Explain why step-up transformers are used in the National Grid before electrical power is transmitted along the transmission cables.

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).

Electricity generated at a power station is passed through a step-up transformer before entering the National Grid's transmission cables.

Model answer (4 marks)

Step‑up transformers raise the voltage of the electricity produced at the power station.

Because power is constant (P = IV), a higher voltage means the current required to carry the same power is lower.

Lower current flowing through the transmission cables reduces the resistive losses (I²R losses) as heat.

Thus the transmission of electrical power becomes more efficient.

Examiner tips

Use the exact phrase ‘step‑up transformer’ and ‘increase the voltage’.
Show the relationship P = IV and explain that higher V gives lower I for constant P.
Mention that lower I reduces I²R losses in the cables.
Keep the answer concise and to the point.

Common mistakes

Confusing voltage with current or power; e.g. saying the transformer increases current. Failing to mention the I²R loss reduction. Using vague terms like ‘better’ or ‘more efficient’ without explaining why.

Mark scheme (4 marks)

The step-up transformer increases the potential difference
Because power is constant (P = IV), increasing potential difference causes the current to decrease
Lower current means less energy is lost (as heat) in the transmission cables
This makes the transmission of electrical power more efficient

Key terms in this question

step-up transformer · National Grid · transmission cables