons. In the same year, he also obtained evidence that radioactivity causes the transformation of one element into another.  
  Biography of A. H. Becquerel
  Presentation of the life and discoveries of Becquerel, who was awarded the Nobel Prize in Physics jointly with Pierre and Marie Curie.  
  Bohr, Niels Henrik David (1885–1962) Danish physicist whose theoretical work in 1913 established the structure of the atom and the validity of quantum theory by showing that the nuclei of atoms are surrounded by shells of electrons, each assigned particular sets of quantum numbers according to their orbits. Bohr's atomic theory was validated in 1922 by the discovery of an element he had predicted, hafnium. He explained the structure and behavior of the nucleus, as well as the process of nuclear fission. He also proposed the doctrine of complementarity, the theory that a fundamental particle is neither a wave nor a particle, because these are complementary modes of description. In 1939 Bohr proposed his liquid-droplet model for the nucleus, in which nuclear particles are pulled together by short-range forces, similar to the way in which molecules in a drop of liquid are attracted to one another. The extra energy produced by the absorption of a neutron causes the nuclear particles to separate into two groups of approximately the same size, thus breaking the nucleus into two smaller nuclei—as happens in nuclear fission. The model was vindicated when Bohr correctly predicted the differing behavior of nuclei of uranium-235 and uranium-238 from the fact that the number of neutrons in each nucleus is odd and even respectively.  
  Born, Max (1882–1970) German-born British physicist. In 1924 Born coined the term "quantum mechanics" and in 1925 he devised a system called matrix mechanics that accounted mathematically for the position and momentum of the electron in the atom. He received a Nobel prize in 1954 for fundamental work on quantum theory, especially his 1926 discovery that the wave function of an electron is linked to the probability that the electron is to be found at any point. He also devised a technique, called the Born approximation method, for computing the behavior of subatomic particles, which is of great use in high-energy physics. In 1953 Born was also able to determine the energies involved in lattice formation, from which the properties of crystals may be derived, and thus laid one of the foundations of solid-state physics.  
  Born, Max
  Biography of the German-born British physicist. The Web site details the work of Born, and his relationships with his contemporaries and colleagues.  
  Broglie, Louis Victor Pierre Raymond de, 7th duc de Broglie (1892–1987) French theoretical physicist. He established that all subatomic particles can be described either by particle equations or by wave equations, thus laying the foundations of wave mechanics. De Broglie's discovery of wave–particle duality enabled physicists to view Einstein's conviction that matter and energy are interconvertible as being fundamental to the structure of matter. The study of matter waves led not only to a much deeper understanding of the nature of the atom but also to explanations of chemical bonds and the practical application of electron waves in electron microscopes.  
  De Broglie, Louis
  Biographical details and a photograph of Louis de Broglie, the famous French physicist and mathematician. There are also links to de Broglie's most famous work on quantum mechanics and to many of his contemporaries.  
  Carnot, (Nicolas Leonard) Sadi (1796–1832) French scientist and military engineer who founded the science of thermodynamics. His pioneering work was Reflexions sur la puissance motrice du feu/On the Motive Power of Fire, which considered the changes that would take place in an idealized, frictionless steam engine. Carnot's theorem showed that the amount of work that an engine can produce depends only on the temperature difference that occurs in the engine. In formulating his theorem, Carnot considered the case of an ideal heat engine following a reversible sequence known as the Carnot cycle. This cycle consists of the isothermal expansion and adiabatic expansion of a quantity of gas, producing work and consuming heat, followed by isothermal compression and adiabatic compression, consuming work and producing heat to restore the gas to its original state of pressure, volume, and temperature. Carnot's law states that no engine is more  




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  efficient than a reversible engine working between the same temperatures.  
  Charles, Jacques Alexandre César (1746–1823) French physicist who studied gases and made the first ascent in a hydrogen-filled balloon, in 1783. His work on the expansion of gases led to the formation of Charles's law.  
  Curie, Marie (born Manya Sklodowska) (1867–1934) Polish scientist who, with her husband Pierre Curie, discovered in 1898, two new radioactive elements in pitchblende ores: polonium and radium. They isolated the pure elements in 1902 and were jointly awarded the Nobel Prize for Physics in 1903. In 1910 with André Debierne (1874–1949), who had discovered actinium in pitchblende in 1899, Marie Curie isolated pure radium metal; she was awarded the Nobel Prize for Chemistry in 1911.  
  Marie and Pierre Curie
  Biographies of the Curies. This full account of their lives not only provides a wealth of personal details but also places their work alongside others working to increase understanding of radiation. Marie Curie's life after the death of Pierre and the hostility she suffered from the French press and scientific establishment is movingly described.  
  Dirac, Paul Adrien Maurice (1902–1984) British physicist who worked out a version of quantum mechanics consistent with special relativity. In 1928 he formulated the relativistic theory of the electron. The model was able to describe many quantitative aspects of the electron, including such properties as the half-quantum spin and magnetic moment. The existence of antiparticles, such as the positron (positive electron), was one of its predictions.  
  Doppler, Christian Johann (1803–1853) Austrian physicist who in 1842 described the Doppler effect (change of observed frequency (or wavelength) of waves due to relative motion between the wave source and observer) and derived the observed frequency mathematically in Doppler's principle.  
  Einstein, Albert (1879–1955) German-born U.S. physicist whose theories of relativity revolutionized our understanding of matter, space, and time. Einstein suggested that packets of light energy are capable of behaving as particles called "light quanta" (later called photons). Einstein used this hypothesis to explain the photoelectric effect, proposing that light particles striking the surface of certain metals cause electrons to be emitted and deduced the photoelectric law, for which he was awarded the Nobel Prize for Physics in 1921. Einstein went on to show in 1907 that mass is related to energy by the famous equation E = mc2, which indicates the enormous amount of energy that is stored as mass, some of which is released in radioactivity and nuclear reactions, for example in the sun. He also investigated Brownian motion, explaining the phenomenon as being due to the effect of large numbers of molecules (in this case, water molecules) bombarding the particles. Einstein's explanation of Brownian motion and its subsequent experimental confirmation was one of the most important pieces of evidence for the hypothesis that matter is composed of atoms. His special theory of relativity started with the premises that the laws of nature are the same for all observers in unaccelerated motion, and that the speed of light is independent of the motion of its source. In the general theory of relativity, the properties of space-time were to be conceived as modified locally by the presence of a body with mass; and light rays should bend when they pass by a massive object. His last conception of the basic laws governing the universe was outlined in his unified field theory, made public in 1953.  
  Life and Theories of Albert Einstein
  Heavily illustrated site on the life and theories of Einstein. There is an illustrated biographical chart, including a summary of his major achievements and their importance to science. The theory of relativity gets an understandably more in-depth coverage, along with photos and illustrations. The pages on his theories on light and time include illustrated explanations and an interactive test. There is also a "time-traveler" game demonstrating these theories.  
  Faraday, Michael (1791–1867) English chemist and physicist. In 1821 he began experimenting with electromagnetism and discovered the induction of electric currents and made the first dynamo, the first electric motor, and the first transformer. Faraday isolated benzene from gas oils and produced the basic laws of electrolysis in 1834. Faraday's laws of electrolysis established the link between electricity and chemical affinity, one of the most fundamental concepts in science. It was Faraday who coined the terms anode, cathode, cation, anion, electrode, and electrolyte. He demonstrated in 1837 that electrostatic force consists of a field of curved lines of force, and that different substances have specific inductive capacities—that is, they take up different amounts of electric charge when subjected to an electric field. He also pointed out that the energy of a magnet is in the field around it and not in the magnet itself, extending this basic conception of field theory to electrical and gravitational systems.  
  Fermi, Enrico (1901–1954) Italian-born U.S. physicist who proved the existence of new radioactive elements produced by bombardment with neutrons, and discovered nuclear reactions produced by low-energy neutrons. This research was the basis for studies leading to the atomic bomb and nuclear energy. Fermi built the first nuclear reactor in 1942 at Chicago University and later took part in the Manhattan Project to construct an atomic bomb.  
