A student rubs a plastic rod with a woollen cloth. The plastic rod becomes negatively charged. Explain why the rod becomes negatively charged and describe the charge left on the woollen cloth after rubbing.

Edexcel GCSE Physics (1PH0) — 11.1 Static electricity (Phys only) · Explain · 4 marks · View as Markdown

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

Plastic and wool are both insulators. When they are rubbed together, a static charge can build up on each material.

Model answer (4 marks)

Electrons are transferred from the woollen cloth to the plastic rod by friction when they are rubbed together.
The plastic rod gains electrons, so it has an excess of negative charge.
The woollen cloth loses electrons, leaving it with a positive static charge.
Because both materials are insulators, the electrons cannot flow away, so the charge remains and builds up.

Examiner tips

  • Use the word "transfer" to show movement of electrons. Mention friction as the cause of transfer. State the sign of the charge on each material. Explain why the charge stays (insulators).

Common mistakes

  • Saying the plastic rod becomes positively charged. Claiming the woollen cloth becomes negatively charged. Forgetting to mention that the materials are insulators.

Mark scheme (4 marks)

  1. Electrons are transferred from the woollen cloth to the plastic rod (by friction / due to rubbing)
  2. The plastic rod gains electrons, giving it an excess of negative charge
  3. The woollen cloth loses electrons, leaving it with a positive static charge
  4. The effect occurs because both materials are insulators, so electrons cannot flow away and the charge remains / builds up

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