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.
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 ]
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?