WJEC A-level Physics definitions
WJEC A-level Physics asks you to define or state these 14 terms and laws in words. A data booklet gives you equations, not these.
Before you learn these
These explanations are written by Revise and are not yet teacher-reviewed. Marks go to the wording in WJEC's specification and mark schemes, so check each one against them.
Motion, forces and Newton's laws
State Newton first, second (as rate of change of momentum) and third laws and draw free-body diagrams
First law: an object stays at rest or moves at constant velocity unless a resultant force acts on it.
Second law: the resultant force equals the rate of change of momentum, F = Δ(mv)/Δt. When the mass is constant this becomes F = ma.
Third law: if body A exerts a force on body B, then B exerts an equal and opposite force on A. The two forces are the same type and act on different bodies, so they never cancel each other.
A free-body diagram shows only the forces acting on one object, each drawn from that object with its direction and label (weight, normal contact, tension, drag, friction).
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Define displacement velocity acceleration and distinguish average from instantaneous values
Displacement is the distance moved in a stated direction (a vector). Velocity is the rate of change of displacement. Acceleration is the rate of change of velocity.
Average velocity = total displacement ÷ total time. Average speed = total distance ÷ total time. They can differ greatly: one lap of a track gives zero average velocity.
Instantaneous velocity is the velocity at one moment: the gradient of the tangent to the displacement–time graph at that time.
A speedometer shows instantaneous speed; a journey time from a timetable gives an average.
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Solids under stress
Define stress, strain and the Young modulus and relate them for an elastic material
Stress σ = force ÷ cross-sectional area = F/A (unit Pa). Strain ε = extension ÷ original length = x/L (no unit).
The Young modulus E = stress ÷ strain = FL/(Ax) (unit Pa). It is a property of the material, not of one sample: a thicker or longer wire of the same steel has the same E.
E applies in the region where stress is proportional to strain. A large E means a stiff material.
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Waves, superposition and optics
State conditions for total internal reflection and apply the critical-angle relation
Total internal reflection happens only when light travels from a higher to a lower refractive index and the angle of incidence exceeds the critical angle.
At the critical angle c the refracted ray runs along the boundary (refraction angle 90°). From Snell's law: sin c = n₂/n₁.
Optical fibres use a core with a slightly higher refractive index than the cladding, so light hitting the boundary at more than c is totally internally reflected along the fibre.
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Momentum and collisions
Define linear momentum and impulse as force multiplied by time
Linear momentum p = mv. It is a vector (kg m s⁻¹), so its direction matters.
Impulse = force × time = FΔt = change in momentum Δp. Its unit, N s, is the same as kg m s⁻¹.
For a bounce, the change in momentum includes the reversal of direction: from +15 to −10 m s⁻¹ is a change of 25 m s⁻¹, not 5.
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Circular motion and simple harmonic motion
Define angular velocity and relate linear and angular speed by v equals omega r
Angular velocity ω is the angle swept per second, in rad s⁻¹: ω = Δθ/Δt = 2π/T = 2πf.
Every point on a rotating object has the same ω, but the linear speed depends on the radius: v = ωr.
One revolution is 2π radians; convert rev min⁻¹ to rad s⁻¹ by × 2π ÷ 60.
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State the defining condition for simple harmonic motion a equals minus omega squared x
Simple harmonic motion is defined by: the acceleration is proportional to the displacement from equilibrium and always directed towards the equilibrium position.
As an equation: a = −ω²x. The minus sign shows the acceleration opposes the displacement.
The period of SHM does not depend on the amplitude (it is isochronous), which is why pendulums keep time.
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Gravitational, electric and magnetic fields
State Newton law of gravitation and Coulomb law and compare their forms
Newton's law of gravitation: F = Gm₁m₂/r². Coulomb's law: F = Q₁Q₂/(4πε₀r²).
Both are inverse-square laws for point masses or charges (or spheres, measured from their centres): doubling r quarters the force.
Differences: gravity is always attractive and acts on mass; the electric force can attract or repel and acts on charge. The electric force between two protons is about 10³⁶ times the gravitational force.
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Define gravitational and electric field strength as force per unit mass or charge
Gravitational field strength g = force per unit mass = F/m (N kg⁻¹). Electric field strength E = force per unit positive charge = F/Q (N C⁻¹).
Field lines show the direction of the force on a test mass or positive test charge; their spacing shows the strength.
Between parallel plates the electric field is uniform: E = V/d.
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Define electric and gravitational potential and relate field strength to potential gradient
Potential at a point is the work done per unit mass (gravitational) or per unit positive charge (electric) in bringing it from infinity to that point.
Gravitational potential V = −GM/r is always negative (zero at infinity, attractive force). Electric potential V = Q/(4πε₀r) has the sign of Q.
Field strength is the negative potential gradient: g = −ΔV/Δr, E = −ΔV/Δx. Equipotentials are always perpendicular to field lines; no work is done moving along one.
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State Faraday law and Lenz law for electromagnetic induction
Magnetic flux Φ = BA (Wb) for a field perpendicular to an area A. Flux linkage = NΦ for a coil of N turns.
Faraday's law: the induced emf is equal to the rate of change of magnetic flux linkage: ε = −Δ(NΦ)/Δt.
Lenz's law: the direction of the induced current is such that it opposes the change that produces it. This is energy conservation: you must do work to keep the change going.
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Thermal physics and gases
Define internal energy as sum of random kinetic and potential energies of particles
Internal energy is the sum of the randomly distributed kinetic and potential energies of all the particles in a body.
Heating raises the kinetic part, which we detect as a rise in temperature. During a change of state, the energy supplied increases the potential part (breaking bonds), so the temperature stays constant.
Internal energy can be increased by heating or by doing work on the system. WJEC writes the first law as ΔU = Q − W, where Q is the heat supplied to the system and W is the work done by it, so work done on the system makes W negative.
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Nuclear and particle physics
Define activity decay constant half-life and use A equals lambda N and N equals N0 exp minus lambda t
Activity A is the number of decays per second (becquerel, Bq). The decay constant λ is the probability per second that a nucleus decays: A = λN.
Decay is random and exponential: N = N₀e^(−λt), and likewise A = A₀e^(−λt).
Half-life t½ is the time for N (or A) to halve: t½ = ln 2 / λ. After n half-lives, a fraction (½)ⁿ remains.
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Capacitance and RC circuits
Define capacitance and explain factors affecting a parallel-plate capacitor
Capacitance is the charge stored per unit potential difference: C = Q/V. Unit: farad (F = C V⁻¹). Practical values are µF, nF or pF.
For a parallel-plate capacitor: C = ε₀εᵣA/d. Capacitance increases with plate area A and the permittivity of the dielectric, and decreases with plate separation d.
A dielectric increases C because its molecules polarise, reducing the field and p.d. between the plates for the same charge.