# Homework Help: How to determine the surface charge density for a parallel plate capacitor

1. Sep 10, 2012

### ppoonamk

1. The problem statement, all variables and given/known data

The problem statement is given in the figure below

2. Relevant equations

E=σ/(2* ε)

3. The attempt at a solution

The top plate is assumed to be negative charge. The bottom plate is assumed to be positive charge. Since negative charge pulls and positive charge pushes.

surface charge density for upper surface of the top plate = σ1
surface charge density for lower surface of the top plate = σ3

surface charge density for upper surface of the bottom plate = σ2
surface charge density for lower surface of the bottom plate = σ4

So I tried to derive σ3 and σ4 in terms of σ1 and σ2

I know that the electric field in the metal plates is 0.
So the electric field in the top plate = σ1/(2* ε)+σ2/(2* ε)+σ4/(2* ε)-σ3/(2* ε)=0

Electric field in the bottom plate = 0= σ1/(2* ε)+σ2/(2* ε)+σ3/(2* ε)-σ4/(2* ε)

How do I get σ3 and σ4 in terms of σ2 and σ1

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