Describe how electricity generated at a power station reaches homes through the National Grid, including the role of transformers.

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

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

A new offshore wind farm has been connected to the National Grid. The electricity it generates must travel hundreds of kilometres before reaching domestic customers.

Model answer (4 marks)

1. At the wind farm a step‑up transformer raises the voltage from the generator to the high‑voltage transmission level.
2. The electricity is carried over long‑distance power cables at this high voltage.
3. High voltage means lower current for the same power, so the I²R losses in the cables are reduced.
4. Near the consumer a step‑down transformer lowers the voltage to the safer domestic level before it enters the household supply.

Examiner tips

Use the exact sequence: step‑up → high‑voltage transmission → reduced losses → step‑down to domestic voltage.
Show the cause–effect link: high voltage → lower current → less heat loss.
Mention the purpose of each transformer (increase/decrease voltage).

Common mistakes

Confusing the order of transformers (step‑down before transmission).
Forgetting to explain why high voltage reduces losses.
Using vague terms like "large voltage" instead of "high voltage".

Mark scheme (4 marks)

A step-up transformer increases the potential difference (voltage) from the power station / wind farm to the power lines
Electricity is transmitted along power cables at very high voltage / potential difference
Transmitting at high voltage reduces the current, which reduces the energy lost as heat in the power cables
A step-down transformer decreases the potential difference to a safer / lower voltage suitable for domestic use

Key terms in this question

National Grid