A petrol station uses earthing cables to prevent sparking when fuel is being transferred between tankers and underground storage tanks. Explain why static charge can build up during fuel transfer and why sparking is dangerous in this situation, and describe how earthing prevents a spark from occurring.

OCR A-Level Physics A (H556) — 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).

During fuel transfer at a petrol station, fuel flows through pipes at high speed. Engineers attach a metal earthing cable between the tanker and the ground before any fuel is transferred.

Model answer (5 marks)

1. When fuel flows through the pipe, electrons are transferred between the fuel and the pipe walls, creating a charge imbalance and static charge on the tanker.
2. The tanker is metal but is insulated from the ground by rubber tyres, so the charge cannot leave the tanker and continues to build up.
3. If the charge becomes large enough, it can jump through the air as a spark between the highly charged tanker and a nearby object of opposite or lower charge.
4. A spark is dangerous because it can ignite the fuel vapour, leading to a fire or explosion.
5. The earthing cable gives a low‑resistance path for electrons to flow from the tanker to the ground, safely removing the excess charge before a spark can form.

Examiner tips

  • Use the exact terms: static charge, charge imbalance, low‑resistance path, spark, ignition.
  • Show the cause–effect chain: flow → charge transfer → insulation → build‑up → spark → danger.
  • Mention the role of the tyre insulation explicitly.
  • Keep the answer concise and point‑wise to match the 5 marks.

Common mistakes

  • Confusing static charge with electric current; not explaining the tyre insulation.
  • Failing to state that the spark can ignite vapour, or that the earthing removes charge before it reaches the spark threshold.
  • Using vague terms like ‘electricity’ instead of ‘static charge’ or ‘spark’.

Mark scheme (5 marks)

  1. Electrons are transferred between the fuel and the pipe walls as fuel flows through, causing a charge imbalance / static charge to build up on the tanker.
  2. The tanker is made of metal but is insulated from the ground by rubber tyres, so the charge cannot flow away and continues to build up.
  3. If enough charge builds up, a spark can occur as charge jumps through the air between the highly charged tanker and a nearby object of opposite / lower charge.
  4. Sparking is dangerous because the spark can ignite the fuel vapour, causing a fire or explosion.
  5. The earthing cable provides a low resistance path for electrons to flow from the tanker to the ground, safely removing the excess charge before it can build up enough to cause a spark.

Key terms in this question

static charge · sparking · earthing

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