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LearnCambridge50542026–2028Paper 2 · TheoryElectricity and magnetism

Paper 2 · Module

Electricity and magnetism

8 subtopics≈ 240 min in total

Start the module: Magnetism and magnetic fields Open graphing calculator

Magnetism and magnetic fields
Magnets have a north and a south pole: like poles repel, unlike poles attract. Magnets attract magnetic materials (iron, steel, nickel, cobalt) by inducing magnetism in them. The space around a magnet where a force acts is its magnetic field, shown by field lines from N to S. Soft iron makes temporary magnets; steel makes permanent ones.
≈ 25 min
Charge, current and potential difference
Electric current is the rate of flow of charge, $I=\dfrac Qt$ , measured with an ammeter in series. Potential difference (and e.m.f.) is the energy transferred per unit charge, $V=\dfrac WQ$ , measured with a voltmeter in parallel. Rubbing insulators transfers electrons, giving them static charge; like charges repel and unlike charges attract.
≈ 30 min
Resistance
Resistance tells you how hard it is for current to flow: $R=\dfrac VI$ , in ohms (Ω). A fixed resistor at constant temperature has a straight-line $I$ – $V$ graph; a filament lamp curves because its resistance rises as it heats; a diode lets current through in one direction only. Longer, thinner wires have more resistance.
≈ 30 min
Series and parallel circuits
In series, the current is the same everywhere, p.d.s add up to the supply, and resistances add. In parallel, each branch gets the full p.d., the branch currents add up to the total, and the combined resistance is less than any single branch. A potential divider shares the supply p.d. in the ratio of its resistances.
≈ 35 min
Electrical power, energy and safety
Electrical power is $P=IV$ , and the energy transferred is $E=IVt$ . Electricity companies charge per kilowatt-hour: the energy used by a $1$ kW appliance in $1$ hour. Fuses and circuit breakers cut off the current if it gets too big; earthing a metal case makes a fault blow the fuse instead of electrocuting you.
≈ 30 min
Electromagnetic induction and generators
Move a wire across a magnetic field, or change the field through a coil, and an e.m.f. is induced. It's bigger with faster movement, a stronger magnet and more turns, and it reverses if the motion reverses. A generator spins a coil in a magnetic field to produce alternating current, whose trace you can read on an oscilloscope.
≈ 30 min
Electromagnets and motors
A current produces a magnetic field: circles round a straight wire, and a bar-magnet pattern from a solenoid. Wind a solenoid round soft iron and you have an electromagnet. A current-carrying wire in a magnetic field feels a force, and in a d.c. motor that force turns a coil, with a split-ring commutator reversing the current every half turn.
≈ 30 min
Transformers
A transformer has two coils wound on a soft iron core. An alternating current in the primary coil makes a changing magnetic field that induces an alternating e.m.f. in the secondary. The voltages are in the same ratio as the turns: $\dfrac{V_p}{V_s}=\dfrac{N_p}{N_s}$ . Power stations step the voltage up for transmission so the current, and the energy wasted heating the cables, is small.
≈ 30 min