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Physics definitions, A to Z.

Use these concise definitions to recall the physics, then practise applying them in questions. Course coverage and accepted wording vary: check your current syllabus and the mark scheme for the question you are answering.

Explore topic notes: IGCSE revision · A-Level revision

From Dr Desouky Physics Academy · About the teacher. Definitions are revision explanations, not an official exam-board glossary. Cambridge's published marking guidance requires scientific meaning in context and allows appropriate alternative wording.

A

Absolute zeroIGCSE
The lowest possible temperature, 0 K (−273.15 °C), at which particles have minimum internal energy.
AccelerationIGCSE
The change in velocity per unit time. A vector, measured in m/s².
ActivityA2
The number of nuclear decays per unit time in a radioactive sample, measured in becquerels (Bq).
AmplitudeIGCSE
The maximum displacement of a wave or oscillation from its rest (equilibrium) position.

B

Binding energyA2
The energy required to separate a nucleus into its individual protons and neutrons.
Boyle's lawIGCSE
For a fixed amount of gas at constant temperature, the pressure is inversely proportional to the volume: p₁V₁ = p₂V₂.
Brownian motionIGCSE
The random movement of microscopic particles caused by collisions with fast-moving, invisible molecules — evidence for the particle model of matter.

C

CapacitanceA2
The charge stored per unit potential difference: C = Q ÷ V, measured in farads (F).
Centre of gravityIGCSE
The point through which the entire weight of an object appears to act.
Centripetal forceA2
The resultant force acting towards the centre of a circle, required to keep a body in circular motion.
CoherenceAS
Two wave sources are coherent when they maintain a constant phase difference, which requires the same frequency.
Conduction (thermal)IGCSE
The transfer of thermal energy through a material by particle vibrations passed between neighbours — and in metals, mainly by free electrons.
ConvectionIGCSE
The transfer of thermal energy in a fluid: heated fluid expands, becomes less dense and rises, while cooler denser fluid sinks to replace it.
CoupleAS
A pair of equal, antiparallel forces with different lines of action, producing rotation only with zero resultant force.
Critical angleIGCSE
The angle of incidence in the medium of higher refractive index for which the angle of refraction is 90°. Total internal reflection occurs above this angle when light travels towards a medium of lower refractive index.
Current (electric)IGCSE
The charge passing a point per unit time: I = Q ÷ t, measured in amperes.

D

de Broglie wavelengthAS
The wavelength associated with a moving particle: λ = h ÷ p, where p is the particle's momentum.
Decay constantA2
The probability per unit time that a given nucleus will decay; relates activity to nuclei present via A = λN.
DensityIGCSE
The mass per unit volume of a substance: ρ = m ÷ V.
DiffractionIGCSE
The spreading of a wave as it passes through a gap or around an obstacle, greatest when the gap is comparable to the wavelength.
DisplacementAS
The change in position from the starting point to the finishing point, measured along a straight line with direction. A vector measured in metres.

E

EfficiencyIGCSE
The ratio of useful energy (or power) output to total energy (or power) input, expressed as a fraction or percentage.
Elastic collisionAS
A collision in which both momentum and total kinetic energy are conserved.
Elastic deformationAS
Deformation in which the object returns to its original shape and size when the deforming force is removed.
Electric fieldIGCSE
A region in which an electric charge experiences a force. Field lines show the direction of the force on a positive charge.
Electric field strengthA2
The force per unit positive charge at a point: E = F ÷ Q, measured in N/C or V/m.
Electric potentialA2
The work done per unit positive charge in bringing a small test charge from infinity to the point.
Electromagnetic inductionIGCSE
The production of an e.m.f. across a conductor when it cuts magnetic field lines or when the magnetic flux through a circuit changes.
Electromotive force (e.m.f.)IGCSE
The energy transferred to each unit of charge by a source; the terminal p.d. when no current flows.
EquilibriumIGCSE
The state in which both the resultant force and the resultant moment on a body are zero.
EvaporationIGCSE
The escape of the most energetic particles from the surface of a liquid at any temperature below boiling, cooling the remaining liquid.

F

Faraday's lawA2
The magnitude of the induced e.m.f. equals the rate of change of magnetic flux linkage; its direction opposes the change, as described by Lenz's law.
FrequencyIGCSE
The number of complete waves or oscillations passing a point per second, measured in hertz (Hz).

G

Gravitational field strengthIGCSE
The gravitational force acting per unit mass at a point: g = F ÷ m, measured in N/kg.
Gravitational potentialA2
The work done per unit mass in bringing a small test mass from infinity to the point: φ = −GM ÷ r. Always negative.

H

Half-lifeIGCSE
The time taken for half the nuclei of a radioactive isotope in a sample to decay, or for the activity to halve.
Hooke's lawIGCSE
The extension of a spring is directly proportional to the load applied, provided the limit of proportionality is not exceeded.

I

ImpulseIGCSE
The product of force and the time for which it acts, equal to the change in momentum it produces: Ft = Δp.
Internal energyA2
The sum of the random kinetic and potential energies of all the molecules in a system.
Internal resistanceAS
The resistance within a source of e.m.f. that causes the terminal p.d. to fall below the e.m.f. when current flows.
IsotopesIGCSE
Atoms of the same element with the same number of protons but different numbers of neutrons.

