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Learn4PH1Paper 1P · Paper 1 (core content)Magnetism and electromagnetismElectromagnetic induction and generators
Magnetism and electromagnetism · Subtopic 2 of 4

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.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 30 minRevision 6 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe how to induce an e.m.f. with a magnet and a coil or wire.
  • State the factors that affect the size and direction of the induced e.m.f.
  • Describe the a.c. generator and sketch its output.
  • Read p.d. and time period from an oscilloscope trace.
Pause and think

Push a magnet into a coil connected to a meter, and the needle flicks. Hold the magnet still inside the coil. What does the needle do now?

Section 2 of 14

Core idea

Start hereElectricity from a moving magnet

Push a magnet into a coil of wire connected to a meter: the needle flicks. Stop: nothing. Pull it out: the needle flicks the other way. It's not the magnet that makes electricity, it's the change.

The big idea

A changing magnetic field through a coil induces an e.m.f. Faster change → bigger e.m.f. The induced current opposes the change (Lenz). Generators use this.

1. Inducing a current

Pushing a magnet into a coil induces a current
Nmeter
Moving in: the needle flicks one way. Moving out: the other way. Not moving: no reading, even with the magnet inside.
  • In: current one way; out: the other; still: none.
  • Bigger e.m.f.: faster motion, stronger magnet, more turns.
Why the induced current opposes the change
  1. N pole pushed in⇒coil end becomes N\text{N pole pushed in}\Rightarrow\text{coil end becomes N}
    Repels the magnet: you must do work.
  2. work done→electrical energy\text{work done}\to\text{electrical energy}
    Energy is conserved.
If the coil attracted the magnet, energy would appear from nothing.

2. Generators

Output of an a.c. generator
24681012−2−1120timevoltageone rotation = one cycle
As the coil spins, the voltage rises, falls and reverses: alternating current. Spinning faster gives a bigger and more frequent output.
Worked exampleSpeeding up a generator

Spin twice as fast.

Goal
Effect on output.
Strategy
Faster change; more turns per second.

  1. Field lines are cut twice as fast: larger peak e.m.f.

  2. Twice as many turns per second: double the frequency.

AnswerHigher peak e.m.f. and double frequency.
✓ CheckPeak occurs when the coil plane is parallel to the field ✓.
Common misconceptionStationary magnet No movement means no change, so no e.m.f., however strong the magnet.

Summary

  • Change of field → e.m.f.
  • Opposes the change.
Key words
Electromagnetic induction
Producing an e.m.f. by a changing magnetic field.
HistoryFaraday's ring. On 2929 August 18311831, Michael Faraday wound two coils on an iron ring. When he switched on the current in one, a galvanometer on the other twitched, then fell back. He had discovered electromagnetic induction, the principle behind every power station.
Fun factYour phone charges by induction. A wireless charging pad has a coil carrying alternating current. Its changing magnetic field passes through a coil in your phone and induces a current that charges the battery, with no metal contacts at all.
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