Hey, I recenty took an exam and the professor asked us to turn in a corrected version of the exam based on the comments he made to us (I guess people didn't do as well as he expected). On one of the problems it asks: How much work does it take for you to move a charge Q from infinity to the center of the sphere (all charge is located at the surface)? I already calculated the potential from infinity to the surface of the sphere as well as the work required to create the sphere. When I originally did the problem, I did this: W=(1/2)*Q*(Work to create sphere) He made a comment circling that and then: Q*delta-V(due to q=4 pi R^2) from infinity to R (R=surface of sphere). I don't really know what the professor means by that and I can't really ask him right now either. Does anyone have an idea what that comment is supposed to mean or how I should be correcting this problem?