A step-down transformer is used to reduce the potential difference from the National Grid before electricity is supplied to homes. Describe how a step-down transformer works, including why the number of turns on each coil is important.

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)

An alternating current in the primary coil creates a changing magnetic field in the iron core. This changing field induces a potential difference in the secondary coil. The secondary coil has fewer turns than the primary coil. Because there are fewer turns, the induced potential difference is smaller, giving a lower output voltage than the input.

Examiner tips

  • Use the word ‘induced’ to show understanding of EMF. Mention the iron core to show why the field is concentrated. State the turn ratio explicitly. Show that fewer turns give a lower voltage, not a higher one.

Common mistakes

  • Saying the transformer increases voltage when it is step‑down. Forgetting to mention the iron core. Confusing the number of turns with the current rather than the voltage.

Mark scheme (4 marks)

  1. Alternating current in the primary coil creates a changing magnetic field (in the iron core)
  2. The changing magnetic field induces a potential difference in the secondary coil
  3. The secondary coil has fewer turns than the primary coil
  4. Fewer turns on the secondary coil means a lower potential difference is produced / the output potential difference is smaller than the input

Key terms in this question

step-down transformer · potential difference · turns

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