# Homework Help: Equipotential, can someone check my work?

1. Sep 20, 2011

### pyroknife

A solid sphere of +20C with radius a=1m. A conducting spherical shell of -30C with radius b=2m and c=3m is concentric with the solid sphere.

1. Find the inner and outer surface charge of the conducting spherical shell.

My attempt: I just eyeballed it and said -20C is on the inner shell b/c the +20C is attracting it and the rest -10C is on the outer shell.

2. Graph V vs r(the radius from the center).

My attempt: I calculated V at each of the regions.
region 1: r<1 the V due to a solid sphere is (kQ/2R)(3-r^2/R^2)=30k-10r^2
region 2: 1<r<2 V=kQ/r=20k/r
region 3: 2<r<3 r=0
region 4: r>3 V=-10k/r

I attached my graph which i drew on paint.
you can't see 0 between 2&3 but it's there.

2. Sep 20, 2011

### pyroknife

Woops I forgot part 3.
Question 3 was to draw the electric field lines. I just drew 4 lines from the solid sphere to the inner radius of the spherical shell with the arrows pointing towards the shell. I drew another 4 lines approaching the outer spherical shell. There is no lines going thru in between the inner&outer radius b/c E=0 inside a conductor.

3. Sep 20, 2011

### pyroknife

112 views and no replies? :(