  Feynman, Richard Phillips
  Part of an archive containing the biographies of the world's greatest mathematicians, this site is devoted to the life and contributions of physicist Richard Feynman.  
  Feynman, Richard P(hillips) (1918–1988) U.S. physicist whose work laid the foundations of quantum electrodynamics, developing a simple and elegant system of Feynman diagrams  




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  to represent interactions between particles and how they moved from one space-time point to another. He had rules for calculating the probability associated with each diagram. He also contributed to the theory of superfluidity and to many aspects of particle physics, including quark theory and the nature of the weak nuclear force.  
  Foucault, Jean Bernard Léon (1819–1868) French physicist who used a pendulum to demonstrate the rotation of the earth on its axis, and invented the gyroscope in 1852. In 1862 he made the first accurate determination of the velocity of light.  
  Franklin, Benjamin (1706–1790) U.S. scientist, statesman, writer, printer, and publisher. He proved that lightning is a form of electricity, distinguished between positive and negative charges, and invented the lightning conductor. Franklin also made a fundamental discovery when he realized that the gain and loss of electricity must be balanced—the concept of conservation of charge. Franklin's interest in atmospheric electricity led him to recognize the aurora borealis as an electrical phenomenon, postulating good conditions in the rarefied upper atmosphere for electrical discharges, and speculating on the existence of what we now call the ionosphere.  
  Autobiography of Benjamin Franklin, The
  Contains the text of Franklin's autobiography, complete with an introduction and notes.  
  Fresnel, Augustin Jean (1788–1827) French physicist who refined the theory of polarized light. Fresnel realized in 1821 that light waves do not vibrate like sound waves longitudinally, in the direction of their motion, but transversely, at right angles to the direction of the propagated wave.  
  Galileo, properly Galileo Galilei (1564–1642) Italian mathematician, astronomer, and physicist. Galileo discovered that freely falling bodies, heavy or light, have the same, constant acceleration and that this acceleration is due to gravity. He also determined that a body moving on a perfectly smooth horizontal surface would neither speed up nor slow down. He invented a thermometer, a hydrostatic balance, and a compass, and discovered that the path of a projectile is made up of two components: one component consists of uniform motion in a horizontal direction, and the other component is vertical motion under acceleration or deceleration due to gravity. Galileo used this explanation to refute objections to the sun-centered theory of Polish astronomer Nicolaus Copernicus. Galileo's work founded the modern scientific method of deducing laws to explain the results of observation and experiment.  
  Geiger, Hans (Wilhelm) (1882–1945) German physicist who produced the Geiger counter. He spent the period 1906–12 in Manchester, England, working with Ernest c0016-01.gifRutherford on radioactivity. In 1908 they designed an instrument to detect and count alpha particles, positively charged ionizing particles produced by radioactive decay.  
  Hawking, Stephen (William) (1942– ) English physicist whose work in general relativity—particularly gravitational field theory—led to a search for a quantum theory of gravity to explain black holes and the Big Bang, singularities that classical relativity theory does not adequately explain. His book A Brief History of Time (1988) gives a popular account of cosmology and became an international bestseller. His latest book is The Nature of Space and Time, written with Roger Penrose.  
  Hawking, Stephen
  Stephen Hawking's own home page, with a brief biography, disability advice, and a selection of his lectures, including "The Beginning of Time" and a series debating the nature of space and time.  
  Heaviside, Oliver (1850–1925) English physicist. In 1902 he predicted the existence of an ionized layer of air in the upper atmosphere, which was known as the Kennelly—Heaviside layer but is now called the E-layer of the ionosphere. Deflection from it makes possible the transmission of radio signals around the world, which would otherwise be lost in outer space. Heaviside's theoretical work had implications for radio transmission. His studies of electricity published in Electrical Papers (1892) had considerable impact on long-distance telephony, and he added the concepts of inductance, capacitance, and impedance to electrical science.  
  Heaviside, Oliver
  Extensive biography of the English physicist. The site contains a description of his contribution to physics, and in particular his simplification of Maxwell's 20 equations in 20 variables, replacing them by two equations in two variables. Today we call these "Maxwell's equations" forgetting that they are in fact "Heaviside's equations."  
  Hertz, Heinrich Rudolf (1857–1894) German physicist who studied electromagnetic waves, showing their behavior resembles that of light and heat waves. He confirmed James Clerk Maxwell's theory of electromagnetic waves. In 1888 he realized that electric waves could be produced and would travel through air, and he confirmed this experimentally. He went on to determine the velocity of these waves (later called radio waves) and, on showing that it was the same as that of light, devised experiments to show that the waves could be reflected, refracted, and diffracted. The unit of frequency, the hertz, is named for him.  
  Heisenberg, Werner (Karl) (1901–1976) German physicist who developed quantum theory and formulated the uncertainty principle, which states that there is a theoretical limit to the precision with which a particle's position and momentum can be measured. In other words, it is impossible to specify precisely both the position and the simultaneous  




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  momentum (mass multiplied by velocity) of a particle. There is always a degree of uncertainty in either, and as one is determined with greater precision, the other can only be found less exactly.  
  Heisenberg, Werner Karl
  Part of an archive containing the details of the world's greatest mathematicians, this site is devoted to the life and contributions of physicist Werner Heisenberg.  
  Huygens (or Huyghens), Christiaan (1629–1695) Dutch mathematical physicist and astronomer. He proposed the wave theory of light, developed the pendulum clock in 1657, discovered polarization, and observed Saturn's rings. He made important advances in pure mathematics, applied mathematics, and mechanics, which he virtually founded.  
  Huygens, Christiaan
  Extensive biography of the Dutch astronomer, physicist, and mathematician. The site contains a description of his contributions to astronomy, physics, and mathematics. Also included are the title page of his book Horologium Oscillatorium (1673) and the first page of his book De Ratiocinüs in Ludo Aleae (1657).  
  Joliot-Curie, Frédéric (Jean) Joliot (1900–1958) and Iréne (born Curie) (1897–1956) French physicists. They made the discovery of artificial radioactivity and the transmutation of elements. In 1934, while bombarding light elements with alpha particles, they noticed that although proton production stopped when the alpha particle bombardment stopped, another form of radiation continued. The alpha particles had produced an isotope of phosphorus not found in nature. This isotope was radioactive and was decaying through beta-decay.  
  Josephson, Brian David (1940– ) Welsh physicist, a leading authority on superconductivity. In 1973 he shared a Nobel prize for his theoretical predictions of the properties of a supercurrent through a tunnel barrier (the Josephson effect), which led to the development of the Josephson junction.  
  Joule, James Prescott (1818–1889) English physicist. His work on the relations between electrical, mechanical, and chemical effects led to the discovery of the first law of thermodynamics. He determined the mechanical equivalent of heat (Joule's equivalent) in 1843, and the SI unit of energy, the joule, is named for him. He also discovered Joule's law, which defines the relation between heat and electricity; and with Irish physicist Lord Kelvin in 1852 the Joule-Kelvin (or Joule-Thomson) effect.  
  Kelvin, William Thomson, 1st Baron Kelvin (1824–1907) Irish physicist who introduced the Kelvin scale, the absolute scale of temperature. His work on the conservation of energy in 1851 led to the second law of thermodynamics. In 1847 he concluded that electrical and magnetic fields are distributed in a manner analogous to the transfer of energy through an elastic solid. From 1849 to 1859, Kelvin also developed the work of English scientist Michael c0016-01.gifFaraday into a full theory of magnetism, arriving at an expression for the total energy of a system of magnets.  
  Maxwell, James Clerk (1831–1879) Scottish physicist. His main achievement was in the understanding of electromagnetic waves: Maxwell's equations bring electricity, magnetism, and light together into one set of relations. He studied gases, optics, and the sensation of color, and his theoretical work in magnetism prepared the way for wireless telegraphy and telephony. In developing the kinetic theory of gases, Maxwell gave the final proof that heat resides in the motion of molecules.  
