# Homework Help: Find the electric field a distance z from the centre of spherical

1. Nov 30, 2011

### skanda9051

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

Find the electric field a distance z from the centre of spherical sphere of radius R which carries uniform density B. treat Z<R (inside) and Z>R (outside)………. By using law of cosine how to solve this problem?

2. Relevant equations

1/4∏εo∫ (σ da/ r^2) cos(theta)
3. The attempt at a solution
substituting r by law of cosin.. r=√R^2+z^2-2Rzcos(theta)
1/4∏εo ∫[σ da/ R^2+z^2-2Rzcos(theta)]cos(theta)
i am stuck here what should i do next

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2. Nov 30, 2011

### danielakkerma

Re: electrodynamics

Hi...
Is the charge density given by a surface density, or a volume density? When talking about a sphere and not a spherical shell, one ought to remember that the inside is filled(in this case with charge).
In any case,
Try the following:
$dA = rdr\sin(\theta)d\theta d\phi$
$dV = r^2dr\sin(\theta)d\theta d\phi$
Phi goes from 0 to 2pi,
Theta goes from 0 to Pi.
r from zero to R.
Try it,
Daniel