A student rubs a glass rod with a silk cloth. The glass rod becomes positively charged. Explain why the glass rod becomes positively charged and describe the charge left on the silk cloth after rubbing.

Eduqas GCSE Physics — 7.3 Static electricity – forces and electric fields · 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 glass rod to the silk cloth during rubbing.
The glass rod loses electrons, leaving it with more protons than electrons, so it becomes positively charged.
The silk cloth gains the transferred electrons, acquiring more electrons than protons.
Hence the silk cloth becomes negatively charged.

Examiner tips

  • Use the word "transfer" to link rubbing to electron movement. Show the charge imbalance for each body. Mention the resulting positive/negative charge explicitly. Keep the answer in 4 clear points to match the 4 marks.

Common mistakes

  • Saying the glass rod gains electrons instead of loses. Forgetting to state that the silk cloth becomes negative. Using vague terms like "more charge" without specifying electrons or protons.

Mark scheme (4 marks)

  1. Electrons are transferred from the glass rod to the silk cloth (during rubbing)
  2. The glass rod loses electrons and so has more positive charge than negative charge / gains a positive charge
  3. The silk cloth gains electrons / has more electrons than protons
  4. The silk cloth is negatively charged

Key terms in this question

positively charged

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