A fuel tanker delivers petrol through a hose to an underground storage tank at a petrol station. As the fuel flows through the hose, a static charge builds up. Explain why this static charge builds up and why it is dangerous, and describe how earthing the hose prevents the danger.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Petrol is an insulating liquid. As it flows rapidly through a rubber hose, electrons are transferred between the fuel and the hose.
Model answer (5 marks)
As the petrol flows through the rubber hose, friction between the liquid and the hose transfers electrons from the petrol to the hose.
Because the hose is an insulator, the transferred electrons cannot leave the hose, so a static charge accumulates on its surface.
When the charge becomes large enough, a spark can jump through the air to neutralise the excess charge.
The spark can ignite the petrol vapour, leading to a fire or explosion.
Earthing the hose gives a low‑resistance path for the excess electrons to flow into the Earth, preventing the charge from reaching the level required for a spark.
Because the hose is an insulator, the transferred electrons cannot leave the hose, so a static charge accumulates on its surface.
When the charge becomes large enough, a spark can jump through the air to neutralise the excess charge.
The spark can ignite the petrol vapour, leading to a fire or explosion.
Earthing the hose gives a low‑resistance path for the excess electrons to flow into the Earth, preventing the charge from reaching the level required for a spark.
Examiner tips
- Use the word ‘friction’ to explain electron transfer. Mention the hose’s insulating property. Explain the spark mechanism and its danger. State that earthing removes excess charge via a low‑resistance path.
Common mistakes
- Confusing the hose with the petrol as the source of charge. Saying the hose is a conductor instead of an insulator. Omitting the spark‑ignition link to the danger.
Mark scheme (5 marks)
- As the fuel flows through the hose, friction causes electrons to be transferred from one material to the other
- Because the hose is an insulator, the electrons cannot flow away, so a static charge builds up on the hose
- When the charge becomes large enough, a spark jumps through the air to balance the charges
- The spark could ignite the petrol vapour, causing a fire or explosion
- Earthing provides a low resistance path for electrons to flow to the Earth, so excess charge is removed and cannot build up to a level that causes a spark
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →