In one of our exercises, there is a conducting sphere (S2) with a cavity inside (S1). A point charge is placed inside the cavity. We were told to try and find the electric field outside by following a set of steps. 1. Imagine that S2 is grounded. In that case, the charge on the cavity wall (S1) is -q and the charge on the outer surface is 0. 2. Imagine that only S2 exists, with no cavity and no point charge. 3. Use super-position from step 1 & 2 to determine the total electric field outside the original shell. a) The charge on the inner cavity wall is -q. b) The charge on the outer wall is +q (I assume this is because the conductor should remain neutral?). c) This is where I get lost. The solution says that: From superposition and uniqueness, it can be inferred that the charge over S2 is uniformly distributed. Hence, it can be treated as though a charge of q was placed at the centre of the sphere. What does this mean, exactly? Superposition and uniqueness? How has the previous parts proved this, exactly?