Revision notes · Magnetism and electromagnetism

Induced potential, transformers and the National Grid (physics only) (HT only)

Induced potential4.7.3.1

Definition: Electromagnetic induction: a potential difference is induced across a conductor whenever there is relative movement between the conductor and a magnetic field — equivalently, whenever the conductor experiences a changing magnetic field.

If the conductor is part of a complete circuit, this induced potential difference drives a current around the circuit. That induced current itself creates its own magnetic field — and this induced field always acts to oppose the change that produced it in the first place.

Uses of the generator effect4.7.3.2

The generator effect is electromagnetic induction used deliberately to generate electricity: a coil of wire is made to rotate within a magnetic field (or a magnet is rotated near a fixed coil), for example by a turbine. As the coil turns, it continually cuts through the magnetic field, inducing a changing potential difference — and a current, if the coil is part of a complete circuit.

Generator type	How current is taken from the rotating coil	Output
Alternator	Slip rings and brushes maintain contact with the coil as it rotates freely	Alternating current (AC) — the induced pd smoothly reverses direction each half-turn as the coil's orientation to the field reverses
Dynamo	A split-ring commutator reverses the connections to the external circuit every half-turn	Direct current (DC) — the output pulses, but stays positive throughout

Microphones4.7.3.3

A dynamic microphone uses the generator effect to convert sound into an electrical signal. It has a fixed permanent magnet with a coil of wire positioned in its magnetic field, free to move.

Incoming sound waves are pressure variations in the air, which push the coil backward and forward. As the coil moves through the magnet's field, its changing position induces a current in the coil — the size and pattern of the induced current varies in exactly the same way as the original sound wave, giving an electrical signal that represents the sound, which can be amplified and sent on to a loudspeaker.

Transformers4.7.3.4

A transformer has a primary (input) coil and a secondary (output) coil, both wound around the same iron core, but not otherwise electrically connected.

1An alternating current (AC) in the primary coil produces a continuously changing magnetic field in the iron core.
2This changing field passes through the secondary coil, inducing an alternating potential difference — and so an alternating current — in it.
3Because a transformer relies on a changing magnetic field, it only works with AC: a constant direct current (DC) in the primary would produce a constant magnetic field, which wouldn't induce anything in the secondary coil.

Equation	Units
turns on primary ÷ turns on secondary = pd across primary ÷ pd across secondary — Nₚ ÷ Nₛ = Vₚ ÷ Vₛ	N = number of turns (no units), V in volts (V)

Transformer

Transformer type	Coil turns	Effect on voltage
Step-up transformer	More turns on the secondary coil than the primary	Increases the voltage
Step-down transformer	Fewer turns on the secondary coil than the primary	Decreases the voltage

⚠️ Common mistake: The equation relating primary/secondary current (Vₚ × Iₚ = Vₛ × Iₛ) only holds if the transformer is 100% efficient — assume it isn't unless a question explicitly says so, and stick to the turns/voltage equation instead unless told otherwise.

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