Exams › JEE Advanced › Physics
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?
- Only the induced charge on the outer surface redistributes.
- Only the induced charge on the inner surface redistributes.
- Both the inner-surface and outer-surface induced charges redistribute.
- 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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