A worker at a petrol station notices a small spark as they reach for the fuel nozzle. Explain why static charge can build up on the worker's body and why this sparking is dangerous at a petrol station. You should also explain how earthing the fuel pump reduces this risk.

OCR A-Level Physics B: Advancing Physics (H557) — 3.2 Forces in action · Explain · 5 marks · View as Markdown

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

Petrol stations require all fuel pumps to be earthed. Workers are advised to touch a metal surface before handling fuel nozzles.

Model answer (5 marks)

Static charge builds up on the worker because electrons are transferred when the worker’s clothing or shoes rub against surfaces such as the seat or the floor, giving the worker a net charge.
The worker’s body becomes charged because the charge cannot flow away – the worker is not earthed and the body acts as an insulator.
Sparking occurs when the potential difference between the charged worker and the fuel nozzle becomes large enough that electrons jump through the air, balancing the charges.
The spark is dangerous because it can ignite petrol vapour in the air, leading to a fire or explosion.
Earthing the fuel pump gives a low‑resistance path for electrons to flow from the pump to the ground, preventing charge from building up on the pump and removing any excess charge safely.

Examiner tips

  • Use the word ‘electrons’ and ‘potential difference’ to show understanding of charge transfer.
  • Explain that the worker is not earthed and the body is an insulator.
  • Mention that the spark can ignite vapour, not just cause a fire.
  • Show that earthing provides a low‑resistance path to ground.

Mark scheme (5 marks)

  1. Static charge builds up on the worker because electrons are transferred when clothing/shoes rub against surfaces (e.g. seat, floor)
  2. The worker's body becomes charged because the charge cannot flow away — the worker is an insulator (or is not earthed)
  3. Sparking occurs when enough charge has built up and electrons jump through the air between the worker and the nozzle to balance the charges
  4. The spark is dangerous because it can ignite petrol vapour, causing a fire or explosion
  5. Earthing provides a low resistance path for electrons to flow away from the pump to the ground, preventing charge from building up / removing excess charge safely

Key terms in this question

static charge · sparking · earthing

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