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## Homework Statement

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)?

## Homework Equations

Electric field generated by a charged ring in distance r from its center

Gauss' law

## 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?