Explain why a transformer only works with alternating current (AC) in the primary coil and not with direct current (DC).

AQA GCSE Physics (8463) — 4.7.3 Induced potential, transformers and the National Grid (Phys only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. AC in the primary coil creates a continuously changing magnetic field.
2. The changing field passes through (cuts through) the secondary coil.
3. This induces a potential difference (and current) in the secondary coil.
4. DC gives a constant magnetic field, so no change occurs and no potential difference is induced in the secondary coil.

Examiner tips

  • Use the word ‘induce’ and mention ‘changing magnetic field’ to show understanding of Faraday’s law.
  • Show the causal chain: AC → changing field → cutting through secondary → induced EMF.
  • Remember to contrast with DC’s steady field to explain why no EMF is produced.
  • Keep the answer concise – 4 points are enough for full marks.

Common mistakes

  • Saying the transformer works with DC because it has a magnetic field – ignores the need for change. Using vague terms like ‘magnetic flux’ without explaining the change. Including unnecessary details such as core material or efficiency.

Mark scheme (4 marks)

  1. AC in the primary coil produces a continuously changing magnetic field
  2. This changing magnetic field passes through (cuts through) the secondary coil
  3. This induces a potential difference (and current) in the secondary coil
  4. DC produces a constant (steady) magnetic field, so no change occurs and no potential difference is induced in the secondary coil

Key terms in this question

alternating current · primary coil · transformer

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