Here's the question I am thinking about: If there is a spherical shell with uniform charge on it, I know the electric field inside the shell is zero and that outside the shell is calculated just the same with a single charge at the center of the sphere, but how about the field just at the shell (r=R)? I try to solve this problem by Gauss' Law. I can draw a gaussian surface just above and below the shell, but when I try to draw just at the shell, how many charges does the surface enclose? The charges are assumed to be infinitely small. When they are just at the gaussian surface, are they enclosed?