1. The problem statement, all variables and given/known data There is a conductor with a spherical cavity. Point charge B is at the center of the cavity. Point charge A is outside the conductor. What is the force on B. 2. Relevant equations 3. The attempt at a solution I know the answer should be zero, but I can't fully prove it to myself. If B were not in the conductor, charge would distribute on the cavity surface to make E inside zero. However, once you add B, how do you know the new distribution of charge on the inner surface won't create an electric field? It seems to me that all you can say is the total charge on the inner surface will equal charge of B. However, you don't know anything about the distribution.