[SOLVED] electrostatics problem 1. The problem statement, all variables and given/known data Two spherical cavities, of radii a and b, are hollowed out from the interior of a (neutral) conducting sphere of radius R. At the center of each cavity a point charge is placed--call these charges q_a and q_b. Why is it necessarily true that the force on q_a and q_b is 0? Why is it true that the force is still zero no matter what kind of charge distribution you have outside the conductor. 2. Relevant equations 3. The attempt at a solution You can use Gauss's Law to find the electric field in each cavity. And you find that it is as the charge were isolated in space. But that doesn't tell you the electric field at the charge...only around it, right?