# Homework Help: Electric field 0.1mm above charged plate.

1. Sep 19, 2009

### megr_ftw

1. The problem statement, all variables and given/known data
A thin 0.08x0.08 meter copper plate is charged with 9*10^9 electrons. What are the strength and direction of the electric field 0.1mm above center of the top surface of the plate.

2. Relevant equations
n=Q/A
E=n/E_0

3. The attempt at a solution\
I found the linear charge density to be 2.25*10^-7 c/m^3 from the first equation above. I multiplied the number of electrons by the charge of a single electron and divided by the area.

Next I took the linear charge density and divided it by epsilon_0 to get a final charge of 25423.72 N/C.
I plugged this into webassign but it says thats wrong? could someone check this for me because I swear this is the right way to go about this problem
thanks

2. Sep 19, 2009

### Redbelly98

Staff Emeritus
That equation is for a pair of oppositely-charged plates.

For a single plate, it's

E = n/2εo

They might want you to watch the number of significant figures in the answer you report, too.

Other than that, things look okay to me.

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