Explain how a charged comb can attract small pieces of uncharged paper, even though the paper has no overall charge.

AQA A-Level Physics (7408) — 3.7.3 Electric Fields · Explain · 5 marks · View as Markdown

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

When a plastic comb is rubbed through hair, it becomes negatively charged. If the comb is held close to small pieces of paper, the paper is attracted to the comb.

Model answer (5 marks)

The negatively charged comb produces an electric field around it.
The field acts on the electrons in the paper, pushing them away from the comb.
This leaves the side of the paper nearest the comb deficient in electrons, i.e. positively charged.
The positive near side of the paper is attracted to the negative comb.
Because the attractive force on the near side is larger than the repulsive force on the far side, a net attraction occurs.

Examiner tips

  • Use the word ‘electric field’ early. Show the chain: field → electron movement → induced charge → attraction. Mention the net force is due to unequal distances (field gradient).

Common mistakes

  • Confusing the comb’s charge with the paper’s charge. Forgetting that the paper is initially neutral. Saying the paper is attracted because it is ‘negative’ rather than because of induced positive charge.

Mark scheme (5 marks)

  1. The charged comb creates an electric field around it
  2. The electric field causes the electrons in the paper to be redistributed / move
  3. Electrons in the paper are repelled away from the comb, leaving the near side of the paper positively charged
  4. The positively charged near side of the paper is attracted to the negatively charged comb
  5. The attractive force on the near side is greater than the repulsive force on the far side, so there is a net attractive force

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