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How is a voltage induced by a changing magnetic field, and how do generators and transformers work?

Electromagnetic induction, the AC generator, the structure and action of a transformer, and the transformer equation.

A CCEA GCSE Physics answer on electromagnetic induction, how an AC generator produces a voltage, the structure and action of a transformer, and how to use the transformer equation.

Generated by Claude Opus 4.811 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

CCEA wants you to describe electromagnetic induction, explain how an AC generator produces a voltage, describe the structure and action of a transformer, and use the transformer equation. Induction is the reverse of the motor effect.

The answer

Electromagnetic induction

This is the reverse of the motor effect: there, a current in a field gives a force; here, movement in a field gives a voltage.

The AC generator

Transformers

The transformer equation

Transformers are vital in the National Grid: step-up transformers raise the voltage for transmission (reducing energy lost as heat in the cables), and step-down transformers lower it again for safe use in homes.

Worked example: a step-up transformer

Examples in context

Example 1. A wind turbine. The blades turn a generator coil in a magnetic field, inducing an alternating voltage. The faster the wind spins the blades, the larger the induced voltage and the more electricity is generated.

Example 2. A phone charger transformer. A step-down transformer (combined with other parts) reduces the high mains voltage to the low voltage the phone needs, using the turns ratio to set the output.

Try this

Q1. State two ways to increase an induced voltage. [2 marks]

  • Cue. Move the magnet faster; use a stronger magnet (or more turns).

Q2. Why must a transformer use alternating current? [1 mark]

  • Cue. It needs a changing magnetic field to induce a voltage in the secondary.

Q3. A transformer has 100100 primary and 300300 secondary turns. Is it step-up or step-down? [1 mark]

  • Cue. Step-up (more turns on the secondary).

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA style4 marksA transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The primary voltage is 230 V. Calculate the secondary voltage and state whether it is a step-up or step-down transformer.
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Use the transformer equation:

VpVs=NpNs⇒Vs=Vp×NsNp=230×50200=57.5 V.\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s} \Rightarrow V_s = V_p \times \dfrac{N_s}{N_p} = 230 \times \dfrac{50}{200} = 57.5\ \text{V}.

The secondary voltage (57.5 V) is lower than the primary, so it is a step-down transformer.

Markers reward the transformer equation, the value 57.5 V, and identifying it as step-down.

CCEA style3 marksState two ways to increase the size of the voltage induced when a magnet is moved into a coil of wire.
Show worked answer →

Two ways: move the magnet faster (increase the rate at which the field changes); and use a stronger magnet.

(Using more turns on the coil also increases the induced voltage.)

Markers reward any two of: move the magnet faster; stronger magnet; more turns on the coil.

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