1. The problem statement, all variables and given/known data I'm having trouble understanding my book's explanation of the electric field within a conductor with a cavity. http://imgur.com/mjHRQoq [Broken] 2. Relevant equations EA = qenc/ε 3. The attempt at a solution So I understand the first two pictures, the electric field is 0 because the Gaussian surface does not contain any charges. Then on the third image I am confused when a positive charge is introduced in the cavity of the conductor. 1. Where do the random negative charges come from in the last picture? 2. Why does the electric field have to be zero for all points on the surface? If you look at the picture in the middle (b), the picture shows the charges all on the outside surface. If you introduce a positive charge, q, then q(enclosed) will not be zero and the electric field is not zero anymore. Instead, the book introduces random negative charges just to make q(enclosed) = 0. Couldn't I just randomly introduced negative charges on the interior of the Gaussian surface for an empty cavity then and say that electric field is not zero?