# Homework Help: Electric field strength

1. Jun 14, 2008

### ntk

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

Two plates each have an area of 80cm2 and are placed facing one another in a vacuum. If the top plate carries a positive charge of 25nC and the bottom plate carrie, a charge of -25nC. Find the electric field strength between them.
Permitivity of free space = 8.9 * 10^-12
2. Relevant equations
Electrical field strength = surface charge density/permitivity of free space

3. The attempt at a solution
Should I take the difference of the charge between two plates, which is 50nC, and divide it by the surface area to find the charge density or just use the charge of one plate which is 25nC ?
Thank you.

2. Jun 14, 2008

### Staff: Mentor

The latter. Surface charge density = charge on one plate/area of plate.

3. Jun 15, 2008

### ntk

Yeah, i think so. But at first, i thought if there is a positive particle between the plates then there will be two forces acting on the particle. Add them up and divide it by the charge then we will get the field strength ??

4. Jun 15, 2008

### Staff: Mentor

The equation you are using already includes the effect of both surface charges. The field from a single sheet of charge would be half that:
$$E = \frac{\sigma}{2\epsilon_0}$$