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How is electricity generated by induction, and how do transformers change voltage?

Electromagnetic induction, the a.c. generator, the transformer, the turns ratio and transformer equation, and why transformers are used in the National Grid.

A focused answer to WJEC GCSE Physics topic 1.9 on electromagnetic induction, covering how a generator induces an alternating voltage, how a transformer changes voltage using the turns ratio, the transformer equation, and why transformers matter for efficient transmission.

Generated by Claude Opus 4.89 min answer

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

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  1. What this topic is asking
  2. Electromagnetic induction
  3. The a.c. generator
  4. Transformers
  5. The transformer equation
  6. Try this

What this topic is asking

WJEC wants you to explain electromagnetic induction, describe how an a.c. generator induces a voltage, and use the transformer to change voltage with the turns ratio and the transformer equation. This is part of topic 1.9 Electromagnetism in Unit 1 of WJEC GCSE Physics (3420).

Electromagnetic induction

The a.c. generator

This is the principle behind power-station generators, where turbines spin the coil.

Transformers

The transformer equation

Try this

Q1. State what must change to induce a voltage in a coil. [1 mark]

  • Cue. The magnetic field through the coil (or there must be relative movement of a magnet and the coil).

Q2. A transformer steps 230 V230\,\text{V} down to 11.5 V11.5\,\text{V} with 400400 primary turns. Find the secondary turns. [2 marks]

  • Cue. Ns=NpΓ—VsVp=400Γ—11.5230=20N_s = N_p \times \dfrac{V_s}{V_p} = 400 \times \dfrac{11.5}{230} = 20 turns.

Exam-style practice questions

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

WJEC 20193 marksA transformer has 200200 turns on the primary coil and 5050 turns on the secondary coil. The primary voltage is 230 V230\,\text{V}. Calculate the secondary voltage.
Show worked answer β†’

A topic 1.9 transformer calculation. Use VpVs=NpNs\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}. Rearrange for the secondary voltage: Vs=VpΓ—NsNp=230Γ—50200=57.5 VV_s = V_p \times \dfrac{N_s}{N_p} = 230 \times \dfrac{50}{200} = 57.5\,\text{V} (2 marks for substitution and answer). Markers reward the correct turns ratio and the answer with units; this is a step-down transformer because there are fewer secondary turns. A common error is to invert the ratio.

WJEC 20214 marksExplain how an a.c. generator produces an alternating voltage, and state two ways to increase the output.
Show worked answer β†’

A topic 1.9 Explain question. As the coil rotates in the magnetic field, the field lines through it change, inducing a voltage across the coil (electromagnetic induction) (1 mark). Each half-turn the coil cuts the field in the opposite sense, so the induced voltage reverses, giving an alternating output collected through slip rings (1 mark). The output is increased by spinning the coil faster, using more turns, or using a stronger magnet (2 marks for any two). Markers reward the rotation inducing a voltage, the reversal each half-turn, and two valid ways to increase the output.

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