# Homework Help: Gaussian spheres

1. Sep 25, 2008

### antingtom

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

Consider a metal sphere and spherical shells. The innermost one is solid with radius R_1. A spherical shell surrounds the sphere and has an inner radius R_2 and an outer radius R_3. The sphere and shell are again surrounded by a shell of inner radius R_4 and outer radius R_5. None of the objects initially have a net charge. A negative charge -Q_0 is placed on the inner sphere and a positive charge +Q_0 is placed on the outermost shell.

i) After the charges have reached equilibrium, what will the direction of the electric field btwn the inner sphere and the middle shell?
ii) What will be the charge on the inner surface of the middle shell?
iii) What will be the charge on the outer surface of the middle shell?
iv) What will be the charge on the inner surface of the outermost shell?
v) What will be the charge on the outer surface of the outermost shell?
vi) What will the electric field plot look like?

2. Relevant equations

E.ds=Q(enclosed)/ε

3. The attempt at a solution

i) Does the fact that it's a metal sphere mean it's a conductor? If so, won't #1 be 0?
ii) inner surface of middle shell: E=1/4πε * -Q_0/(R_2)^2 ? (not 0 because of negative charge from inner sphere?)
iii) outer surface of middle shell: E=1/4πε * +Q_0/(R_3)^2 (I'm not sure if it would "feel" the Q_0 charge since r<R)
iv) inner surface of outermost shell: E=0
v) outer: E=1/4πε * +Q_0/(R_5)^2

I am really trying to understand this so I'd appreciate some help.

2. Sep 25, 2008

### Defennder

They're asking for the E-field in between conductors metal sphere and metal shell. So it's not the E-field in a conductor
They want the charge, not the E-field. Keep in mind the E-field in a conductor. Then apply Gauss law such the Gaussian surface is concentric within the middle conductor. What should the charge on the inner surface of the middle conductor be?
This follows from the previous part. Now that you have the charge on the inner surface of the middle conductor, what charge would have to be on its outer surface?
This is almost the same as ii). Just use an appropriate gaussian surface.
Use symmetry to deduce the direction of the E flux lines.