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## Homework Statement

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.

## Homework 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 ]

## 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?