Physics Encyclopedia

Physics Encyclopedia

If you would like to prepare for school subjects or simply increase your general knowledge, then enjoy our physics encyclopedia. We tried to focus only on very important terms and definitions. We also kept our terminology very brief so that you absorb the concept more quickly and easily.

Physics Glossary (Page 1)


AB (ATOBARN): 10-18 barns. A unit used to measure cross-section. The inverse of this unit is used to measure integrated luminosity.
ABSCISSA: The value corresponding to the horizontal distance of a point on a graph from the Y axis. The X coordinate.
ABSOLUTE DEVIATION: The difference between a single measured value and the average of several measurements made in the same way.
ABSOLUTE ERROR: The actual difference between a measured value and its accepted value.
ABSOLUTE ZERO: The lowest theoretical temperature a material can have, where the molecules that make up the material have no kinetic energy. Absolute zero is reached at 0 K or -273º C.
ABSORPTION SPECTRUM: A continuous spectrum interrupted by dark lines or bands that are characteristic of the medium through which the radiation has passed.
ACCELERATION: The rate of change of velocity with respect to time.
ACCELERATOR: Accelerators are ring-shaped or linear devices that accelerate charged particles. More powerful than any other microscope, high-energy accelerators allow physicists to study matter at the smallest scale human beings have ever seen, exposing the quarks inside a proton. At the same time, high-energy accelerators can produce collisions that recreate the conditions of the early universe though in a much smaller volume. Creating tiny fireballs of high density and high temperature, physicists produce the particles that were abundant in the early universe, a trillionth of a second after the Big Bang.
ACCEPTOR: An element with three valence electrons per atom which when added to a semiconductor crystal provides electron "holes" in the lattice structure of the crystal.
ACCURACY: Closeness of a measurement to the accepted value for a specific physical quantity; expressed in terms of error.
ACTIVITY: In radioactive substances, the number of nuclei that decay per second. Activity, A, will be larger in large samples of radioactive material, since there will be more nuclei.
ADHESION: The force of attraction between unlike molecules.
ADIABATIC PROCESS: A thermal process in which no heat is added to or removed from a system.
AGS (ALTERNATING GRADIENT SYNCHROTRON): An accelerator based at Brookhaven National Laboratory (BNL). AGS.html.
ALPHA DECAY: A form of radioactive decay where a heavy element emits an alpha particle and some energy, thus transforming into a lighter, more stable, element.
ALPHA PARTICLE: A helium-4 nucleus, especially when emitted from the nucleus of a radioactive atom.
ALTERNATING CURRENT: An electric current that has one direction during one part of a generating cycle and the opposite direction during the remainder of the cycle.
AMANDA: (ANTARTIC MUON AND NEUTRINO DETECTOR ARRAY) A Cherenkov detector, embedding in the ice located at the south pole, designed to look at very high energy neutrinos. Amanda.html.
AMMETER: An electric meter designed to measure current.
AMPERE: The unit of electric current; one coulomb per second.
AMPÈRE'S LAW: Relates a circulating magnetic field to an electric current passing through a loop.
AMPLIFIER: A device consisting of one or more vacuum tubes (or transistors) and associated circuits, used to increase the strength of a signal.
AMPLITUDE: In reference to oscillation, amplitude is the maximum displacement of the oscillator from its equilibrium position. Amplitude tells how far an oscillator is swinging back and forth. In periodic motion, amplitude is the maximum displacement in each cycle of a system in periodic motion. The precise definition of amplitude depends on the particular situation: in the case of a stretched string it would be measured in meters, whereas for sound waves it would be measured in units of pressure.
ANGLE OF INCIDENCE: When a light ray strikes a surface, the angle of incidence is the angle between the incident ray and the normal.
ANGLE OF REFLECTION: The angle between a reflected ray and the normal.
ANGLE OF REFRACTION: The angle between a refracted ray and the line normal to the surface.
ANGSTROM: A unit of linear measure equal to 10-10 m.
ANGULAR ACCELERATION: The time rate of change of angular velocity.
ANGULAR FREQUENCY: A frequency, f, defined as the number of revolutions a rigid body makes in a given time interval. It is a scalar quantity commonly denoted in units of Hertz (Hz) or s-1.
ANGULAR IMPULSE: The product of a torque and the time interval during which it acts.
ANGULAR MOMENTUM: The product of the rotational inertia of a body and its angular velocity.