  Maxwell, James Clerk
  Extensive biography of the Scottish physicist and mathematician. The site contains a description of his contribution to physics, in particular his work on electricity and magnetism.  
  Michelson, Albert Abraham (1852–1931) German-born U.S. physicist. With his colleague Edward Morley (1838–1923), he performed in 1887 the Michelson-Morley experiment to detect the motion of the earth through the postulated ether (a medium believed to be necessary for the propagation of light). The negative result of the experiment demonstrated that the velocity of light is constant whatever the motion of the observer. The failure of the experiment indicated the nonexistence of the ether, and led Einstein to his theory of relativity. Michelson invented the Michelson interferometer to detect any difference in the velocity of light in two directions at right angles.  
  Newton, Isaac (1642–1727) English physicist and mathematician who laid the foundations of physics as a modern discipline. During 1665–66, he discovered the binomial theorem, differential and integral calculus, and that white light is composed of many colors. He developed the three standard laws of motion (Newton's laws of motion) and the universal law of gravitation, set out in Philosophiae naturalis principia mathematica (1687, usually referred to as the Principia). Newton's greatest achievement was to demonstrate that scientific principles are of universal application. He clearly defined the nature of mass, weight, force, inertia, and acceleration. In 1679 he calculated the moon's motion on the basis of his theory of gravity and also found that his theory explained the laws of planetary motion that had been derived by the German astronomer Johannes Kepler on the basis of observations of the planets.  
  Oersted, Hans Christian (1777–1851) Danish physicist who founded the science of electromagnetism. In 1820 he discovered the magnetic field associated with an electric current.  
  Ohm, Georg Simon (1789–1854) German physicist who studied electricity and discovered the fundamental law that bears his name. The SI unit of electrical resistance, the ohm, is named for him, and the unit of conductance (the inverse of resistance) was formerly called the mho, which is "ohm" spelled backwards.  




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  Planck, Max Karl Ernst (1858–1947) German physicist who framed the quantum theory in 1900. His research into the manner in which heated bodies radiate energy led him to report that energy is emitted only in indivisible amounts, called "quanta," the magnitudes of which are proportional to the frequency of the radiation. His discovery ran counter to classical physics that radiation consisted of waves and is held to have marked the commencement of the modern science.  
  Planck, Max Karl Ernst Ludwig
  Biography of the eminent German physicist Max Planck. Planck is thought of by many to have been more influential than any other in the foundation of quantum physics, and partly as a consequence has a fundamental constant named for him.  
  Röntgen (or Roentgen), Wilhelm Konrad (1845–1923) German physicist. He discovered X-rays in 1895. While investigating the passage of electricity through gases, he noticed the fluorescence of a barium platinocyanide screen. This radiation passed through some substances opaque to light, and affected photographic plates. Developments from this discovery revolutionized medical diagnosis.  
  Röntgen, Wilhelm Conrad
  Biography of Wilhelm Conrad Röntgen, the German physicist who first realized the huge potential of the electromagnetic field of X-rays. The presentation includes sections on Röntgen's early years and education, his academic career and scientific experiments, and the miraculous coincidences that led him to his great discovery of X-rays.  
  Rutherford, Ernest, 1st Baron Rutherford of Nelson (1871–1937) New Zealand-born British physicist. His main research was in the field of radioactivity, and he discovered alpha, beta, and gamma rays. He was the first to recognize the nuclear nature of the atom in 1911. In 1919 he produced the first artificial transformation, changing one element to another by bombarding nitrogen with alpha particles and getting hydrogen and oxygen. After further research he announced that the nucleus of any atom must be composed of hydrogen nuclei; at Rutherford's suggestion, the name "proton" was given to the hydrogen nucleus in 1920. He speculated that uncharged particles (neutrons) must also exist in the nucleus. In 1934, using heavy water, Rutherford and his co-workers bombarded deuterium with deuterons and produced tritium. This may be considered the first nuclear fusion reaction.  
  Schrödinger, Erwin
  Part of an archive containing the biographies of the world's greatest mathematicians, this site is devoted to the life and contributions of physicist Erwin Schrödinger.  
  Schrödinger, Erwin (1887–1961) Austrian physicist. He advanced the study of wave mechanics to describe the behavior of electrons in atoms. In 1926 he produced a solid mathematical explanation of the quantum theory and the structure of the atom.  
  Tesla, Nikola (1856–1943) Serbian-born U.S. physicist and electrical engineer who invented fluorescent lighting, the Tesla induction motor (1882–87), and the Tesla coil, and developed the alternating current (A.C.) electrical supply system. The Tesla coil is an air core transformer with the primary and secondary windings tuned in resonance to produce highfrequency, high-voltage electricity.  
  Tesla, Nikola
  Short biography of the electrical inventor plus quotations by and about Tesla, anecdotes, a photo gallery, and links to other sites of interest.  
  Thomson, J(oseph) J(ohn) (1856–1940) English physicist. He discovered the electron in 1897. His work inaugurated the electrical theory of the atom, and his elucidation of positive rays and their application to an analysis of neon led to the discovery of isotopes. Using magnetic and electric fields to deflect positive rays, Thomson found in 1912 that ions of neon gas are deflected by different amounts, indicating that they consist of a mixture of ions with different charge-to-mass ratios. English chemist Frederick Soddy had earlier proposed the existence of isotopes and Thomson proved this idea correct when he identified, also in 1912, the isotope neon-22.  
  Volta, Alessandro Giuseppe Antonio Anastasio, Count (1745–1827) Italian physicist who invented the first electric cell (the voltaic pile, in 1800), the electrophorus (an early electrostatic generator, in 1775), and an electroscope. Volta also produced a list of metals in order of their electricity production based on the strength of the sensation they made on his tongue, thereby deriving the electromotive series. In about 1795, Volta recognized that the vapor pressure of a liquid is independent of the pressure of the atmosphere and depends only on temperature. The volt is named for him.  
  Weber, Wilhelm Eduard (1804–1891) German physicist who studied magnetism and electricity. Working with the German mathematician Karl Gauss (1777–1855), he made sensitive magnetometers to measure magnetic fields, and instruments to measure direct and alternating currents. He also built an electric telegraph. The SI unit of magnet flux, the weber, is named for him.  
  Young, Thomas (1773–1829) British physicist who revived the wave theory of light and identified the phenomenon of interference in 1801. He also established many important concepts in mechanics. Young assumed that light waves are propagated in a similar way to sound waves, and proposed that different colors consist of different frequencies. He obtained experimental proof for the principle of interference by passing light through extremely narrow openings and  




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  observing the interference patterns produced. In mechanics, Young was the first to use the terms "energy" for the product of the mass of a body with the square of its velocity and "labor expended" for the product of the force exerted on a body "with the distance through which it moved." He also stated that these two products are proportional to each other. He introduced an absolute measurement in elasticity, now known as Young's modulus.  
