1. The problem statement, all variables and given/known data A hollow insulating sphere of radius R is charged uniformly to a charge of Q. There is a small hole on this sphere. What is the electric field strength at the centre of this hole? 2. Relevant equations 3. The attempt at a solution I think I have to use Gauss's law here but what should be the Gaussian surface here? I am thinking of a small cylinder around the hole. Correct? Please see the attachment. ##E_1## is the electric field just above the hole (outside) and ##E_2## is the electric field inside. From Gauss's Law, ##E_1S-E_2S=0 \Rightarrow E_1=E_2##. Am I going in the right direction? What should be the next step?