This page lists every key definition in the Cambridge IGCSE Physics 0625 syllabus. Use it for revision and self-testing.
Topic 1: Motion, Forces and Energy
Speed: the distance travelled per unit time.
Velocity: the speed of an object in a given direction; displacement per unit time.
Acceleration: the change in velocity per unit time.
Mass: a measure of the quantity of matter in an object.
Weight: the gravitational force acting on an object; W = mg.
Density: the mass per unit volume of a substance.
Force: a push or pull that can change the motion, shape or direction of an object.
Resultant force: the single force that has the same effect as all the forces acting on an object combined.
Moment: the turning effect of a force; moment = force x perpendicular distance from the pivot.
Centre of gravity: the point at which the entire weight of an object may be considered to act.
Momentum: the product of mass and velocity of an object; p = mv.
Impulse: the change in momentum of an object; impulse = force x time.
Work: the energy transferred when a force moves an object through a distance in the direction of the force; W = Fd.
Power: the rate of doing work or the rate of energy transfer; P = W/t.
Kinetic energy: the energy an object has because of its motion; KE = 0.5mv^2.
Gravitational potential energy: the energy an object has because of its position in a gravitational field; GPE = mgh.
Efficiency: the ratio of useful energy output to total energy input, expressed as a percentage.
Pressure: the force acting per unit area; P = F/A.
Topic 2: Thermal Physics
Internal energy: the total kinetic energy and potential energy of the particles in a substance.
Temperature: a measure of the average kinetic energy of the particles in a substance.
Specific heat capacity: the energy required to raise the temperature of 1 kg of a substance by 1 degree C (or 1 K).
Specific latent heat of fusion: the energy required to change 1 kg of a substance from solid to liquid at constant temperature.
Specific latent heat of vaporisation: the energy required to change 1 kg of a substance from liquid to gas at constant temperature.
Thermal conduction: the transfer of thermal energy through a material by the vibration of particles, without the particles moving from their mean positions.
Convection: the transfer of thermal energy through a fluid by the movement of the fluid itself.
Thermal radiation: the transfer of energy by electromagnetic waves (infrared), which does not require a medium.
Topic 3: Waves
Wave: a disturbance that transfers energy from one place to another without transferring matter.
Wavelength: the minimum distance between two points on a wave that are in phase (e.g. crest to crest).
Frequency: the number of complete waves passing a point per unit time.
Amplitude: the maximum displacement of a point on the wave from its rest position.
Period: the time for one complete wave to pass a point.
Transverse wave: a wave in which the oscillations are perpendicular to the direction of energy transfer.
Longitudinal wave: a wave in which the oscillations are parallel to the direction of energy transfer.
Refraction: the change in direction of a wave as it passes from one medium to another, caused by a change in speed.
Diffraction: the spreading of a wave as it passes through a gap or around an obstacle.
Critical angle: the angle of incidence in the denser medium at which the angle of refraction is 90 degrees.
Total internal reflection: the complete reflection of light at the boundary between two media when the angle of incidence exceeds the critical angle.
Refractive index: the ratio of the speed of light in a vacuum to the speed of light in the medium; also sin(i)/sin(r).
Topic 4: Electricity and Magnetism
Electric current: the rate of flow of electric charge; I = Q/t.
Potential difference (voltage): the energy transferred per unit charge between two points; V = W/Q.
Resistance: the opposition to current flow; the ratio of potential difference to current; R = V/I.
Electromotive force (e.m.f.): the energy transferred per unit charge by a source of electrical energy.
Power (electrical): the rate at which electrical energy is transferred; P = IV.
Magnetic field: a region where a magnetic force acts on a magnetic material or a current-carrying conductor.
Electromagnetic induction: the generation of an e.m.f. (and hence a current in a closed circuit) when a conductor moves through a magnetic field or when the magnetic field through a conductor changes.
Transformer: a device that uses electromagnetic induction to change the voltage of an alternating current supply.
Topic 5: Nuclear Physics
Atom: the smallest particle of an element that retains the chemical properties of that element.
Nucleus: the dense central part of an atom, containing protons and neutrons.
Isotope: atoms of the same element that have the same number of protons but different numbers of neutrons.
Radioactive decay: the spontaneous and random emission of radiation from an unstable nucleus.
Half-life: the time taken for half of the radioactive nuclei in a sample to decay, or the time for the activity to halve.
Alpha particle: a particle consisting of 2 protons and 2 neutrons (a helium-4 nucleus), emitted during alpha decay.
Beta particle: a high-speed electron emitted from the nucleus when a neutron changes into a proton.
Gamma ray: a high-frequency electromagnetic wave emitted from the nucleus.
Nuclear fission: the splitting of a large nucleus into two smaller nuclei, with the release of energy and neutrons.
Nuclear fusion: the joining of two small nuclei to form a larger nucleus, with the release of energy.
Topic 6: Space Physics
Galaxy: a large collection of stars, gas and dust held together by gravity.
Red-shift: the increase in wavelength (and decrease in frequency) of light from galaxies moving away from us.
Hubble’s Law: the recessional velocity of a galaxy is proportional to its distance from us.
Big Bang theory: the theory that the universe began as a very hot, dense point approximately 13.8 billion years ago and has been expanding ever since.
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