A student charges a balloon by rubbing it against their hair. The balloon is then held close to, but not touching, a thin stream of water from a tap. Explain why the stream of water is attracted towards the balloon.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a balloon is rubbed against hair, electrons are transferred from the hair to the balloon. The balloon is then held near a thin stream of water without touching it.
Model answer (5 marks)
Rubbing the balloon against hair transfers electrons from the hair to the balloon, giving the balloon a negative static charge.
The negative charge on the balloon creates an electric field around it.
The electric field repels electrons in the water, so the surface of the water nearest the balloon becomes positively charged (induced charge or charge separation).
Opposite charges attract, therefore the positively charged region of the water is attracted to the negatively charged balloon.
Thus the stream of water is drawn towards the balloon by a non‑contact electrostatic force – the balloon need not touch the water to exert a force on it.
The negative charge on the balloon creates an electric field around it.
The electric field repels electrons in the water, so the surface of the water nearest the balloon becomes positively charged (induced charge or charge separation).
Opposite charges attract, therefore the positively charged region of the water is attracted to the negatively charged balloon.
Thus the stream of water is drawn towards the balloon by a non‑contact electrostatic force – the balloon need not touch the water to exert a force on it.
Examiner tips
- Use the word "induced" or "charge separation" to show understanding of the mechanism.
- Show the chain: transfer of electrons → negative balloon → electric field → repulsion of electrons in water → positive region → attraction to balloon.
- Mention that the force is non‑contact to cover the last point.
- Keep the answer concise and use UK spelling.
Common mistakes
- Forgetting to state that the balloon becomes negatively charged.
- Confusing the direction of the electric field or the sign of the induced charge.
- Saying the water is attracted because it is negatively charged, rather than because a positive region is induced.
Mark scheme (5 marks)
- Rubbing transfers electrons from the hair to the balloon, giving the balloon a negative static charge
- The negatively charged balloon creates an electric field around it
- The negative charge on the balloon repels electrons in the water, so the surface of the water nearest the balloon becomes positively charged (induced charge / charge separation / induction)
- Unlike (opposite) charges attract, so the positive region of the water is attracted to the negative balloon
- This is a non-contact (electrostatic) force — the balloon does not need to touch the water to exert a force on it
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →