1. The problem statement, all variables and given/known data A sphere has outer radius 15 and inner radius 5. Between r= 5 and 15, the sphere is solid and contains a total charge of 20 C. At r < 5, the sphere is hollow. Calculate the electric field at r = 8. 2. Relevant equations Gauss' Law --> EA = (q)/epsilon zero 3. The attempt at a solution What I'm confused about is the hollow sphere part. But ignoring that, here's how I would do the problem. The volume of the entire sphere is (4/3)pi(15^3) = 14137.16694 m^3. The volume of the hollow part is (4/3)pi(5^3)= 523.598 m^3. Therefore, the volume of just the solid part is 14137 - 523 = 13613.568 m^3. The charge density per volume is then 20 / 13613 = 0.001469. To calculate the electric field at r = 8, I need to find the volume of such a sphere. Same procedure as above, take the volume minus the empty part to find 1621.06. Multiply this by the charge density... 1621.06 * 0.001469 to find 2.38133 C. This is the internal charge. Take a gaussian surface of radius 8. It will have surface area (4pi)(8^2) E = (internal charge)/ (epsilon zero)(area) E= 2.38 / (eps. zero)(804.24) = huge number, 3.34 x 10 ^ 8. What's confusing me is whether it's ok to take the surface area of a sphere with radius 8, because of that hollow part. Do I need to subtract the hollow part's surface area or something?