  Biographies of Physicists
  Valuable compilation of biographies of the most famous physicists of all time, including Aristotle, Da Vinci, Kepler, Galilei, Newton, Franklin, Curie, Feynman, and Oppenheimer. Be careful to bookmark it properly because you can easily get lost in the many pages of this site.  
phenomenon by which a substance retains radiation of particular wavelengths; for example, a piece of blue glass absorbs all visible light except the wavelengths in the blue part of the spectrum; it also refers to the partial loss of energy resulting from light and other electromagnetic waves passing through a medium. In nuclear physics, absorption is the capture by elements, such as boron, of neutrons produced by fission in a reactor.
experimental and theoretical science of sound and its transmission; in particular, that branch of the science that has to do with the phenomena of sound in a particular space such as a room or theater.
term used to describe a process that occurs without loss or gain of heat, especially the expansion or contraction of a gas in which a change takes place in the pressure or volume, although no heat is allowed to enter or leave.
taking up of a gas or liquid at the surface of another substance, most commonly a solid (for example, activated charcoal adsorbs gases). It involves molecular attraction at the surface, and should be distinguished from absorption (in which a uniform solution results from a gas or liquid being incorporated into the bulk structure of a liquid or solid).
branch of fluid physics that studies the forces exerted by air or other gases in motion. Examples include the airflow around bodies moving at speed through the atmosphere (such as land vehicles, bullets, rockets, and aircraft), the behavior of gas in engines and furnaces, air conditioning of buildings, the deposition of snow, the operation of aircushion vehicles (hovercraft), wind loads on buildings and bridges, bird and insect flight, musical wind instruments, and meteorology. For maximum efficiency, the aim is usually to design the shape of an object to produce a streamlined flow, with a minimum of turbulence in the moving air.
  alternating current (A.C.)
electric current that flows for an interval of time in one direction and then in the opposite direction, that is, a current that flows in alternately reversed directions through or around a circuit. Electric energy is usually generated as alternating current in a power station, and alternating currents may be used for both power and lighting. The advantage of alternating current over direct current (D.C.), as from a battery, is that its voltage can be raised or lowered economically by a transformer: high voltage for generation and transmission, and low voltage for safe utilization.
instrument that measures electric current (flow of charge per unit time), usually in amperes, through a conductor. It should not to be confused with a voltmeter, which measures potential difference between two points in a circuit.
SI unit (symbol A) of electrical current. Electrical current is measured in a similar way to water current, in terms of an amount per unit time; one ampere represents a flow of about 6.28 × 10
18 electrons per second, or a rate of flow of charge of one coulomb per second.
ion carrying a negative charge. During electrolysis, anions in the electrolyte move toward the anode (positive electrode).
  Archimedes' principle
principle that the weight of the liquid displaced by a floating body is equal to the weight of the body. The principle is often stated in the form: "an object totally or partially submerged in a fluid displaces a volume of fluid that weighs the same as the apparent loss in weight of the object (which, in turn, equals the upward force, or upthrust, experienced by that object)." It was discovered by the Greek mathematician Archimedes.
  atmosphere, or standard atmosphere
unit (symbol atm) of pressure equal to 760 torr, 1013.25 millibars, or 1.01325 × 10
5 newtons per square meter. The actual pressure exerted by the atmosphere fluctuates around this value, which is assumed to be standard at sea level and 0°C/32°F, and is used when dealing with very high pressures.
  Avogadro's number, or Avogadro's constant
the number of carbon atoms in 12 g of the carbon-12 isotope (6.022045 × 10
23). The atomic weight of any element, expressed in grams, contains this number of atoms.
study of the motion and impact of projectiles such as bullets, bombs, and missiles. For projectiles from a gun, relevant exterior factors include temperature, barometric pressure, and wind strength; and for nuclear missiles these extend to such factors as the speed at which the earth turns.
SI unit (symbol Bq) of radioactivity, equal to one radioactive disintegration (change in the nucleus of an atom when a particle or ray is given off) per second.
  binding energy
amount of energy needed to break the nucleus of an atom into the neutrons and protons of which it is made.
  boiling point
for any given liquid, the temperature at which the application of heat raises the temperature of the liquid no further, but converts it into vapor.
  Boltzmann constant
constant (symbol k) that relates the kinetic energy (energy of motion) of a gas atom or




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  molecule to temperature. Its value is 1.38066 × 10–23 joules per kelvin. It is equal to the gas constant R, divided by c0016-01.gifAvogadro's number.  
elementary particle whose spin can only take values that are whole numbers or zero. Bosons may be classified as gauge bosons (carriers of the four fundamental forces) or mesons. All elementary particles are either bosons or fermions. Unlike fermions, more than one boson in a system (such as an atom) can possess the same energy state.
form of carbon, made up of molecules (buckyballs) consisting of 60 carbon atoms arranged in 12 pentagons and 20 hexagons to form a perfect sphere. It was named for the U.S. architect and engineer Richard Buckminster Fuller (1895–1983) because of its structural similarity to the geodesic dome that he designed.
  capacitor, or condenser
device for storing electric charge, used in electronic circuits; it consists of two or more metal plates separated by an insulating layer called a dielectric.
part of an electronic device in which electrons are generated. In a thermionic valve, electrons are produced by the heating effect of an applied current; in a photocell, they are produced by the interaction of light and a semiconducting material. The cathode is kept at a negative potential relative to the device's other electrodes (anodes) in order to ensure that the liberated electrons stream away from the cathode and toward the anodes.
  cathode ray
stream of fast-moving electrons that travel from a cathode (negative electrode) toward an anode (positive electrode) in a vacuum tube. They carry a negative charge and can be deflected by electric and magnetic fields. Cathode rays focused into fine beams of fast electrons are used in cathode-ray tubes, the electrons" kinetic energy being converted into light energy as they collide with the tube's fluorescent screen.
  centrifugal force
useful concept in physics, based on an apparent (but not real) force. It may be regarded as a force that acts radially outward from a spinning or orbiting object, thus balancing the centripetal force (which is real). For an object of mass m moving with a velocity v in a circle of radius r, the centrifugal force F equals mv
2/r (outward).
  centripetal force
force that acts radially inward on an object moving in a curved path. For example, with a weight whirled in a circle at the end of a length of string, the centripetal force is the tension in the string. For an object of mass m moving with a velocity v in a circle of radius r, the centripetal force F equals mv
2/r (inward). The reaction to this force is the c0016-01.gifcentrifugal force.
  chain reaction
in nuclear physics, a fission reaction that is maintained because neutrons released by the splitting of some atomic nuclei themselves go on to split others, releasing even more neutrons. Such a reaction can be controlled (as in a nuclear reactor) by using moderators to absorb excess neutrons. Uncontrolled, a chain reaction produces a nuclear explosion (as in an atomic bomb).
arrangement of electrical components through which a current can flow. There are two basic circuits, series and parallel. In a series circuit, the components are connected end to end so that the current flows through all components one after the other. In a parallel circuit, components are connected side by side so that part of the current passes through each component. A circuit diagram shows in graphical form how components are connected together, using standard symbols for the components.
  cloud chamber
apparatus for tracking ionized particles. It consists of a vessel fitted with a piston and filled with air or other gas, saturated with water vapor. When the volume of the vessel is suddenly expanded by moving the piston outward, the vapor cools and a cloud of tiny droplets forms on any nuclei, dust, or ions present. As fast-moving ionizing particles collide with the air or gas molecules, they show as visible tracks.
  cold fusion
in nuclear physics, the fusion of atomic nuclei at room temperature. If cold fusion were possible it would provide a limitless, cheap, and pollution-free source of energy, and it has therefore been the subject of research around the world. In 1989, Martin Fleischmann (1927– ) and Stanley Pons (1943– ) of the University of Utah, United States, claimed that they had achieved cold fusion in the laboratory, but their results could not be substantiated and in 1998 their patent was allowed to lapse.
quality or wavelength of light emitted or reflected from an object. Visible white light consists of electromagnetic radiation of various wavelengths, and if a beam is refracted through a prism, it can be spread out into a spectrum, in which the various colors correspond to different wavelengths. From long to short wavelengths (from about 700 to 400 nanometers) the colors are red, orange, yellow, green, blue, indigo, and violet.
  cosmic radiation
streams of high-energy particles from outer space, consisting of protons, alpha particles, and light nuclei, which collide with atomic nuclei in the earth's atmosphere, and produce secondary nuclear particles (chiefly mesons, such as pions and muons) that shower the earth.
SI unit (symbol C) of electrical charge. One coulomb is the quantity of electricity conveyed by a current of one c0016-01.gifampere in one second.
  critical mass
in nuclear physics, the minimum mass of fissile material that can undergo a continuous c0016-01.gifchain reaction. Below this mass, too many neutrons escape from the surface for a chain reaction to carry on; above the critical mass, the reaction may accelerate into a nuclear explosion.
science of very low temperatures (approaching c0016-01.gifabsolute zero), including the production of very low temperatures and the exploitation of special properties associated with them, such as the disappearance of electrical resistance (superconductivity).
