Explain how a transformer is used in the National Grid to reduce energy loss when electricity is transmitted over long distances.

WJEC GCSE Physics (Wales) — 1.9 Electromagnetism · Explain · 4 marks · View as Markdown

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

Electricity generated at a power station is transmitted across the country through power cables at very high voltages before being supplied to homes and businesses.

Model answer (4 marks)

A step‑up transformer is used at the power station to increase the voltage of the electricity before it is transmitted.

Increasing the voltage reduces the current that is required to carry the same amount of power (P=VI).

With a lower current the resistive losses in the transmission cables (P_loss=I²R) are much smaller, so less energy is lost as heat.

At the distribution end a step‑down transformer lowers the high voltage back to a safe level for use in homes and businesses.

Examiner tips

  • Use the sequence: step‑up → higher V, lower I, less I²R loss, step‑down → lower V for consumers
  • Mention the power equation P=VI and the loss equation P_loss=I²R
  • Show the logical flow from transmission to distribution

Common mistakes

  • Failing to explain that the transformer changes voltage, not power
  • Confusing the roles of step‑up and step‑down transformers
  • Ignoring the I²R loss relationship

Mark scheme (4 marks)

  1. A step-up transformer increases the voltage (potential difference) before transmission
  2. Increasing voltage causes the current to decrease (for the same power)
  3. Lower current means less energy is lost (as heat) in the power cables
  4. A step-down transformer reduces the voltage to a safe level for use in homes/businesses

Key terms in this question

transformer

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