A positively charged rod is held near a small, uncharged metal sphere resting on an insulating stand. The sphere is then touched on the far side by a grounded wire, and the wire is removed before the rod is taken away. Explain why the metal sphere ends up with a negative charge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The positively charged rod attracts free electrons in the metal sphere towards the near side, creating a negative charge on the near side and a positive charge on the far side (induced charge separation).
2. The far side becomes electron‑deficient, so electrons flow from the ground through the wire into the sphere.
3. When the wire is removed the extra electrons remain on the sphere, giving it an excess of electrons.
4. After the rod is taken away the excess electrons spread over the whole sphere, leaving it with an overall negative charge.
2. The far side becomes electron‑deficient, so electrons flow from the ground through the wire into the sphere.
3. When the wire is removed the extra electrons remain on the sphere, giving it an excess of electrons.
4. After the rod is taken away the excess electrons spread over the whole sphere, leaving it with an overall negative charge.
Examiner tips
- Use the word ‘induced’ to show understanding of charge separation.
- Explain the flow of electrons from ground to sphere.
- Show the sequence: induction → grounding → removal → final charge.
- Mention that the rod’s field causes the redistribution of electrons.
Common mistakes
- Forgetting that the far side becomes positively charged and attracts electrons from ground.
- Confusing the direction of electron flow (writing electrons move to the rod instead of from ground).
- Not linking the removal of the wire to the trapping of excess electrons.
Mark scheme (4 marks)
- The positive rod attracts free electrons in the metal sphere towards the near side / repels positive ions away from the near side (induced charge separation / redistribution of electrons)
- Electrons flow from the ground/Earth into the sphere (through the wire) because the far side of the sphere is positively charged / electron-deficient
- When the wire is removed, the extra electrons are trapped on the sphere (the sphere now has an excess of electrons)
- When the rod is taken away, the excess electrons spread over the sphere, leaving it with an overall negative charge
Related
- All Eduqas GCSE Physics revision notes →
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- Decode the mark scheme abbreviations →
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