A step-up transformer is used at a power station to increase the voltage before electricity is transmitted through the National Grid. Explain why transmitting electricity at high voltage reduces energy losses in the transmission cables.

Eduqas GCSE Physics — 8.3 Induced potential and transformers · Explain · 4 marks · View as Markdown

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

Power stations generate electricity at around 25 000 V. Step-up transformers increase this to 400 000 V before the electricity enters the transmission cables of the National Grid.

Model answer (4 marks)

Increasing the voltage reduces the current required to transmit the same power, because P=VI.

The heating loss in a cable is I²R, so a lower current gives a much smaller loss.

With less power lost as heat, more of the generated energy reaches consumers.

Thus high‑voltage transmission makes the system more efficient and reduces wasted energy.

Examiner tips

  • Show the P=VI relationship and the I²R loss formula. Explain the chain: high V → low I → low I²R loss → more efficient. Use the exact terms “current”, “power loss”, “heat”, “efficient”.
  • common_mistakes
  • :
  • Mixing up voltage and current effects, e.g. saying high voltage increases current. Forgetting to mention the I²R dependence of losses. Using vague phrases like “less loss” without linking to current.

Mark scheme (4 marks)

  1. Increasing voltage causes a decrease in current (for the same power transmitted)
  2. Heating effect / power loss in cables depends on the current
  3. Lower current means less power/energy is lost as heat in the cables
  4. This makes the transmission more efficient (less energy wasted / greater proportion of energy reaches consumers)

Key terms in this question

step-up transformer · National Grid · voltage · energy loss · transmission

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