10C of charges are placed on a spherical conducting shell. A particle with a charge of –3C is placed at the center of the cavity. The net charge on the inner surface of the shell is:

Answer: +3C

In a conductor, the electric field inside the conductor (in the material itself) must be zero. So:

The inner surface of the shell must carry a charge that exactly cancels the field from the charge inside the cavity.

Since there's a –3 C charge at the center, the inner surface must have a charge of +3C to neutralize the field inside the conductor.

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