  Curie temperature
temperature above which a magnetic material cannot be strongly magnetized. Above the Curie temperature, the energy of the atoms is too great for them to join together to form the small areas of magnetized material, or c0016-01.gifdomains, which combine to produce the strength of the overall magnetization.
any directly measurable physical quantity such as mass (M), length (L), and time (T), and the derived units




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  obtainable by multiplication or division from such quantities. For example, acceleration (the rate of change of velocity) has dimensions (LT–2), and is expressed in such units as km s–2. A quantity that is a ratio, such as relative density or humidity, is dimensionless.  
combination of a cold anode and a heated cathode, or the semiconductor equivalent, which incorporates a p–n junction. Either device allows the passage of direct current in one direction only, and so is commonly used in a rectifier to convert alternating current (A.C.) to direct current (D.C.).
  direct current (D.C.)
electric current that flows in one direction, and does not reverse its flow as c0016-01.gifalternating current does. The electricity produced by a battery is direct current.
small area in a magnetic material that behaves like a tiny magnet. The magnetism of the material is due to the movement of the electrons in the atoms of the domain. In an unmagnetized sample of material, the domains point in random directions, or form closed loops, so that there is no overall magnetization of the sample. In a magnetized sample, the domains are aligned so that their magnetic effects combine to produce a strong overall magnetism.
  Doppler effect
change in the observed frequency (or wavelength) of waves due to relative motion between the wave source and the observer. The Doppler effect is responsible for the perceived change in pitch of a siren as it approaches and then recedes, and for the red shift of light from distant galaxies. It is named for the Austrian physicist Christian Doppler.
repetition of a sound wave, or of a radar or sonar signal, by reflection from a surface. By accurately measuring the time taken for an echo to return to the transmitter, and by knowing the speed of a radar signal (the speed of light) or a sonar signal (the speed of sound in water), it is possible to calculate the range of the object causing the echo (echolocation).
  electric field
region in which a particle possessing electric charge experiences a force owing to the presence of another electric charge. The strength of an electric field, E, is measured in volts per meter (V m
–1). It is a type of c0016-01.gifelectromagnetic field.
any terminal by which an electric current passes in or out of a conducting substance; for example, the anode or c0016-01.gifcathode in a battery or the carbons in an arc lamp. The terminals that emit and collect the flow of electrons in thermionic valves (electron tubes) are also called electrodes: for example, cathodes, plates, and grids.
  electromagnetic field
region in which a particle with an electric charge experiences a force. If it does so only when moving, it is in a pure magnetic field; if it does so when stationary, it is in an electric field. Both can be present simultaneously.
  electromotive force (emf)
loosely, the voltage produced by an electric battery or generator in an electrical circuit or, more precisely, the energy supplied by a source of electric power in driving a unit charge around the circuit. The unit is the volt.
substance that cannot be split chemically into simpler substances. The atoms of a particular element all have the same number of protons in their nuclei (their atomic number). Elements are classified in the periodic table of the elements. Of the known elements, 92 are known to occur in nature (those with atomic numbers 1–92). Those elements with atomic numbers above 96 do not occur in nature and are synthesized only, produced in particle accelerators. Of the elements, 81 are stable; all the others, which include atomic numbers 43, 61, and from 84 up, are radioactive.
in thermodynamics, a parameter representing the state of disorder of a system at the atomic, ionic, or molecular level; the greater the disorder, the higher the entropy. Thus the fast-moving disordered molecules of water vapor have higher entropy than those of more ordered liquid water, which in turn have more entropy than the molecules in solid crystalline ice. In a closed system undergoing change, entropy is a measure of the amount of energy unavailable for useful work. At absolute zero (–273.15°C/–459.67°F/0 K), when all molecular motion ceases and order is assumed to be complete, entropy is zero.
process in which a liquid turns to a vapor without its temperature reaching boiling point. A liquid left to stand in a saucer eventually evaporates because, at any time, a proportion of its molecules will be fast enough (have enough kinetic energy) to escape through the attractive intermolecular forces at the liquid surface into the atmosphere. The temperature of the liquid tends to fall because the evaporating molecules remove energy from the liquid. The rate of evaporation rises with increased temperature because as the mean kinetic energy of the liquid's molecules rises, so will the number possessing enough energy to escape.
increase in size of a constant mass of substance caused by, for example, increasing its temperature (thermal expansion) or its internal pressure. The expansivity, or coefficient of thermal expansion, of a material is its expansion (per unit volume, area, or length) per degree rise in temperature.
SI unit (symbol F) of electrical capacitance (how much electric charge a c0016-01.gifcapacitor can store for a given voltage). One farad is a capacitance of one c0016-01.gifcoulomb per volt. For practical purposes the microfarad (one millionth of a farad, symbol mF) is more commonly used.
subatomic particle whose spin can only take values that are half-integers, such as 1/2 or 3/2. Fermions may be classified as leptons, such as the electron, and baryons, such as the proton and neutron. All elementary particles are either fermions or c0016-01.gifbosons.
  fiber optics
branch of physics dealing with the transmission of light and images through glass or plastic fibers known as optical fibers.
region of space in which an object exerts a force on another separate object because of certain properties they both possess. For example, there is a force of attraction between any two objects that have mass when one is in the gravitational field of the other. Other fields of force include c0016-01.gifelectric fields (caused by electric charges) and magnetic fields (caused by circulating electric currents), either of which can involve attractive or repulsive forces.




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any influence that tends to change the state of rest or the uniform motion in a straight line of a body. The action of an unbalanced or resultant force results in the acceleration of a body in the direction of action of the force, or it may, if the body is unable to move freely, result in its deformation (see c0016-01.gifHooke's law). Force is a vector quantity, possessing both magnitude and direction; its SI unit is the newton.
change from liquid to solid state, as when water becomes ice. For a given substance, freezing occurs at a definite temperature, known as the freezing point, that is invariable under similar conditions of pressure, and the temperature remains at this point until all the liquid is frozen. The amount of heat per unit mass that has to be removed to freeze a substance is a constant for any given substance, and is known as the latent heat of fusion.
force that opposes the relative motion of two bodies in contact. The coefficient of friction is the ratio of the force required to achieve this relative motion to the force pressing the two bodies together.
  fundamental constant
physical quantity that is constant in all circumstances throughout the whole universe. Examples are the electric charge of an electron, the speed of light, Planck's constant, and the gravitational constant.
  Geiger counter
any of a number of devices used for detecting nuclear radiation and/or measuring its intensity by counting the number of ionizing particles produced. It detects the momentary current that passes between c0016-01.gifelectrodes in a suitable gas when a nuclear particle or a radiation pulse causes the ionization of that gas. The electrodes are connected to electronic devices that enable the number of particles passing to be measured. The increased frequency of measured particles indicates the intensity of radiation. The device is named for the German physicist Hans Geiger.
subatomic particle that experiences the strong nuclear force. Each is made up of two or three indivisible particles called quarks. The hadrons are grouped into the baryons (protons, neutrons, and hyperons) and the mesons (particles with masses between those of electrons and protons).
  heat capacity
quantity of heat required to raise the temperature of an object by one degree. The specific heat capacity of a substance is the heat capacity per unit of mass, measured in joules per kilogram per kelvin (J kg
–1 K–1).
  Hooke's law
law stating that the deformation of a body is proportional to the magnitude of the deforming force, provided that the body's elastic limit is not exceeded. If the elastic limit is not reached, the body will return to its original size once the force is removed. The law was discovered by the English scientist Robert Hooke (1635–1703) in 1676.
phenomenon where a changing current in a circuit builds up a magnetic field which induces an c0016-01.gifelectromotive force either in the same circuit and opposing the current (self-inductance) or in another circuit (mutual inductance). The SI unit of inductance is the henry (symbol H). A component designed to introduce inductance into a circuit is called an inductor (sometimes inductance) and is usually in the form of a coil of wire. The energy stored in the magnetic field of the coil is proportional to its inductance and the current flowing through it.
device included in an electrical circuit because of its inductance.
  integrated circuit (IC), or silicon chip
miniaturized electronic circuit produced on a single crystal, or chip, of a semiconducting material—usually silicon.
