# Homework Help: Calculating electric fields

1. Sep 15, 2008

### ibaraku

1. The problem statement, all variables and given/known data
Two concentric plastic spherical shells carry uniformly distributed charges, Q on the inner shell and -Q on the outer shell. Find the electric field (a)Inside the smaller shell, (b)between the shells, and (c) outside the larger shell

2. Relevant equations

integration(E * n)dA = (4pi) (k) (Qenclosed)

3. The attempt at a solution

In order to get hteh electric field between the shells, we can say that

[abs(E) * abs(n) cos delta](4pi) r^2 = (4pi) (k) (Qenclosed)

and working out the algebra it comes out to be

E = kQ/pi r^2

but what about for the charge outside the larger shell and the charge inside the smaller charge???
Is it safe to say that the we need to divide by 2 for the smaller shell and multiply by 2 for the large shell???
Thanks

2. Sep 16, 2008

### Defennder

Yes that's right.
You do this the same way as you did the above, except now the Gaussian surface for (a) cuts through the smaller sphere, and in (c) the Gaussian surface encloses both spheres. There isn't any way you can use the result obtained above to compute for the other two cases. The method used is still valid, though because of symmetry.