# Physics GCSE Dictionary

 Acceleration Change in velocity per second (in metres per second per second, m/s2). Alpha radiation The process in which a large unstable nucleus becomes stable by emitting alpha particles, each composed of two protons and two neutrons, emitted by unstable nucleus. Amplitude The magnitude of the change of the oscillation in a wave. Background radioactivity Radioactivity from substances around us, such as rocks. Beta radiation A process in which a neutron-rich nucleus becomes stable as a result of a neutron changing into a proton, creating and emitting a beta particle (i.e. an electron). Big Bang theory The theory that the Universe was created in a giant explosion (the Big Bang) and that the Universe has been expanding ever since. Centre of mass The point where an object’s mass may be considered to be concentrated. Centripetal force The resultant force towards the centre of a circle that act on an object moving in a circular path. Conduction Heat transfer in a substance due to movement of particles in the substance. Convection Heat transfer in a liquid or gas due to convection currents, the flow of a fluid due to differences in temperature. E.g. circulation of the upper part of the Earth’s mantle. Converging lens A lens that makes light rays parallel to the principal axis converge to a point; also referred to as a convex lens. Coulomb The unit of electrical charge (C), equal to the charge passing a point in a (direct current) circuit in 1 second when the current is 1A. Diverging lens A lens that makes light rays parallel to the axis diverge (i.e. spread out) as if coming from a single point; also referred to as a concave lens. Doppler effect The change in wavelength (and frequency) of the waves from a moving source due to the motion of the source. Efficiency This is defined as: useful energy output / total energy input. Efficiency is never 100% due to energy losses (e.g. heat loss due to friction). Elastic potential energy The energy stored in an elastic object when work is done to change its shape. Electric current The rate of flow of electric charge (amperes, A), equals 1 C/s. Electric potential energy The energy of a charged object due to its charge (in joules, J). Electrical energy Energy transferred by the movement of charge. 1 kilowatt hour (kW h) is the electrical energy supplied to a 1 kW electrical device in 1 hour. Electromagnetic induction effect in which a potential difference is generated in a wire or coil when the wire or coil cuts across the lines of a magnetic field, also known as the dynamo effect. Electromagnetic waves Electric and magnetic disturbances that transfer energy from one place to another. The spectrum of electromagnetic waves in order of increasing wavelength: gamma and X-rays, ultraviolet radiation, visible light, infra-red radiation, microwaves, radio waves. Energy Ways in which energy is stored or transferred, including chemical energy, elastic (or strain energy), electrical energy, gravitational potential energy, kinetic energy, thermal energy. The conservation of energy states that energy cannot be created or destroyed, it can only be transformed or transferred. Force Force can change the motion of objects (in newtons, N). Frequency The number of complete waves passing a point each second (in hertz, Hz). Friction force A force that opposes the relative motion of two surfaces where they are in contact with each other. Gamma radiation Electromagnetic radiation that is emitted from unstable nuclei in radioactive substances. Gravitational field strength The force of gravity on an object of mass 1 kg (in newtons per kilogram, N/kg). Has a value of 9.8 N/kg near the Earth. Gravitational potential energy Energy of an object due to its position in a gravitational field. Near the Earth’s surface, change of g.p.e. (in joules, J) = (in newtons, N) x v distance moved (in metres, m). Half-life of a radioactive isotope Time taken for the number of nuclei of the isotope (or mass of the isotope) in a sample of radioactive material to halve. Kinetic energy Energy of a moving object due to its motion. Kinetic energy = ½mv2. Moment The turning effect of a force. Moment of a force (in newton metres) = force (in newtons) x perpendicular distance from the pivot to the line of action of the force (in metres). Momentum Mass (in kilograms, kg) x velocity (in m/s). The conservation of momentum states that momentum conserved in any collision or explosion provided no external forces act on the objects that collide or explode. Motor effect When a current is passed through a wire in a magnetic field and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field. Nuclear fission The process in which certain nuclei (uranium 235 and plutonium 239) split into two smaller fragments when struck by a neutron, releasing energy and two or three neutrons in the event. Nuclear fusion The process in which small nuclei are forced together so they fuse with each other to form a larger nucleus, releasing energy in the event. Ohm’s law The current through a resistor at a constant temperature is directly proportional to the potential difference across the resistor. Oscilloscope A device that is used to display the shape of an electrical wave. Period The time taken for a satellite to orbit the Earth once, or the time for one wave. Potential difference The difference in electric potential energy per unit charge between two charged objects (in volts, V). Power The energy transformed per second (in watts, W). Reflection of light When light rays are reflected from a surface, the angle of incidence is equal to the angle of reflection. Refraction The change of direction and wavelength of a light ray when it passes across a boundary between two transparent substances (including air). Renewable energy Energy from sources that never run out, including wind energy, wave energy, tidal energy, solar energy, hydroelectricity and geothermal energy. Resistance Resistance (in ohms, Ω) = potential difference (in volts, V) / current (in amperes, A). Resonance When waves build up in an object causing the amplitude of the wave to increase. Speed Distance travelled per second (in metres/second, m/s). Velocity is speed in a given direction. Static electricity Charge contained in an insulator or an insulated conductor. Transformer Electrical device used to change an alternating voltage. A step-up transformer used to step the voltage up, e.g. from a power station to the grid voltage. A step-down transformer used to step the voltage down, e.g. from the grid voltage to the mains voltage used in homes and offices. Wavelength The distance from one wave peak to the next wave peak along the wave. Work Energy transferred by a force. Work done (in joules, J) = force (in newtons, N) x distance moved in the direction of the force (in metres, m).