# Electric Field problem.

1. Apr 6, 2014

### Prashasti

Question : Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude 17.0*10^-22. What is E (Electric field) : (a) in the outer region of the first plate, (b) in the outer region of the second plate?

Answer: According to the book, answers are (a) 0 (b) 0 And the reason they've given is - "Net electric field is Zero because electric fields due to both the plates will cancel out each other".

My objection : How? How do they cancel each other's effect when that particular region is NOT EQUIDISTANT from each other, and as Electric field varies inversely with the square of distance, HOW ARE THEY EQUAL IN MAGNITUDE?

2. Apr 6, 2014

### Simon Bridge

... this is only correct for sherical charge distributions - i.e. a sphere or a point charge.

Why is it important that the plates are "large"?

The electric field from two sheets of charge like that is the sum of the fields due to the single sheets alone.
What is the electric field due to a single large sheet of charge?

3. Apr 6, 2014

### Prashasti

I got it. Thank you so much. I think it is E = sigma/2×vacuum permittivity?!

4. Apr 6, 2014

### Simon Bridge

Well done - yeah: the field due to a large plate (except near the edges) is uniform.
Since the plates have opposite signs, their fields cancel everywhere except in between them (and by the edges) where they add together.