Explain why electrical power is transmitted across the National Grid at a high potential difference rather than a low potential difference.

AQA GCSE Physics (8463) — 4.2.5 The National Grid · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

High voltage reduces the current for a given power, so power loss in the cables (P_loss=I^2R) is much lower. Step‑up transformers raise the voltage at the source, step‑down transformers lower it at the consumer.

Examiner tips

  • Use the word ‘high voltage’ to start the answer
  • Explain the I^2R relationship
  • Mention step‑up and step‑down transformers
  • Keep the answer concise and to the point

Common mistakes

  • Failing to mention the I^2R loss formula
  • Using ‘low voltage’ instead of ‘high voltage’
  • Not linking transformers to voltage change

Mark scheme (4 marks)

  1. Increasing the potential difference (using a step-up transformer) causes the current to decrease
  2. Power loss in the transmission cables is reduced when current is lower
  3. This is because power loss is related to (the square of) the current, so a smaller current results in significantly less power lost as heat
  4. Step-down transformers are used to reduce the potential difference to safe, usable levels for consumers at the end of the transmission

Key terms in this question

National Grid · potential difference

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