# Physics A-Level Dictionary

 Absolute zero The lowest possible temperature at which a gas would have a zero volume. Absorption spectrum The spectrum of electromagnetic radiation absorbed by matter when radiation of all frequencies is passed through it. Activity The number of decays per second of a radioactive substance. Antinode The point of maximum displacement of a stationary wave. Binding energy The negative of the amount of energy needed to separate a nucleus into individual nucleons. Boson The carrier particle that mediates the weak nuclear force. Capacitance The ratio of charge stored per increase in potential difference. Capacitor An electrical device used to store charge and energy in the electrical field. Circular motion Motion with constant radius of curvature caused by a centripetal force and acceleration that is perpendicular to velocity. Coherent waves Waves that have a constant phase difference. De Broglie wavelength Length of de Broglie wave of a particle, equal to Planck’s constant divided by the momentum of the particle. Elastic collision Interaction between two objects in which kinetic energy is conserved. Electric field strength The ratio of force exerted by an electric field on a test charge to the magnitude of the charge. Emission spectrum The spectrum produced by radiation from excited atoms. Energy levels The discrete amounts of energy that an electron in an atom may have. Entropy A measure of disorder in a system. Excited state An energy level in an atom that is higher than the ground state. Farad The unit of capacitance. One farad equals one coulomb per volt. First law of thermodynamics The change in internal energy is equal to heat added and work done on system. Fundamental particles The smallest particles (quarks and leptons) of which all materials are composed. Gluon The carrier particle of the strong nuclear force. Graviton The particle that carries the gravitational force. Not yet observed. Heat engine A device that converts thermal energy to mechanical energy. Heisenberg uncertainty principle The more accurately the position of a particle is measured, the less accurately the momentum can be known, and vice versa. Impulse The product of force and time interval over which it acts. Equals the change in momentum of an object. Inelastic collision Collision in which kinetic energy is not conserved. Laser A device that produces coherent light by stimulated emission of radiation. Law of universal gravitation The gravitational force between two objects depends directly on the product of their masses and inversely on the square of their separation. Lenz’s law The magnetic field generated by an induced current opposes the change in field that caused the current. Lepton A “light” particle that does not interact with the strong nuclear force. Includes the electron and the neutrino. Linear accelerator A device that accelerates subatomic particles by applying successive electric field. Neutrino Chargeless, massless, subatomic particle emitted in beta decay. A type of lepton. Photoelectric effect The ejection of electrons from surface of metal exposed to electromagnetic radiation. Photon A quantum of electromagnetic waves and a mediator of the electromagnetic interaction.. Photovoltaic cell A device that converts electromagnetic radiation into electrical energy. Planck’s constant The ratio of energy of photon to its frequency. A fundamental constant. Principle of superposition When two waves interact, the resulting amplitude is equal to the sum of the individual amplitudes of the two waves. Quark The basic building blocks of protons, neutrons, other baryons, and mesons. Resistivity A measure of how strongly a material opposes the flow of electric current. Second law of thermodynamics Heat always flows from a region of high temperature to a region of lower temperature. Simple harmonic motion Motion caused by linear restoring force that is proportional to the square of the displacement, and that has a period independent of the amplitude of motion. Superconductor An electrical conductor that has no resistance and low temperatures. Strangeness A property of strange quarks. Strangeness is always conserved in strong interactions. Strong nuclear force Force of very short range that holds neutrons and protons in nucleus together. Mediated by gluons. Synchrotron A device to accelerate particles in which particles move in circular path. Uses magnetic fields. Transistor A semiconductor device that controls large current by means of small voltage changes. Wave-particle duality The principle that matter can exhibit both particle and wave-like properties. Weak nuclear force The force involved in beta decay of the neutron and atomic nuclei. Work function Energy needed to remove an electron from metal. Mediated by bosons. Young’s modulus A constant of proportionality associated with the change in length of a material according to its elastic properties.