# Electrostatics - infinite plates

1. Sep 30, 2009

### Gil-H

1. The problem statement, all variables and given/known data
Consider two infinite conducting plates of some thickness with charge 2q and -q.
The system is in equilibrium, which means there's no charged particles in motion.
Find the charge distribution on the surfaces of each plate.
(designated as q1 q2 q3 and q4)

2. Relevant equations

3. The attempt at a solution
All I know about infinite plates is that the electric field is σ/2ε0.
But that doesn't seem to help me here

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2. Oct 1, 2009

### Staff: Mentor

Use that to figure out the field between the plates.

Hint: What's the field within the conducting material? Apply Gauss's law.

3. Oct 1, 2009

### Gil-H

Thanks. You gave something to think about.
But I'm not sure I follow completely.

Because the field inside each plate is zero,
I used gaussian surface which encloses q2 and q3,
and that leads me to that:
q2 + q3 = 0,
and together with:
q1 + q2 = 2q
q3 + q4 = -q
I have three equations.

I need to get one more. How?

4. Oct 1, 2009

### Staff: Mentor

How about using some gaussian surfaces that enclose only one of the charges at a time?

5. Oct 2, 2009

### Gil-H

Now I got that!
Thank you very much!

By doing that I achieved that:
q1 + q2 + q3 = -q4
q2 + q3 + q4 = -q1

and with:
q1 + q2 = 2q
q3 + q4 = -q

I ended up with:
q1 = 0.5q
q2 = 1.5q
q3 = -1.5q
q4 = 0.5q

which is the correct answer! :-)