1. The problem statement, all variables and given/known data A square plate of edge length 9.0 cm and negligible thickness has a total charge of 6.3 x 10-6 C. (a) Estimate the magnitude E of the electric field just off the center of the plate (at, say, a distance of 0.50 mm) by assuming that the charge is spread uniformly over the two faces of the plate. (b) Estimate E at a distance of 62 m (large relative to the plate size) by assuming that the plate is a point charge. 2. Relevant equations E = magnitude of electric field Omega = Surface Charge Density e = 8.85 x 10^-12 C^2/Nm^2 pi = pi (ie 3.14...) r = radius q = Charge E = (Omega)/(2e) and then I used E = [ 1/(4pie) ] [ q/r^2 ] 3. The attempt at a solution Well, I first used the second equation there with q = 6.3 x 10^-6 C and r = .5 mm = .0005 m to find the magnitude of the electric field. And then I plugged what I got there into E = (Omega)/(2e) to find Omega. That was wrong :(. What am I doing wrong? Is there another equation?