Explain why electricity is transmitted across the National Grid at very high voltages rather than at lower voltages.

WJEC GCSE Physics (Wales) — 1.2 Generating electricity · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

High voltage reduces the current required to deliver a given power, because P=IV. Lower current means less I²R loss in the conductors, so less energy is dissipated as heat. This makes the transmission of electricity more efficient. The power cables are metal conductors with resistance, so a smaller current reduces the heating effect.

Examiner tips

Use the relation P=IV to explain why higher V gives lower I for the same power.
Mention I²R losses and how they relate to heat.
Show that lower losses mean higher efficiency.
Keep the answer concise and use the exact terminology.

Common mistakes

Confusing voltage with power or current.
Forgetting to mention the I²R loss mechanism.
Using vague terms like "less energy" without linking to resistance and heat.

Mark scheme (4 marks)

High voltage reduces the current in the power cables
Lower current means less energy is lost / dissipated (as heat) in the power cables
This makes the transmission of electricity more efficient
The power cables are metal wires / conductors that have resistance, so a smaller current reduces the heating effect

Key terms in this question

National Grid