A student rubs a perspex rod with a silk cloth. Explain why the perspex rod becomes positively charged during this process.

AQA GCSE Physics (8463) — 4.2.6 Static electricity (Phys only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Electrons are transferred from the perspex rod to the silk cloth when they are rubbed together.
The perspex rod therefore loses electrons.
With fewer electrons it has more positive charges than negative charges, giving it a net positive charge.
Because perspex is an insulator, the remaining charge cannot flow away and the rod stays positively charged.

Examiner tips

  • Use the word ‘transfer’ to show the movement of electrons. Show the sequence: transfer → loss of electrons → net positive charge. Mention that perspex is an insulator to justify why the charge remains.
  • Use the exact terminology: electrons, positive charge, insulator.

Common mistakes

  • Saying the rod gains electrons instead of loses. Forgetting to explain that perspex is an insulator. Using vague terms like ‘becomes charged’ without specifying positive or negative.

Mark scheme (4 marks)

  1. Electrons are transferred from the perspex rod to the silk cloth (during rubbing)
  2. The perspex rod loses electrons
  3. The perspex rod has more positive charges than negative charges (so it has a net positive charge)
  4. Perspex is an insulator so the remaining charge cannot flow away / electrons cannot flow through it

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