A step-up transformer is used at a power station before electricity enters the National Grid. Describe how a step-up transformer works, from the alternating current entering the primary coil to the higher potential difference produced across the secondary coil.

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

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

Model answer (4 marks)

1. The AC flowing through the primary coil creates a time‑varying magnetic field in the iron core.
2. This changing magnetic field links the secondary coil, cutting through its turns.
3. By Faraday’s law, a potential difference is induced across the secondary coil.
4. Because the secondary has more turns than the primary, the induced voltage is larger – the transformer steps up the potential difference.

Examiner tips

  • Use the exact phrase ‘time‑varying magnetic field’ and ‘Faraday’s law’ to show understanding.
  • Show the sequence: primary AC → magnetic field → secondary → induced voltage → higher V due to more turns.
  • Keep the answer concise – one sentence per point for 4 marks.

Common mistakes

  • Confusing the roles of primary and secondary coils; describing the secondary as the source of the magnetic field.
  • Omitting the iron core or the concept of a changing magnetic field.
  • Using vague terms like ‘more voltage’ without explaining the turn ratio.

Mark scheme (4 marks)

  1. The alternating current in the primary coil produces a changing magnetic field (in the iron core).
  2. The changing magnetic field passes through / cuts through the secondary coil.
  3. A potential difference is induced across the secondary coil.
  4. The secondary coil has more turns than the primary coil, so the induced potential difference is greater (stepped up).

Key terms in this question

step-up transformer · primary coil · secondary coil · potential difference · alternating current

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