1. The problem statement, all variables and given/known data A non-conducting disk with a 4-mm thickness is lying flat. It has a -4 C/m^2 surface charge on the upper surface and a surface charge on the lower surface. In terms of epsilon naught, what is the approximate field strength 1 mm above the upper surface? 2. Relevant equations E(disk)z = η/2ε0 * [1- ( z /√(z^2 +R^2)) ] 3. The attempt at a solution The radius R and charge are unknowns, I have no idea how to aproach this problem, Im stuck. I would like some help to interpret the known data so I can start to build an answer! Please, and thanks in advance for any suggestions.