SI unit (symbol J) of work and energy, replacing the calorie (one joule equals 4.2 calories).
  Kelvin scale
temperature scale used by scientists. It begins at—absolute zero (–273.15°C) and increases by the same degree intervals as the Celsius scale; that is, 0°C is the same as 273.15 K and 100°C is 373.15 K.
  kinetic energy
energy of a body resulting from motion. It is contrasted with c0016-01.gifpotential energy.
branch of dynamics dealing with the action of forces producing or changing the motion of a body; kinematics deals with motion without reference to force or mass.
  kinetic theory
theory describing the physical properties of matter in terms of the behavior—principally movement—of its component atoms or molecules. The temperature of a substance is dependent on the velocity of movement of its constituent particles, increased temperature being accompanied by increased movement. A gas consists of rapidly moving atoms or molecules and, according to kinetic theory, it is their continual impact on the walls of the containing vessel that accounts for the pressure of the gas. The slowing of molecular motion as temperature falls, according to kinetic theory, accounts for the physical properties of liquids and solids, culminating in the concept of no molecular motion at c0016-01.gifabsolute zero (0K/–273°C). By making various assumptions about the nature of gas molecules, it is possible to derive from the kinetic theory the various gas laws (such as Avogadro's hypothesis, Boyle's law, and Charles's law).
  latent heat
heat absorbed or released by a substance as it changes state (for example, from solid to liquid) at constant temperature and pressure.
emission of light from a body when its atoms are excited by means other than raising its temperature. Short-lived luminescence is called fluorescence; longer-lived luminescence is called phosphorescence. When exposed to an external source of energy, the outer electrons in atoms of a luminescent substance absorb energy and "jump" to a higher energy level. When these electrons "jump" back to their former level they emit their excess energy as light.
  melting point
temperature at which a substance melts, or changes from solid to liquid form. A pure substance under standard conditions of pressure (usually one atmosphere) has a definite melting point. If heat is supplied to a solid at its melting point, the temperature does not change until the melting process is complete. The melting point of ice is 0°C or 32°F.
SI unit (symbol N) of c0016-01.gifforce. One newton is the force needed to accelerate an object with mass of one kilogram by one meter per second per second. The weight of a medium size (100 g/3 oz) apple is one newton.
in particle physics, either a proton or a neutron, when present in the atomic nucleus. Nucleon number is an alternative name for the mass number of an atom.




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  nuclear physics
study of the properties of the nucleus of the atom, including the structure of nuclei; nuclear forces; the interactions between particles and nuclei; and the study of radioactive decay.
SI unit (symbol W) of electrical c0016-01.gifresistance (the property of a conductor that restricts the flow of electrons through it).
study of light and vision—for example, shadows and mirror images, lenses, microscopes, telescopes, and cameras.
  particle physics
study of the particles that make up all atoms, and of their interactions. More than 300 subatomic particles have now been identified by physicists, categorized into several classes according to their mass, electric charge, spin, magnetic moment, and interaction.
  Peltier effect
change in temperature at the junction of two different metals produced when an electric current flows through them. The extent of the change depends on what the conducting metals are, and the nature of change (rise or fall in temperature) depends on the direction of current flow. It is the reverse of the c0016-01.gifSeebeck effect. It is named for the French physicist Jean Charles Peltier (1785–1845) who discovered it in 1834.
  perpetual motion
idea that a machine can be designed and constructed in such a way that, once started, it will continue in motion indefinitely without requiring any further input of energy (motive power). Such a device would contradict at least one of the two laws of thermodynamics that state that (1) energy can neither be created nor destroyed (the law of conservation of energy) and (2) heat cannot by itself flow from a cooler to a hotter object. As a result, all practical (real) machines require a continuous supply of energy, and no heat engine is able to convert all the heat into useful work.
  piezoelectric effect
property of some crystals (for example, quartz) to develop an electromotive force or voltage across opposite faces when subjected to tension or compression, and, conversely, to expand or contract in size when subjected to an electromotive force. Piezoelectric crystal oscillators are used as frequency standards (for example, replacing balance wheels in watches), and for producing ultrasound.
  Planck's constant
fundamental constant (symbol h) that relates the energy (E) of one quantum of electromagnetic radiation (the smallest possible ''packet" of energy) to the frequency (f) of its radiation by E = hf. Its value is 6.6260755 × 10
  potential difference (pd)
difference in the electrical potential (see c0016-01.gifpotential, electric) of two points, being equal to the electrical energy converted by a unit electric charge moving from one point to the other. The SI unit of potential difference is the volt (V). The potential difference between two points in a circuit is commonly referred to as voltage. In equation terms, potential difference V may be defined by: V = W/Q, where W is the electrical energy converted in joules and Q is the charge in coulombs.
  potential, electric
relative electrical state of an object. The potential at a point is equal to the energy required to bring a unit electric charge from infinity to the point. The SI unit of potential is the volt (V). Positive electric charges will flow "downhill" from a region of high potential to a region of low potential. The difference in potential—c0016-01.gifpotential difference (pd)—is expressed in volts so, for example, a 12 V battery has a pd of 12 volts between its negative and positive terminals.
  potential energy (PE)
energy possessed by an object by virtue of its relative position or state (for example, as in a compressed spring or a muscle). It is contrasted with kinetic energy, the form of energy possessed by moving bodies. An object that has been raised up is described as having gravitational potential energy.
rate of doing work or consuming energy. It is measured in watts (joules per second) or other units of work per unit time. If the work done or energy consumed is W joules and the time taken is t seconds, then the power P is given by the formula P = W/t.
in a fluid, the force that would act normally (at right angles) per unit surface area of a body immersed in the fluid. The SI unit of pressure is the pascal (Pa), equal to a pressure of one newton per square meter. In the atmosphere, the pressure declines with height from about 100 kPa at sea level to zero where the atmosphere fades into space. Pressure is commonly measured with a barometer, manometer, or Bourdon gauge. Other common units of pressure are the bar and the torr.
triangular block of transparent material (plastic, glass, silica) commonly used to "bend" a ray of light or split a beam into its spectral colors. Prisms are used as mirrors to define the optical path in binoculars, camera viewfinders, and periscopes. The dispersive property of prisms is used in the spectroscope.
  quantum theory
theory that energy does not have a continuous range of values, but is, instead, absorbed or radiated discontinuously, in multiples of definite, indivisible units called quanta. Just as earlier theory showed how light, generally seen as a wave motion, could also in some ways be seen as composed of discrete particles (photons), quantum theory shows how atomic particles such as electrons may also be seen as having wavelike properties. Quantum theory is the basis of particle physics, modern theoretical chemistry, and the solid-state physics that describes the behavior of the silicon chips used in computers.
that property of a conductor that restricts the flow of electricity through it, associated with the conversion of electrical energy to heat; also the magnitude of this property. Resistance depends on many factors, such as the nature of the material, its temperature, dimensions, and thermal properties; degree of impurity; the nature and state of illumination of the surface; and the frequency and magnitude of the current. The SI unit of resistance is the ohm. Resistance = voltage/current.
rapid amplification of a vibration when the vibrating object is subject to a force varying at its natural frequency. In a trombone, for example, the length of the air column in the instrument is adjusted until it resonates with the note being sounded. Resonance effects are also produced by many electrical circuits. Tuning a radio, for example, is done by




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  adjusting the natural frequency of the receiver circuit until it coincides with the frequency of the radio waves falling on the aerial.  
  Seebeck effect
generation of a voltage in a circuit containing two different metals, or semiconductors, by keeping the junctions between them at different temperatures. Discovered by the German physicist Thomas Seebeck (1770–1831), it is also called the thermoelectric effect, and is the basis of the thermocouple. It is the opposite of the c0016-01.gifPeltier effect (in which current flow causes a temperature difference between the junctions of different metals).
  short circuit
unintended direct connection between two points in an electrical circuit. Resistance is proportional to the length of wire through which current flows. By bypassing the rest of the circuit, the short circuit has low resistance and a large current flows through it. This may cause the circuit to overheat dangerously.
