Hi, 1. The problem statement, all variables and given/known data I'd like to find the electrical field at the center of a hole made in a hollow sphere of radius R0 with uniform charge density σ. The radius of the disc-shaped hole is a << R0. 2. Relevant equations I know that the electrical field of the sphere is Q/r2 for r>R and 0 for r<R. I also know that the electrical field of the disc-shaped hole is -2πσ(1-r/SQRT(a2+r2). I am advised NOT to calculate the electrical field at the center of the hole directly, but to use the above data and examine the value as R -> R0, once as R>R0 and once as R<R0. 3. The attempt at a solution As R->R0, the sphere itself would contribute 4πσ to the electrical field, whereas the contribution of the hole should be -2πσ, hence the electrical field at the center of the disc-shaped hole should be 2πσ. I am quite positive this is not how it should be done, moreover am inclined to believe that is not the correct value of the requested field. I'd appreciate some guidance, please.