WJEC A-level Physics data booklet
Every WJEC A-level Physics paper comes with a data booklet. Here are the physical constants and the key relationships by topic, so you know what to look up and what to learn.
This is not WJEC's booklet
CODATA 2018 values from NIST. Your real paper comes with WJEC's own Data Booklet, which may round these differently: check it before the exam. Get it from WJEC's AS and A level Physics page.
Physical constants
| Constant | Symbol | Value | Unit |
|---|---|---|---|
| Speed of light in a vacuum | c | 2.99792458 × 10⁸ | m s⁻¹ |
| Elementary charge | e | 1.602176634 × 10⁻¹⁹ | C |
| Planck constant | h | 6.62607015 × 10⁻³⁴ | J s |
| Boltzmann constant | k | 1.380649 × 10⁻²³ | J K⁻¹ |
| Avogadro constant | NA | 6.02214076 × 10²³ | mol⁻¹ |
| Molar gas constant | R | 8.314462618 | J K⁻¹ mol⁻¹ |
| Gravitational constant | G | 6.67430 × 10⁻¹¹ | N m² kg⁻² |
| Permittivity of free space | ε₀ | 8.8541878128 × 10⁻¹² | F m⁻¹ |
| Permeability of free space | μ₀ | 1.25663706212 × 10⁻⁶ | H m⁻¹ |
| Stefan constant | σ | 5.670374419 × 10⁻⁸ | W m⁻² K⁻⁴ |
| Wien constant | W | 2.897771955 × 10⁻³ | m K |
| Electron mass | mₑ | 9.1093837015 × 10⁻³¹ | kg |
| Proton mass | mₚ | 1.67262192369 × 10⁻²⁷ | kg |
| Neutron mass | mₙ | 1.67492749804 × 10⁻²⁷ | kg |
| Unified atomic mass unit | u | 1.66053906660 × 10⁻²⁷ | kg |
Key relationships by topic
Revise's own summaries of each topic's main equations, with what they mean. Learn what each symbol stands for: the booklet will not tell you which equation a question needs.
Motion, forces and Newton's laws
- Newton's second law in full: force equals rate of change of momentum, F = Δp/Δt, reducing to F = ma at constant mass.
- Revise this topic
Energy, work and power
- W = Fs cosθ — only the component of force along the displacement does work.
- Efficiency = useful output ÷ total input, always ≤ 1.
- P = Fv for a constant force at constant velocity.
- Revise this topic
Solids under stress
- Stress = F/A, strain = Δl/l, Young modulus E = stress/strain.
- Elastic strain energy = ½Fx = area under a force–extension graph.
- Revise this topic
Waves, superposition and optics
- v = fλ, and the frequency of a wave is unchanged when it refracts.
- Two-source interference: dλ = ax/D for Young's slits; nλ = d sinθ for a grating.
- Total internal reflection requires light in the denser medium and an angle beyond the critical angle, sin C = 1/n.
- Revise this topic
Photons and the photoelectric effect
- E = hf = hc/λ, and hf = φ + ½mv²max.
- de Broglie: λ = h/p, confirmed by electron diffraction.
- Revise this topic
Electric circuits
- ε = I(R + r): the terminal p.d. falls as current rises because of lost volts Ir.
- Resistivity ρ = RA/L — a material property, unlike resistance.
- Revise this topic
Momentum and collisions
- Impulse = FΔt = Δp = the area under a force–time graph.
- Revise this topic
Circular motion and simple harmonic motion
- a = v²/r = ω²r, directed toward the centre; the centripetal force is provided by a real force, not an extra one.
- SHM requires a = −ω²x: acceleration proportional to displacement and directed toward equilibrium.
- T = 2π√(l/g) for a simple pendulum; T = 2π√(m/k) for a mass–spring system.
- Revise this topic
Gravitational, electric and magnetic fields
- F = BIL sinθ for a current-carrying conductor; F = BQv for a moving charge, giving circular motion.
- Faraday: induced EMF = rate of change of flux linkage; Lenz: the induced effect opposes the change causing it.
- Revise this topic
Thermal physics and gases
- Q = mcΔT for a temperature change; Q = mL for a change of state at constant temperature.
- pV = nRT = NkT, with temperature always in kelvin.
- Kinetic theory: ½m⟨c²⟩ = (3/2)kT, so absolute temperature is a measure of mean molecular kinetic energy.
- Revise this topic
Nuclear and particle physics
- Activity A = λN and N = N₀e^(−λt), with half-life t½ = ln2/λ.
- ΔE = Δmc²; binding energy per nucleon peaks near iron-56, which is why both fission and fusion can release energy.
- Revise this topic
Capacitance and RC circuits
- Capacitance is charge stored per unit potential difference, C = Q/V; a larger plate area or smaller separation increases capacitance for a parallel-plate capacitor.
- The stored energy is E = ½QV = ½CV² = Q²/(2C), so raising voltage has a squared effect on energy.
- With constant R and C, capacitor charge and voltage rise towards their final values during charging, reaching about 63% after τ = RC; charging current falls to about 37%. During discharge, charge, voltage and current magnitude all fall to about 37% after τ.
- Revise this topic
Alternating currents and AC circuits
- For a sinusoidal supply, Vᵣₘₛ = V₀/√2 and Iᵣₘₛ = I₀/√2; rms values give the equivalent DC heating effect.
- Revise this topic
Orbits and the wider universe
- For a circular orbit gravity supplies the centripetal force, so GMm/r² = mv²/r and v = √(GM/r); a larger orbit has a lower orbital speed.
- Kepler's third law follows from the same model: T² = 4π²r³/(GM), so period increases strongly with orbital radius and a synchronous orbit has a chosen period matching the rotation of the body.
- Redshift of distant galaxies and the Hubble relationship v = H₀d provide evidence for expansion, but distance calibration, peculiar velocities and uncertainty must be considered.
- Revise this topic
Electromagnetic induction and transformers
- Magnetic flux through a coil is Φ = BA cos θ, and induced emf is proportional to the rate of change of flux linkage NΦ; no change means no induced emf.
- For an ideal transformer Vp/Vs = Np/Ns and power is conserved; step-up transmission reduces current and therefore reduces I²R loss in cables, while real transformers have heating and flux losses.
- Revise this topic
What a data booklet will not give you
Definitions and laws you are asked to state in words. Learn the WJEC Physics definitions.