SI unit (symbol S) of electrical conductance, the reciprocal of the c0016-01.gifresistance of an electrical circuit. One siemens equals one ampere per volt. It was formerly called the mho or reciprocal ohm.
SI unit (symbol Sv) of radiation dose equivalent. It replaces the rem (1 Sv =100 rem). Some types of radiation do more damage than others for the same absorbed dose—for example, an absorbed dose of alpha radiation causes 20 times as much biological damage as the same dose of beta radiation. The equivalent dose in sieverts is equal to the absorbed dose of radiation in grays multiplied by the relative biological effectiveness. Humans can absorb up to 0.25 Sv without immediate ill effects; 1 Sv may produce radiation sickness; and more than 8 Sv causes death.
coil of wire, usually cylindrical, in which a magnetic field is created by passing an electric current through it This field can be used to move an iron rod placed on its axis.
  specific gravity or relative density
the density (at 20°C/68°F) of a solid or liquid relative to (divided by) the maximum density of water (at 4°C/39.2°F). The specific gravity of a gas is its density divided by the density of hydrogen (or sometimes dry air) at the same temperature and pressure.
  specific heat capacity
quantity of heat required to raise unit mass (1 kg) of a substance by one kelvin (1 K). The unit of specific heat capacity in the SI system is the c0016-01.gifjoule per kilogram kelvin (J kg
–1 K–1).
study of spectra (see c0016-01.gifspectrum) associated with atoms or molecules in solid, liquid, or gaseous phase. Spectroscopy can be used to identify unknown compounds and is an invaluable tool in science, medicine, and industry (for example, in checking the purity of drugs).
(plural spectra) arrangement of frequencies or wavelengths when electromagnetic radiations are separated into their constituent parts. Visible light is part of the electromagnetic spectrum and most sources emit waves over a range of wavelengths that can be broken up or "dispersed"; white light can be separated into red, orange, yellow, green, blue, indigo, and violet. There are many types of spectra, both emission and absorption, for radiation and particles, used in spectroscopy. An incandescent body gives rise to a continuous spectrum where the dispersed radiation is distributed uninterruptedly over a range of wavelengths. An element gives a line spectrum—one or more bright discrete lines at characteristic wavelengths. Molecular gases give band spectra in which there are groups of close-packed lines. In an absorption spectrum dark lines or spaces replace the characteristic bright lines of the absorbing medium. The mass spectrum of an element is obtained from a mass spectrometer and shows the relative proportions of its constituent isotopes.
  speed of light
speed at which light and other electromagnetic waves travel through empty space. Its value is 299,792,458 m/186,281 mi per second. The speed of light is the highest speed possible, according to the theory of relativity, and its value is independent of the motion of its source and of the observer. It is impossible to accelerate any material body to this speed because it would require an infinite amount of energy.
  speed of sound
speed at which sound travels through a medium, such as air or water. In air at a temperature of 0°C/32°F, the speed of sound is 331 m/1,087 ft per second. At higher temperatures, the speed of sound is greater; at 18°C/64°F it is 342 m/1,123 ft per second. It is greater in liquids and solids; for example, in water it is around 1,440 m/4,724 ft per second, depending on the temperature.
intrinsic angular momentum of a subatomic particle, nucleus, atom, or molecule, which continues to exist even when the particle comes to rest. A particle in a specific energy state has a particular spin, just as it has a particular electric charge and mass. According to c0016-01.gifquantum theory, this is restricted to discrete and indivisible values, specified by a spin quantum number. Because of its spin, a charged particle acts as a small magnet and is affected by magnetic fields.
cooling of a liquid below its freezing point without freezing taking place; or the cooling of a saturated solution without crystallization taking place, to form a supersaturated solution. In both cases supercooling is possible because of the lack of solid particles around which crystals can form. Crystallization rapidly follows the introduction of a small crystal (seed) or agitation of the supercooled solution.
  surface tension
property that causes the surface of a liquid to behave as if it were covered with a weak elastic skin; this is why a needle can float on water. It is caused by the exposed surface's tendency to contract to the smallest possible area because of cohesive forces between c0016-01.gifmolecules at the surface. Allied phenomena include the formation of droplets, the concave profile of a meniscus, and the capillary action by which water soaks into a sponge.
degree or intensity of heat of an object and the condition that determines whether it will transfer heat to another object or receive heat from it, according to the laws of thermodynamics. The temperature of an object is a measure of the average kinetic energy possessed by the atoms or molecules of which it is composed. The SI unit of temperature is the kelvin (symbol K) used with the Kelvin scale. Other measures of temperature in common use are the Fahrenheit scale and the Celsius scale.
  thermal conductivity
ability of a substance to conduct heat. Good thermal conductors, like good electrical conductors, are




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  generally materials with many free electrons (such as metals). Thermal conductivity is expressed in units of joules per second per meter per kelvin (J s–1 m–1 K–1). For a block of material of cross-sectional area a and length l, with temperatures T1 and T2 at its end faces, the thermal conductivity l equals Hl/at(T2—T1), where H is the amount of heat transferred in time t.  
device that converts one form of energy into another. For example, a thermistor is a transducer that converts heat into an electrical voltage, and an electric motor is a transducer that converts an electrical voltage into mechanical energy. Transducers are important components in many types of sensor, converting the physical quantity to be measured into a proportional voltage signal.
device in which, by electromagnetic induction, an alternating current (A.C.) of one voltage is transformed to another voltage, without change of frequency. Transformers are widely used in electrical apparatus of all kinds, and in particular in power transmission where high voltages and low currents are utilized.
solid-state electronic component, made of semiconductor material, with three or more electrodes, that can regulate a current passing through it. A transistor can act as an amplifier, oscillator, photocell, or switch, and usually operates on a very small amount of power.
branch of physics dealing with the theory and application of ultrasound: sound waves occurring at frequencies too high to be heard by the human ear (that is, above about 20 kHz).
  uncertainty principle, or indeterminacy principle
in quantum mechanics, the principle that it is impossible to know with unlimited accuracy the position and momentum of a particle. The principle arises because in order to locate a particle exactly, an observer must bounce light (in the form of a photon) off the particle, which must alter its position in an unpredictable way. It was established by the German physicist Werner Heisenberg, and gave a theoretical limit to the precision with which a particle's momentum and position can be measured simultaneously: the more accurately the one is determined, the more uncertainty there is in the other.
in general, a region completely empty of matter; in physics, any enclosure in which the gas pressure is considerably less than atmospheric pressure (101,325 pascals).
  vapor density
density of a gas, expressed as the mass of a given volume of the gas divided by the mass of an equal volume of a reference gas (such as hydrogen or air) at the same temperature and pressure. It is equal approximately to half the relative molecular weight (mass) of the gas.
  vapor pressure
pressure of a vapor given off by (evaporated from) a liquid or solid, caused by vibrating atoms or molecules continuously escaping from its surface. In an enclosed space, a maximum value is reached when the number of particles leaving the surface is in equilibrium with those returning to it; this is known as the saturated vapor pressure or equilibrium vapor pressure.
resistance of a fluid to flow, caused by its internal friction, which makes it resist flowing past a solid surface or other layers of the fluid. It applies to the motion of an object moving through a fluid as well as the motion of a fluid passing by an object.
SI unit (symbol V) of electromotive force or electric potential. A small battery has a potential of 1.5 volts, while a hightension transmission line may carry up to 765,000 volts. The domestic electricity supply in the U.K. is 230 volts (lowered from 240 volts in 1995); it is 110 volts in the United States.
commonly used term for c0016-01.gifpotential difference (pd) or c0016-01.gifelectromotive force (emf).
SI unit (symbol W) of power (the rate of expenditure or consumption of energy) defined as one joule per second. A light bulb, for example, may use 40, 60, 100, or 150 watts of power; an electric heater will use several kilowatts (thousands of watts). The watt is named for the Scottish engineer James Watt (1736–1819).
