1. The problem statement, all variables and given/known data A cylinder (hollow) with radius R is charged with charge Q on its outer side. What is the electric field at a point A on its center axis (inside the cylinder)? 2. Relevant equations Electric field generated by a charged ring in distance r from its center Gauss' law 3. The attempt at a solution I seem to have misunderstood something here..I could look at a ring of width dz and calculate its contribution to the electric field at the point A on the center axis of the cylinder. This will definitely sum up to something, meaning there will be a field at point A. On the other hand, if I use Gauss' law, I take a cylinder surface of radius r<R surrounding the center axis. Obviously Qin=0, so by conclusion the electric field in any point where r<R is 0. where's my mistake?