A student uses a gold-leaf electroscope to investigate static electricity. The metal cap of the electroscope is touched with a negatively charged rod. The gold leaf rises and stays raised after the rod is removed. Explain why the gold leaf rises and remains raised.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A gold-leaf electroscope consists of a metal cap connected by a metal rod to a thin gold leaf. When the leaf carries the same type of charge as the rod, it rises away from the rod.
Model answer (5 marks)
Electrons are transferred from the negatively charged rod to the metal cap of the electroscope. The cap, rod and leaf therefore become negatively charged. The gold leaf now carries the same type of charge as the rod, so like charges repel. The repulsive force pushes the leaf away from the rod and it rises. After the rod is removed the electroscope is isolated; the excess electrons cannot escape to the surroundings, so the leaf remains raised.
Examiner tips
- Use the phrase ‘electrons are transferred’ to show charge transfer.
- Show that the leaf and rod have the same sign and that like charges repel.
- Explain that the electroscope is isolated so the charge remains.
- Use the word ‘repel’ rather than ‘push’ for clarity.
Common mistakes
- Saying the leaf rises because of attraction to the rod.
- Claiming the leaf stays raised because of a magnetic field.
- Forgetting to mention that the electroscope is isolated from earth.
Mark scheme (5 marks)
- Electrons transfer from the charged rod to the metal cap (and through the electroscope)
- The electroscope (cap, rod and leaf) becomes negatively charged / gains excess electrons
- The rod and the gold leaf both carry the same type of charge (both negative)
- Like charges repel, so the gold leaf is pushed away from the rod
- The leaf remains raised because the charge cannot escape / the electroscope is an electrical insulator from its surroundings / there is no conducting path to earth
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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