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A point charge q sits inside a cavity of an electrically neutral conductor. An external electric field is then turned on. Which of the following happens?

1. Only the induced charge on the outer surface redistributes.
2. Only the induced charge on the inner surface redistributes.
3. Both the inner-surface and outer-surface induced charges redistribute.
4. The force on the cavity charge q changes.

Correct answer: Only the induced charge on the outer surface redistributes.

Solution

Inside a conductor the field is zero, so the cavity (and the charge q) are completely shielded from the external field. The inner-surface induced charge is fixed by q alone (it must total -q with a distribution set by q's position) and does not change. The external field only rearranges the free charge on the outer surface. The force on q remains unchanged.

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