Explain why electricity is transmitted across the National Grid at very high voltages rather than at low voltages.

OCR GCSE Physics A: Gateway Science (J249) — P6.2 Producing electricity · Explain · 4 marks · View as Markdown

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

The National Grid carries electricity from power stations to homes and businesses across the UK. Power cables stretch for hundreds of kilometres across the country.

Model answer (4 marks)

1. High voltage transmission reduces the current in the power cables.
2. A lower current means less energy is lost as heat in the cables.
3. This makes the transmission of electricity more efficient.
4. Step‑up transformers at the power station raise the voltage for transmission, and step‑down transformers reduce it to a safe level for domestic use.

Examiner tips

  • Use the exact phrase "high voltage transmission reduces the current". Show the logical chain: high V → low I → less I²R loss → higher efficiency. Mention transformers as the means of changing voltage.
  • common_mistakes
  • :
  • Confusing voltage with current; writing that high voltage increases current. Omitting the role of transformers. Using vague terms like "better" instead of "more efficient".

Mark scheme (4 marks)

  1. High voltage transmission reduces the current in the power cables
  2. A lower current results in less energy lost as heat in the power cables
  3. This makes transmission of electricity more efficient
  4. Step-up transformers are used at the power station to increase the voltage for transmission, and step-down transformers reduce the voltage to a safe level for domestic use

Key terms in this question

National Grid

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