# Finding the electric field in all regions given uniform charge occupying a<r<b

1. Feb 21, 2009

### Joan

Hi, I hope this is the appropriate place and format etc. One of the questions I have to answer is this:

Use Gauss's law to obtain the field, everywhere, of charge of uniform density roe, occupying the region a<r<b, where r is the distance from the origin.

I am completely flummoxed as to what assumptions I can make..

I am assuming that since we are asked to use Gauss that I should use a spherical symmetry..
I also think that where r<a the field should be zero, but can I assume that the charge lies on the inner surface only for this region?
And where r>b should I treat the whole dstribution as a point charge?
Also, I have started trying to find E inside the a<r<b, using Q= integral(roe dV),and I end up with a term involving b^3-a^3.

If anybody could tell me if i'm on the right track of give me any pointers it would be very much appreciated.

Many thanks.

Last edited: Feb 21, 2009
2. Feb 21, 2009

### clem

For r<a, Gauss shows that E=0.
For r>b, you aare right about the point charge.
Just use Gauss carefully for a<r<b.
Avoid integral rho dV.

3. Feb 21, 2009

### Joan

Hey thanks. In that case should I just leave integral (E.da) = rho/epsilon, ie leaving rho as is?

4. Feb 22, 2009

### clem

No. Read up on using Gauss's law in your textbook. You are not using it correctly.

5. Feb 22, 2009