WJEC A-Level Physics Unit 4 Fields and their Applications: a complete overview of capacitance, electrostatic and gravitational fields, orbits, magnetic fields and induction

What Unit 4 actually demands

Unit 4 Fields and their Applications draws the A2 year together around the idea of a field. It starts with the capacitor, then treats electrostatic and gravitational fields side by side to highlight their shared inverse-square structure, applies that to orbits and the wider universe, and finishes with magnetic fields and electromagnetic induction. The unit is calculation-heavy and rewards confident use of exponentials and the field equations.

This guide walks through the five topics of the unit in specification order, then sets out the exam patterns WJEC repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Capacitance

A capacitor stores charge, $Q = CV$, and energy $E = \frac{1}{2}QV = \frac{1}{2}CV^2$, the area under a charge-voltage graph. Charging and discharging through a resistor are exponential, governed by the time constant $\tau = RC$, with $Q = Q_0 e^{-t/RC}$ on discharge. Charging and discharging a capacitor is a specified practical.

Electrostatic and gravitational fields

WJEC treats the two inverse-square fields together. Coulomb's law is $F = \frac{Q_1 Q_2}{4\pi\varepsilon_0 r^2}$ and Newton's law of gravitation is $F = \frac{Gm_1 m_2}{r^2}$. Each has a field strength (force per unit charge or mass) and a potential (energy per unit charge or mass), with the parallels being a favourite exam theme.

Orbits and the wider universe

Applying gravity to orbits gives the link between orbital speed, radius and period, and Kepler's third law $T^2 \propto r^3$. WJEC also covers ideas from the wider universe such as the evidence for dark matter from rotation curves, and the use of fields to understand satellites and planetary motion.

Magnetic fields

A current in a magnetic field feels a force, $F = BIL$, the motor effect, with direction from Fleming's left-hand rule. A moving charge feels $F = Bqv$. WJEC covers magnetic flux density $B$, the force between conductors, and the operation of devices such as the motor.

Electromagnetic induction

A changing magnetic flux linkage induces an EMF. Faraday's law gives $\varepsilon = -N\frac{d\Phi}{dt}$ and Lenz's law (the minus sign) states the induced current opposes the change, conserving energy. This underpins generators and transformers.

How Unit 4 is examined

WJEC Unit 4 is an A2 written paper of about 1 hour 45 minutes carrying 80 marks. Questions follow the specification statements, around 40 per cent of marks are mathematical, and the specified practical on capacitor discharge recurs. Drill the field equations and the exponential discharge until both are automatic, and rehearse the gravitational-electric analogies.

The five topics, dot point by dot point

Each topic has a dot-point answer page with worked exam questions and cross-links. Browse them from this unit overview and the subject hub.

For the official specification

WJEC publishes the full specification, past papers and mark schemes at wjec.co.uk. Always revise from the current specification and WJEC's own past papers, because question style is board-specific.