# WJEC A-level Physics definitions to learn

> The 14 definitions and laws WJEC A-level Physics asks you to state, written out with equations and units, each linked to practice.

Canonical: https://revise-gray.vercel.app/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).

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.kinematics-dynamics&checkpoint=wjec-alevel-physics.kinematics-dynamics.sp-04)

### 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.kinematics-dynamics&checkpoint=wjec-alevel-physics.kinematics-dynamics.sp-05)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.materials&checkpoint=wjec-alevel-physics.materials.sp-01)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.waves&checkpoint=wjec-alevel-physics.waves.sp-04)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.momentum&checkpoint=wjec-alevel-physics.momentum.sp-01)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.circular-shm&checkpoint=wjec-alevel-physics.circular-shm.sp-01)

### 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.circular-shm&checkpoint=wjec-alevel-physics.circular-shm.sp-03)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields&checkpoint=wjec-alevel-physics.fields.sp-01)

### 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields&checkpoint=wjec-alevel-physics.fields.sp-02)

### 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields&checkpoint=wjec-alevel-physics.fields.sp-03)

### 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields&checkpoint=wjec-alevel-physics.fields.sp-07)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.thermal&checkpoint=wjec-alevel-physics.thermal.sp-01)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.nuclear&checkpoint=wjec-alevel-physics.nuclear.sp-03)

## 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.

[Learn and practise this in Revise](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.capacitance&checkpoint=wjec-alevel-physics.capacitance.sp-01)

## 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)
- [Data booklet](https://revise-gray.vercel.app/wjec-a-level-physics/data-booklet)
- [What Revise can and cannot do yet](https://revise-gray.vercel.app/welcome)