# WJEC A-level Physics data booklet: constants and equations > The 15 physical constants and the key equations by topic for WJEC A-level Physics, so you know what the data booklet gives you and what to learn. Canonical: https://revise-gray.vercel.app/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](https://www.wjec.co.uk/qualifications/physics-as-a-level/). ## 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.kinematics-dynamics) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.energy-power) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.materials) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.waves) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.quantum) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.electric-circuits) ### Momentum and collisions - Impulse = FΔt = Δp = the area under a force–time graph. - [Revise this topic](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.momentum) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.circular-shm) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.fields) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.thermal) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.nuclear) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.capacitance) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.alternating-currents) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.orbits-universe) ### 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](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.electromagnetic-induction) ## What a data booklet will not give you Definitions and laws you are asked to state in words. [Learn the WJEC Physics definitions](https://revise-gray.vercel.app/wjec-a-level-physics/definitions). ## Related - [WJEC A-level Physics](https://revise-gray.vercel.app/wjec-a-level-physics) - [Past paper questions by topic](https://revise-gray.vercel.app/wjec-a-level-physics/past-papers) - [Definitions](https://revise-gray.vercel.app/wjec-a-level-physics/definitions) - [What Revise can and cannot do yet](https://revise-gray.vercel.app/welcome)