All Definitions Needed For A Level

This post contains the key definitions you need for GCE A Level Physics (9478 H2, 2027) in the same order as the study portal.

Use it alongside:

• A Level Physics (9478) Portal (syllabus order + hubs)
• A Level Physics Formula List
• Paper 4 Practical Hub

1. Foundations of Physics

Quantities & Measurement

• A scalar has magnitude only.
• A vector has magnitude and direction.
• A random error causes scatter in readings; reduced by repeats and averaging.
• A systematic error causes a consistent shift from the true value (e.g. zero error); affects accuracy.
• Accuracy is closeness to the true value.
• Precision is closeness of repeated measurements to each other (small spread).
• Absolute uncertainty, Δ x, is the uncertainty written in the same unit as x.
• Percentage uncertainty is (; Delta x/x); times 100; %.
• Dimensional homogeneity: a valid physical equation has the same base units on both sides.

Kinematics

• Displacement, s, is the vector change in position.
• Velocity, v, is the rate of change of displacement: v = ds/dt.
• Acceleration, a, is the rate of change of velocity: a = dv/dt.

Forces & Dynamics

• Resultant force, Fₙₑₜ, is the single force equivalent to all forces acting.
• Momentum, p, is p = mv (vector).
• Impulse, J, is the change in momentum: J =; Delta p = Fₐᵥg; Delta t.
• Newton’s 2nd law (general form): Fₙₑₜ = dp/dt (direction of Fₙₑₜ).
• Conservation of momentum: total momentum of an isolated system remains constant (no external resultant force).
• A couple is two equal and opposite parallel forces whose lines of action do not coincide (causes rotation only).

Work, Energy & Power

• Work done, W (J), is energy transferred by a force through a displacement in the direction of the force.
• Power, P (W), is the rate of energy transfer (rate of doing work): P = dW/dt.
• The work–energy theorem: net work done equals the change in kinetic energy, Wₙₑₜ =; Delta Eₖ.
• Hooke’s law: F; propto x (within the limit of proportionality), often written F = kx.
• Elastic potential energy is energy stored due to deformation (for Hooke’s law, E =; tfrac12kx²).

2. Mechanics

Circular Motion

• Angular displacement, ; theta, is the angle swept (radians).
• Angular velocity, ; omega, is the rate of change of angular displacement: ; omega = d; theta/dt.
• Centripetal acceleration, ac, is acceleration towards the centre of the circular path: ac = v²/r =; omega² r.
• Centripetal force is the resultant force towards the centre that provides centripetal acceleration.
• Period, T, is time for one revolution; frequency, f, is revolutions per second; f = 1/T.

Gravitational Fields

• Gravitational field strength, g, is gravitational force per unit mass.
• Gravitational potential, ; phi, is work done per unit mass in bringing a small mass from infinity to that point.
• Gravitational potential energy, U, is related by U = m; phi.

Oscillations

• Simple harmonic motion (SHM) is motion where acceleration is proportional to displacement and directed towards the equilibrium position: a; propto -x.
• Amplitude, x₀, is the maximum displacement from equilibrium.
• Damping is the loss of energy from an oscillating system.
• Resonance is large-amplitude oscillation when a system is driven at (or near) its natural frequency.

3. Waves

Waves & Superposition

• A wave is a travelling disturbance that transfers energy without net transfer of matter.
• Wavelength, ; lambda, is the distance between adjacent points in phase.
• Frequency, f (Hz), is oscillations per second; period, T, is time for one oscillation.
• Wave speed, v, is v = f; lambda.
• Phase difference, ; Delta; phi, describes how far one oscillation is ahead/behind another (in radians).
• The principle of superposition: resultant displacement is the vector sum of individual displacements.
• Coherent sources have constant phase difference and same frequency.
• Interference is superposition giving reinforcement/cancellation (constructive/destructive).
• Diffraction is the spreading of waves when passing through a gap or past an obstacle (most significant when gap size is comparable to ; lambda).
• A stationary wave is formed by superposition of two waves of the same frequency travelling in opposite directions, producing nodes and antinodes.
• Intensity is power per unit area perpendicular to the direction of travel.

4. Thermal Physics

Thermal Physics

• Internal energy, U, is the sum of the microscopic kinetic and potential energies of the particles in a system.
• First law of thermodynamics (work done on system): ; Delta U = Q + W.
• An ideal gas obeys pV = nRT (equation of state).
• Specific latent heat, l (J kg⁻¹), is thermal energy needed per unit mass for a change of state at constant temperature.
• Specific heat capacity, c (J kg⁻¹ K⁻¹), is thermal energy needed per unit mass per unit temperature rise.

5. Electricity & Magnetism

Electric Fields

• Electric field strength, E, is force per unit positive charge: E = F/q.
• Electric potential, V, is work done per unit positive charge in bringing a small test charge from infinity to that point.
• Potential difference is work done per unit charge between two points.
• Capacitance, C, is charge stored per unit potential difference: C = Q/V.

Currents & Circuits

• Electric current, I, is rate of flow of charge: I = dQ/dt.
• Electromotive force (e.m.f.), ; varepsilon, is energy supplied by a source per unit charge.
• Resistance, R, is R = V/I.
• Resistivity, ; rho, is a material property defined by R =; rho L/A.
• Ohm’s law: V; propto I for an ohmic conductor at constant physical conditions (usually constant temperature).
• Root-mean-square (r.m.s.) value is the d.c.-equivalent value that gives the same average power in a resistor.

Electromagnetic Forces

• Magnetic flux density, B (T), is defined via the force on a current-carrying conductor: F = BIl; sin; theta.
• The force on a moving charge in a magnetic field is F = qvB; sin; theta (direction is perpendicular to v and B).

Electromagnetic Induction

• Magnetic flux, ; Phi, is ; Phi = BA; cos; theta (for uniform B over area A).
• Flux linkage is N; Phi.
• Faraday’s law: induced e.m.f. is proportional to the rate of change of flux linkage: ; varepsilon; propto -d(N; Phi)/dt.
• Lenz’s law: induced current is in a direction that opposes the change that produces it.

6. Modern Physics

Quantum Physics

• A photon is a quantum of electromagnetic radiation with energy E = hf.
• The photoelectric effect is emission of electrons from a metal surface when electromagnetic radiation above a threshold frequency is incident.
• The work function, ; Phi, is the minimum energy needed to remove an electron from the metal surface.
• Threshold frequency, f₀, is the minimum frequency needed for photoemission: hf₀ =; Phi.
• de Broglie wavelength: ; lambda = h/p.
• A wavefunction, ; psi, describes a quantum state; |; psi|² is a probability density.

Nuclear Physics

• Activity, A (Bq), is the number of decays per second.
• Decay constant, ; lambda, is the probability per unit time that a nucleus decays.
• Half-life, t₁/₂, is the time for activity (or number of undecayed nuclei) to fall to half.
• Mass defect is the difference between the mass of separated nucleons and the mass of the nucleus.
• Binding energy is the energy needed to separate a nucleus into its nucleons: E = (; Delta m)c².

7. Paper 4 (Practical) Definitions

Practical Hub

• Repeatable results: same method and equipment gives consistent results.
• Reproducible results: different people/methods/equipment give consistent results.
• Best-fit line: line/curve that represents the overall trend of the data.
• Gradient is ; Delta y/; Delta x (include units and interpret it as a physical constant when asked).
• Linearisation: rearranging to Y = mX + c to extract constants from gradient/intercept.

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