ANGULAR PERIOD: The time, T, required for a rigid body to complete one revolution.
ANGULAR VELOCITY: A vector quantity, , that reflects the change of angular displacement with time, and is typically given in units of rad/s. To find the direction of the angular velocity vector, take your right hand and curl your fingers along the particle or body's direction of rotation. Your thumb then points in the direction of the body's angular velocity.
ANODE: (1) The positive electrode of an electric cell. (2) The positive electrode or plate of an electronic tube. (3) "The electron-poor electrode.
ANTIMATTER: All particles of ordinary matter (electrons, protons, neutrons) have anti-matter partners that appear identical in all respects (e.g. Mass, spin) except that they have the opposite electric charge. We believe that in the Big Bang equal quantities of matter and antimatter were created. The fact that the universe now contains matter and not anti-matter is known as the matter-anti-matter asymmetry. Understanding how this asymmetry was produced is a major goal in particle physics and astrophysics.
ANTINODE: The points midway between nodes on a standing wave, where the oscillations are largest.
ANTIPARTICLE: The generic term for an anti-matter partner of a particle.
ANTIPROTON: The anti-matter partner of the proton.
ANTIQUARK: The anti-matter partner of the quark.
APERTURE: Any opening through which radiation may pass. The diameter of an opening that admits light to a lens or.
APPARENT POWER: The product of the effective values of alternating voltage and current.
ARC TANGENT: The inverse function to the tangent. Symbol: arctan or tan-l. Interpretation: "An angle whose tangent is.
ARMATURE: A coil of wire formed around an iron or steel core that rotates in the magnetic field of a generator or motor.
ATLAS (A TOROIDAL LHC APPARATUS): This multi-purpose experiment is currently under construction for use as a detector at the Large Hadron Collider (LHC) in Geneva Switzerland. Welcome.html.
ATOM: Smallest unit of a chemical element, the limit of classical physics on the small length scales.
ATOMIC MASS UNIT: One-twelfth of the mass of carbon-12, or 1.6605655 x 10-27 kg.
ATOMIC NUMBER: A number, Z, associated with the number of protons in the nucleus of an atom. Every element can be defined in s of its atomic number, since every atom of a given element has the same number of protons.
ATOMIC WEIGHT: The weighted average of the atomic masses of an element's isotopes based on their relative abundance.
AUDIO SIGNAL: The alternating voltage proportional to the sound pressure produced in an electric circuit.
AVERAGE VELOCITY: Total displacement divided by elapsed time.
AXIS OF ROTATION: The line that every particle in the rotating rigid body circles about.
B PHYSICS: The study of particles containing the bottom (b) quark. The b quark is the second heaviest quark, and is found only at particle accelerators. B-Mesons are ideal objects to study the tiny differences between matter and anti-matter.
BABAR: This experiment, which focuses on B-mesons, uses the PEP II storage ring at the Stanford Linear Accelerator Center (SLAC). One of its primary goals is to investigate why the universe appears to be made entirely from matter, and not from anti-matter. There is a competing experiment, BELLE, located at the KEK laboratory. .
BACK EMF: An induced emf in the armature of a motor that opposes the applied voltage.
BAND SPECTRUM: An emission spectrum consisting of fluted bands of color. The spectrum of a substance in the molecular state.
BARN: A unit of cross-section, a barn is equal to 10-28 m2.
BAROMETER: A device used to measure the pressure of the atmosphere.
BARYON: A subatomic particle with a large rest mass, e.g., the proton.
BARYONS: A hadron composed of three quarks. Examples include the protons and neutrons found in ordinary nuclei.
BASIC EQUATION: An equation that relates the unknown quantity with known quantities in a problem.
BASIC LAW OF ELECTROSTATICS: Similarly charged objects repel each other. Oppositely charged objects attract each other.
BEAM: Several parallel rays of light considered collectively.
BEAT: The interference effect resulting from the superposition of two waves of slightly different frequencies propagating in the same direction. The amplitude of the resultant wave varies with time.
BEATS: When two waves of slightly different frequencies interfere with one another, they produce a "beating" interference pattern that alternates between constructive (in-phase) and destructive (out-of-phase). In the case of sound waves, this sort of interference makes a "wa-wa-wa" sound, and the frequency of the beats is equal to the difference in the frequencies of the two interfering waves.
BECQUEREL: The rate of radioactivity equal to one disintegration per second.
