A student rubs a plastic ruler with a dry cloth. The ruler becomes negatively charged. Explain why the ruler becomes negatively charged and describe what charge, if any, remains on the cloth after rubbing.

OCR GCSE Physics A: Gateway Science (J249) — P3.1 Static and charge · Explain · 4 marks · View as Markdown

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

When certain insulating materials are rubbed together, electrons can be transferred from one material to the other, causing both objects to become electrically charged.

Model answer (4 marks)

Electrons are transferred from the cloth to the ruler during rubbing.
The ruler gains electrons, so it has more electrons than protons and becomes negatively charged.
The cloth loses electrons to the ruler.
The cloth is left with a positive charge, equal and opposite to the charge on the ruler.

Examiner tips

  • Use the word ‘transfer’ to show the movement of electrons. State that the ruler gains electrons and therefore has a negative charge. Mention that the cloth loses electrons and becomes positively charged. Show the charge on the cloth is equal and opposite to that on the ruler.

Common mistakes

  • Saying the ruler loses electrons instead of gains. Forgetting to mention the cloth becomes positively charged. Using ‘charge’ without specifying positive or negative.

Mark scheme (4 marks)

  1. Electrons are transferred from the cloth to the ruler (during rubbing)
  2. The ruler gains electrons and therefore has more electrons than protons, giving it a negative charge
  3. The cloth loses electrons (to the ruler)
  4. The cloth is left with a positive charge (equal and opposite to the charge on the ruler)

Key terms in this question

negatively charged · charge

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