# Homework Help: Electric field, hollow sphere

1. Apr 17, 2012

### underground

Hey im currently studying for my final and im stuck on a question i have the solution but im not sure what he did, could someone explain

A hollow sphere of the inner radius R1 and outer radius R2 is uniformly charges with total charge Q. Calculate the electruc fiels in the three regions shown in the cross-section view below 1) r<R1, 2) R1 < r <R2, and 3) r >R2

What i dont get is part two, i know he is using gauess law Q inclosed/e = E(r)*A
but i dont know how he got Q and reduced it

2. Apr 17, 2012

### tiny-tim

welcome to pf!

hi underground! welcome to pf!
the total charge is Q, so the charge density is Q/volume = Q/{4π/3(R23 - R13)},

so the charge inside radius r is the density times the volume within radius r, ie 4π/3(r3 - R13)

3. Apr 17, 2012

### underground

Thanks for the respond im getting there but can you explain one thing

I get what your saying but i dont see where the Q overall charge disappeard

This is my logic of the question and the Q which is overall charge remained in the equation

4. Apr 17, 2012

### tiny-tim

suppose R1 was 0 (ie, a solid sphere) …

then the charge within radius r would be Q(r/R2)3, wouldn't it?

this is similar … the charge is Q times (volume/total-volume)

5. Apr 17, 2012

### underground

Thanks so much for your help :)