BELLE: This experiment, which focuses on B-mesons, is located at the KEK laboratory in Japan. One of its primary goals is to investigate why the universe appears to be made entirely from matter, and not from anti-matter. There is a competing experiment, babar, located at the Stanford Linear Accelerator Center. .
BEPC: Beijing Electron Positron Collider.
BETA DECAY: A form of radioactive decay where a heavy element ejects a beta particle and a neutrino, becoming a lighter element in the process.
BETA PARTICLE: An electron emitted from the nucleus of a radioactive atom.
BETATRON: A device that accelerates electrons by means of the transformer principle .
BEVATRON: A high-energy synchrotron.
BINDING ENERGY: Energy that must be applied to a nucleus to break it up.
B-MESON: A meson containing a bottom (b) quark, and one lighter anti-quark. The b quark is the second heaviest quark, and is found only at particle accelerators. Only the top quark is heavier.
BNL (BROOKHAVEN NATIONAL LABORATORY): A national laboratory located in Long Island, New York. This multidisciplinary laboratory, operated by the Department of Energy, is the site of the AGS and RHIC (relativistic heavy-ion collider) accelerators. .
BOHR ATOMIC MODEL: A model for the atom developed in 1913 by Niels Bohr. According to this model, the electrons orbiting a nucleus can only orbit at certain particular radii. Excited electrons may jump to a more distant radii and then return to their ground state, emitting a photon in the process.
BOILING POINT: The temperature at which a material will change phase from liquid to gas or gas to liquid.
BOLTZMANN CONSTANT: A physical constant relating temperature to energy.
BOONE (BOOSTER NEUTRINO EXPERIMENT): A proposed experiment to be based at Fermilab. It is the expanded version of the Mini-boone experiment, which is currently under construction. .
BOSON: A subatomic particle with zero charge and rest mass, e.g., the photon.
BOYLE’S LAW: For a gas held at a constant temperature, pressure and volume are inversely proportional.
BOYLE'S LAW: The volume of a dry gas varies inversely with the pressure exerted upon it, provided the temperature is constant.
BRANCHING RATIO/FRACTION: When a particle decays, it often can decay in several ways. The likelihood of it decaying to a particular mode is known as its branching ratio for that decay mode.
BREEDER REACTOR: A nuclear reactor in which a fissionable material is produced at a greater rate than the fuel is consumed.
BROWNIAN MOVEMENT: The irregular and random movement of small particles suspended in a fluid, known to be a consequence of the thermal motion of fluid molecules.
BTEV: A proposed experiment to take place at the Fermilab Tevatron. It would study B-Meson decay in detail. Btev.html.
BUBBLE CHAMBER: Instrument used for making the paths of ionizing particles visible as a trail of tiny bubbles in a liquid.
CALORIE: The amount of heat needed to raise the temperature of one gram of water by one degree Celsius. 1 cal = 4.19 J.
CALORIMETER: A heat-measuring device consisting of nested metal cups separated by an air space.
CANDLE: The unit of luminous intensity of a light source.
CAPACITANCE: The ratio of the charge on either plate of a capacitor to the potential difference between the plates. Capacitive reactance. Reactance in an a-c circuit containing capacitance which causes a lagging voltage.
CAPACITOR: A combination of conducting plates separated by layers of a dielectric that is used to store an electric charge.
CAPILLARITY: The elevation or depression of liquids in small-diameter tubes.
CATHODE: (1) The negative electrode of an electric cell. (2) The electron-emitting electrode of an electronic tube. (3) The electron-rich electrode.
CATHODE RAYS: Particles emanating from a cathode; electrons.
CDF (COLLIDER DETECTOR AT FERMILAB): This is one of the two large multi-purpose experiments located at the Fermilab Tevatron. Along with D0, it discovered the top quark in 1995. .
CELSIUS: A scale for measuring temperature, defined such that water freezes at 0ºc and boils at 100ºc. 0ºc = 273 K.
CELSIUS SCALE: The temperature scale using the ice point as 0o and the steam point as 100o, with 100 equal divisions, or degrees, between; formerly the centigrade scale.
CENTER OF CURVATURE: With spherical mirrors, the center of the sphere of which the mirror is a part. All of the normals pass through it.
CENTER OF GRAVITY: The point at which all of the weight of a body can be considered to be concentrated.
CENTER OF MASS: Given the trajectory of an object or system, the center of mass is the point that has the same acceleration as the object or system as a whole would have if its mass were concentrated at that point. In terms of force, the center of mass is the point at which a given net force acting on a system will produce the same acceleration as if the system's mass were concentrated at that point.

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