Explain why electricity generated at a power station is transmitted across the National Grid at very high voltages, rather than at the lower voltages used in homes.

OCR GCSE Physics A: Gateway Science (J249) — P6.1 Energy resources · Explain · 4 marks · View as Markdown

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

Britain's National Grid distributes electricity from power stations to homes and businesses across the country. The electricity is transmitted along overhead power cables at very high voltages before being reduced to 230 V for domestic use.

Model answer (4 marks)

1. Electricity is stepped up to a high voltage at the power station using a step‑up transformer.
2. The high voltage reduces the current that must flow through the transmission cables.
3. A lower current means less resistive (I²R) heating, so less energy is lost as heat in the cables.
4. At the consumer end a step‑down transformer reduces the voltage to the 230 V required for domestic use.

Examiner tips

  • Use the exact terms ‘step‑up transformer’ and ‘step‑down transformer’.
  • Explain the relationship between voltage, current and power loss (P=I²R).
  • Show the logical sequence: high V → low I → low loss → low V for safety.

Common mistakes

  • Confusing voltage with current or power. Misstating that high voltage is used to increase power rather than reduce loss. Omitting the role of transformers in changing voltage levels.

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. Step-up transformers are used at the power station to increase the voltage for transmission
  4. Step-down transformers are used to reduce the voltage to a safe level for domestic use

Key terms in this question

National Grid · voltage

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