1. The problem statement, all variables and given/known data A square insulating sheet 75.0cm on a side is held horizontally. The sheet has 7.40nC of charge spread uniformly over its area. Calculate the electric field at a point 0.240mm above the center of the sheet. 2. Relevant equations [itex]\Phi[/itex] = 2EA E = [itex]\sigma[/itex] / 2[itex]\epsilon[/itex] 3. The attempt at a solution With the formulas above I managed to find that [itex]\sigma[/itex] = 1.316 * 10^-8, but I do not know that [itex]\epsilon[/itex] represents or how to find it. I cannot find the electric field until epsilon has been found.