Source: High school physics (Chinese)
Problem Sets:
Problem
A small, positively charged sphere is brought near an insulated, electrically neutral conductor.
- Does the sphere experience an attractive or repulsive force?
- What happens if the end of the conductor closer to the sphere is momentarily grounded?
- Instead of grounding, what happens if the conductor and sphere briefly touch?
[Q1] Attractive force [Q2] The conductor acquires a net negative charge, increasing attraction. [Q3] Repulsive force
[Q1] The positive sphere induces a charge separation in the neutral conductor via electrostatic induction. Free electrons in the conductor are attracted towards the sphere, creating a net negative charge on the near side and a net positive charge on the far side. Since the attractive force to the closer (negative) side is stronger than the repulsive force from the farther (positive) side, the net force on the sphere is attractive.
[Q2] Grounding the conductor allows electrons to flow from the Earth onto the conductor, attracted by the nearby positive sphere. If the ground connection is removed while the sphere is still nearby, the conductor is left with a net negative charge, and the attractive force becomes stronger.
[Q3] If the sphere touches the conductor (charging by conduction), positive charge is transferred from the sphere to the conductor until they reach a similar potential. After they are separated, both objects will have a net positive charge and will therefore exert a repulsive force on each other.