SI unit (symbol Wb) of magnetic flux (the magnetic field strength multiplied by the area through which the field passes). It is named for German chemist Wilhelm Weber. One weber equals 10
8 maxwells.
force exerted on an object by gravity. The weight of an object depends on its mass—the amount of material in it—and the strength of the earth's gravitational pull, which decreases with height. Consequently, an object weighs less at the top of a mountain than at sea level. On the surface of the moon, an object has only one-sixth of its weight on earth, because the moon's surface gravity is one-sixth that of the earth. If the mass of a body is m kilograms and the gravitational field strength is g newtons per kilogram, its weight W in newtons is given by W = mg.
  W particle
elementary particle, one of the weakons responsible for transmitting the weak nuclear force.
  Z particle
elementary particle, one of the weakons responsible for carrying the weak nuclear force.
  Further Reading  
  Baeyer, Hans Taming the Atom: The Emergence of the Visible Microworld (1994)  
  Bailey, George The Making of Andrei Sakharov (1989)  
  Balfour, Mark The Sign of the Serpent: the Key to Creative Physics (1990)  
  Bernstein, Jeremy Einstein (1973)  
  Berry, A. J. Henry Cavendish: His Life and Scientific Work (1960)  
  Bowe, Frank Comeback (1981)  
  Brown, Andrew The Neutron and the Bomb: A Biography of Sir James Chadwick (1997)  
  Bunge, Mario, and Shea, William (eds.) Rutherford and Physics at the Turn of the Century (1979)  




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  Burt, Philip Barnes Quantum Mechanics and Nonlinear Waves (1981)  
  Calder, Nigel Einstein's Universe (1979)  
  Carrigan, Richard, and Trower, Peter (eds.) Particles and Forces: At the Heart of the Matter (1990)  
  Cassidy, D. C. Uncertainty (1992)  
  Chadwick, J. (ed.) The Collected Papers of Lord Rutherford of Nelson (1962–65)  
  Christianson, G. E. In the Presence of the Creator: Isaac Newton and His Times (1984)  
  Close, Frank The Cosmic Onion: Quarks and the Nature of the Universe (1983)  
  Close, Frank; Marten, Michael; and Sutton, Christine The Particle Explosion (1987)  
  Coleman, James A. Relativity for the Layman (1969)  
  Cox, P. A. Introduction to Quantum Theory and Atomic Structure (1996)  
  Croll, J. G. A. Force Systems and Equilibrium (1974)  
  Davies, D. A. Waves, Atoms, and Solids (1978)  
  Davies, P. C. W. Superforce: the Search for a Grand Unified Theory of Nature (1995)  
  Davies, Paul The Forces of Nature (1986)  
  Draganic, Ivan G. Radiation and Radioactivity on earth and Beyond (second edition, 1993)  
  Drake, Ellen Tan Restless Genius: Robert Hooke and his earthly Thoughts (1996)  
  Drake, J. Electrochemistry and Clean Energy (1994)  
  Einstein, Albert, and Infeld, L. The Evolution of Physics (1938)  
  Everitt, C. W. F. James Clerk Maxwell: Physicist and Natural Philosopher (1975)  
  Faraday, Michael Experimental Researches in Electricity (1839–55)  
  Feather, N. Lord Rutherford (1973)  
  Fermi, Laura Atoms in the Family: My Life with Enrico Fermi (1954)  
  Feynman, Richard Surely You're Joking, Mr. Feynman! (memoirs) (1985)  
  Fogden, Edward Energy (1990)  
  Fölsing, Alexander Albert Einstein (1997)  
  Fritsch, Harald Quarks: The Stuff of Matter (1984)  
  Giroud, F. Marie Curie: A Life (trs 1986)  
  Gjertson, Derek The Newton Handbook (1987)  
  Gleick, James Genius (1992), The Life and Science of Richard Feynman (1992)  
  Goddard, Peter (ed.) Paul Dirac: the Man and His Work (1998)  
  Goldstein, M., and Goldstein, I. F. The Refrigerator and the Universe (1993)  
  Goodchild, P. J. Robert Oppenheimer: Shatterer of Worlds (1985)  
  Gooding, David, and James, Frank (eds.) Faraday Rediscovered (1986)  
  Goodstein, David L. States of Matter (1975)  
  Gribbin, John Schrödinger's Kittens (1995), Q is for Quantum: the A–Z of Particle Physics (1998)  
  Harbison, James P., and Nahory, Robert E. Lasers: Harnessing the Atom's Light (1998)  
  Hesse, Mary B. Forces and Fields: the Concept of Action at a Distance in the History of Physics (1970)  
  Hoffmann, Banesh Albert Einstein: Creator and Rebel (1973)  
  Jammer, Max The Conceptual Development of Quantum Mechanics (1966)  
  Kane, Gordon The Particle Garden (1994)  
  Lederman, Leon, and Schramm, David From Quarks to the Cosmos: Tools of Discovery (1989)  
  MacDonald, D. K. C. Faraday, Maxwell and Kelvin (1965)  
  Maxwell, James Clerk Matter and Motion (1996)  
  Mendelssohn, K. The Quest for Absolute Zero: Meaning of Low Temperature Physics (1977)  
  Michelmore, Peter Einstein: Profile of the Man (1963)  
  Milgrom, Lionel R. The Colours of Life (1998)  
  Moore, Ruth E. Niels Bohr: The Man and the Scientist (1967)  
  Pais, Abraham "Subtle is the Lord": The Life and Science of Albert Einstein (1982), Niels Bohr's Times (1991)  
  Peierls, R. E. The Laws of Nature (1955)  
  Planck, Max Scientific Autobiography and Other Papers (trs 1949)  
  Quinn, Susan Marie Curie: A Life (1995)  
  Rabi, I., and others Oppenheimer (1969)  
  Rayleigh, R. J. The Life of Sir J. J. Thomson (1942)  
  Reid, Robert Marie Curie (1974)  
  Rhodes, R. The Making of the Atomic Bomb (1986)  
  Rogers, E. M. Physics for the Inquiring Mind (1960)  
  Romer, Alfred Restless Atom (1983)  
  Rozental, S. (ed.) Niels Bohr: His Life and Work (1967)  
  Schwartz, Joseph, and McGuinness, Michael Einstein for Beginners (1993)  
  Segrè, Emilio Enrico Fermi, Physicist (1970), From Falling Bodies to Radio Waves (1984)  
  Sharlin, H. and T. Lord Kelvin: The Dynamic Victorian (1978)  
  Smith, A. K., and Weiner, C. (eds.) Robert Oppenheimer: Letters and Recollections (1981)  
  Smith, C., and Wise, M. N. Energy and Empire (1989)  
  Stayer, Marcia Newton's Dream (1988)  




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  Stehle, P. Order, Chaos, Order (1994)  
  Sykes, Christopher (ed.) No Ordinary Genius (1994)  
  Tabor, David Gases, Liquids, and Solids: and Other States of Matter (third edition, 1991)  
  Thompson, S. P. The Life of Lord Kelvin (1977)  
  Thomson, George J. J. Thomson and the Cavendish Laboratory of His Day (1965)  
  Thorning, William Harris Motion and Forces (1974)  
  Tolstoy, Ivan James Clerk Maxwell (1982)  
  Walton, Alan John Three Phases of Matter (second edition, 1985)  
  Weinberg, Steven The Discovery of Subatomic Particles (1983), Dreams of a Final Theory (1993)  
  Weissbluth, Mitchel Atoms and Molecules (1978)  
  Westfall, R. The Life of Isaac Newton (1993)  
  White, Michael Stephen Hawking: a Life in Science (1992), Isaac Newton: The Last Sorcerer (1998)  
  Williams, D. J. Force, Matter, and Energy (1974)  
  Williams, Leslie Michael Faraday: A Biography (1965)  
  Wilson, David Rutherford: Simple Genius (1983)  
  Zohar, Danah The Quantum Self (1990)  
  Zohar, Danah, and Marshall, lan The Quantum Society (1994)