1. The problem statement, all variables and given/known data A sphere with excess charge of 1.5 x10 -7 Coulombs, An outer radius of 2.5cm and inner radius of 1.5 cm ( a cavity ). The question is to find the surface charge density on the inside surface. I'm totally at a loss on this one and couldn't find how to solve it. 2. Relevant equations Well surface charge density would be C/area of inner sphere. 3. The attempt at a solution I couldnt at all figure out how this is done and didnt fiond anythig on it in HRW any help please?