# Electric Field between two metal plates

1. Mar 8, 2010

### skibum143

1. The problem statement, all variables and given/known data
Two square metal plates are placed parallel to each other, separated by a distance d= 2.34 cm. The plates have sides of length L= 0.750 m. One of the plates has charge Q=+ 2.70 x10-3 Coulombs, while the other plate has charge -Q. What is the magnitude of the electric field between the plates, not close to the edge?

2. Relevant equations
C = AE0 / d
E = kQ/r

3. The attempt at a solution
I know how to find the capacatance of the plates, but I don't know how to translate that to the electric field?
I tried to just use kQ/r, but that was incorrect. I'm not sure where I'm going wrong, could someone help?
Thanks!

2. Mar 8, 2010

### collinsmark

Hello skibum143,

E = kQ/r won't help you with this problem. That equation is more akin to the electric field for a line (wire) charge. You should start with an equation that applies to the electric field of a uniformly charged plate (plane). If you can't find one, you can use Guass' law to derive it for an infinite plane, and use that as an approximation here. Then use superposition to combine the respective electric fields of each plate.

3. Mar 8, 2010

### skibum143

If I use the equation for the E field of a sheet, it's Esheet = sigma (q/area) / 2E0.
If I do 2 * the answer for the two sheets, I get 5.4E8 N/C.

However, don't I need to factor in the distance between the plates? I'm not sure how to do that...

4. Mar 8, 2010

### collinsmark

The distance between the plates is small compared to the length of each side of the plates. That means the approximation of using an infinite plane should be pretty good in the region "between the plates, not close to the edge."

5. Mar 8, 2010

### skibum143

I see! Thank you for your help!!