A plastic rod is rubbed with a cloth. After rubbing, the rod is found to carry a negative charge. Explain how the rod became negatively charged and state the charge on the cloth after rubbing.

Pearson Edexcel International GCSE Physics (4PH1) — 2.3 Electric charge · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Friction between the plastic rod and the cloth causes electrons to be transferred from the cloth to the rod.
The rod therefore gains electrons and has an excess of negative charge.
The cloth loses electrons and becomes positively charged.
The magnitude of the charge on the cloth is equal to the magnitude of the charge on the rod, because charge is conserved.

Examiner tips

  • Use the word ‘transfer’ to show the mechanism of electron movement.
  • Mention that the rod becomes negative and the cloth positive.
  • State that the charges are equal in magnitude but opposite in sign.
  • Keep the answer concise – 4 marks are awarded for a clear, brief explanation.

Common mistakes

  • Saying the rod becomes positive or the cloth negative – the signs are reversed.
  • Omitting the conservation of charge – the magnitude equality is a key point.
  • Using vague terms like ‘gain’ or ‘lose’ without specifying electrons or charge transfer.

Mark scheme (4 marks)

  1. Friction causes electrons to be transferred from the cloth to the rod (during rubbing).
  2. The rod gains electrons, giving it an excess of negative charge.
  3. The cloth loses electrons and therefore becomes positively charged.
  4. The magnitude of the charge on the cloth is equal to the magnitude of the charge on the rod (charge is conserved).

Key terms in this question

charge

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