K

Kinetic energyIGCSE
The energy an object has due to its motion: Ek = ½mv².
Kirchhoff's first lawAS
The sum of currents entering a junction equals the sum of currents leaving it — conservation of charge.
Kirchhoff's second lawAS
Around any closed circuit loop, the sum of the e.m.f.s equals the sum of the potential differences — conservation of energy.

L

Latent heatIGCSE
The energy required to change the state of a substance without a change in temperature.
Lenz's lawA2
The direction of an induced e.m.f. is such as to oppose the change causing it — a consequence of conservation of energy.
Longitudinal waveIGCSE
A wave in which the vibrations are parallel to the direction of energy transfer, e.g. sound.

M

Magnetic fluxA2
The product of magnetic flux density and the area at right angles to the field: Φ = BA, measured in webers (Wb).
Magnetic flux densityA2
The force per unit current per unit length on a conductor at right angles to a magnetic field, measured in tesla (T).
MassIGCSE
A measure of the quantity of matter in an object, measured in kilograms and unchanged by location.
Moment of a forceIGCSE
The turning effect of a force: moment = force × perpendicular distance from the pivot to the line of action of the force.
MomentumIGCSE
The product of an object's mass and velocity: p = mv. A vector, measured in kg m/s.

N

Newton's first lawAS
An object remains at rest or moves at constant velocity unless acted on by a resultant force.
Newton's second lawAS
The resultant force on an object equals the rate of change of its momentum: F = Δp ÷ Δt.
Newton's third lawAS
When body A exerts a force on body B, body B exerts a force on body A of equal magnitude, opposite direction, and the same type.
NodeAS
A point on a stationary wave where the amplitude is always zero.

P

PhotonAS
A discrete packet (quantum) of electromagnetic energy: E = hf.
Potential differenceIGCSE
The work done (energy transferred) per unit charge passing between two points: V = W ÷ Q.
PowerIGCSE
The rate of doing work or transferring energy: P = W ÷ t, measured in watts (W).
PressureIGCSE
The force acting per unit area at right angles to a surface: p = F ÷ A, measured in pascals (Pa).
Principle of conservation of energyIGCSE
Energy cannot be created or destroyed; it is only transferred between stores. The total energy of a closed system is constant.
Principle of conservation of momentumIGCSE
The total momentum of interacting bodies is constant, provided the resultant external force on the system is zero.
Principle of momentsIGCSE
For a body in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point.
Principle of superpositionAS
When two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements.

R

Radiation (thermal)IGCSE
The transfer of energy by infrared electromagnetic waves; the only transfer mechanism that requires no medium.
RadianA2
The angle subtended at the centre of a circle by an arc equal in length to the radius; 2π radians = 360°.
Radioactive decayIGCSE
The spontaneous and random emission of radiation from an unstable nucleus, unaffected by external conditions.
RefractionIGCSE
The change in direction of a wave when it crosses a boundary between media, caused by a change in wave speed.
Refractive indexIGCSE
The ratio n = sin i ÷ sin r for light crossing a boundary; equal to the ratio of the wave speeds in the two media.
ResistanceIGCSE
The opposition of a component to electric current: R = V ÷ I, measured in ohms (Ω).
ResistivityAS
A material property defined by ρ = RA ÷ L, measured in ohm-metres (Ω m).
ResonanceA2
The condition in which a system oscillates with maximum amplitude, occurring when the driving frequency equals the system's natural frequency.
Resultant forceIGCSE
The single force that has the same effect as all the forces acting on an object combined.

S

ScalarIGCSE
A quantity with magnitude only, e.g. speed, mass, energy, time.
Simple harmonic motionA2
Motion in which the acceleration is proportional to the displacement from a fixed point and always directed towards that point: a = −ω²x.
Specific heat capacityIGCSE
The energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K).
SpeedIGCSE
The distance travelled per unit time. A scalar, measured in m/s.
Stationary waveAS
A wave pattern formed by the superposition of two progressive waves of equal frequency and amplitude travelling in opposite directions, storing energy rather than transferring it.
StrainAS
The extension per unit original length: ε = x ÷ L. It has no unit.
StressAS
The force applied per unit cross-sectional area: σ = F ÷ A, measured in pascals.

T

Terminal velocityIGCSE
The constant velocity reached when the resultant force on a moving object becomes zero. For a falling object, upward drag and any upthrust balance its weight.
Threshold frequencyAS
The minimum frequency of electromagnetic radiation that will eject electrons from a given metal surface.
Time constantA2
For a capacitor-resistor circuit, τ = RC: the time for the charge to fall to 1/e (about 37%) of its initial value.
Transverse waveIGCSE
A wave in which the vibrations are perpendicular to the direction of energy transfer, e.g. light and water waves.

U

UltrasoundIGCSE
Sound with a frequency above 20,000 Hz, beyond the upper limit of human hearing.

V

VectorIGCSE
A quantity with both magnitude and direction, e.g. velocity, force, momentum, displacement.
VelocityIGCSE
The rate of change of displacement; speed in a given direction.

W

WeightIGCSE
The gravitational force acting on an object: W = mg, measured in newtons.
Work doneIGCSE
The energy transferred when a force moves an object: W = Fs cos θ, where θ is the angle between the force and the displacement.
Work functionAS
The minimum energy required to remove an electron from the surface of a metal.

Y

Young modulusAS
The ratio of stress to strain for a material within the region of proportionality: E = σ ÷ ε.

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