Gaussian Surface Electric Field Problem

1. Feb 4, 2014

wompkins

1. The problem statement, all variables and given/known data
An uncharged spherical thin metallic shell has a point charge q at its center. Find expressions for the electric field
1. Inside the shell
2. Outside the shell
3. has the shell any effect on the field due to q?
4. has the presence of q any effect on the shell?
5. if a second point charge is held outside the shell, does this outside chage feel a force
6. does the inside charge feel a force?
7. what is the apparent contradiction to Newton's 3rd Law
8. What is the resolution to the apparent contradiction?

2. Relevant equations

∫E*dA = Q/ε_0 where Q is net enclosed charge

3. The attempt at a solution

1. E-field inside metallic shell is 0
2. Outside it is like that of a point charge E = kQ/4πε_0
3. Yes the shell "shields" the field inside the shell to zero but outside acts like a normal E-field
4. It will polarize the metallic shell creating the E-field from the surface of the shell
5. Yes it will feel a force and it will be equal to qE where q is the second charge outside the shell and E is from part 2
6. The inside force will NOT feel a force, otherwise it would move
7. How can one feel a force but the other cant is the contradiction
8. The fact that the metallic shell "shields" the charge and get polarized by both charges allows the inner charge to feel no force

I feel like I get the first 3 parts, but 4-8 are just my intuition and was wondering if it was correct train of thought. If not just give me a hint in the right directions please!

2. Feb 5, 2014

ehild

Wrong. Surround the central charge with a concentric sphere of radius r less then the radius of the metal sphere. What is the electric field at r?

It shields the central charge from outside fields, but not from its own field.

[5]Yes it will feel a force and it will be equal to qE where q is the second charge outside the shell and E is from part 2
[6]The inside [STRIKE]force[/STRIKE] charge will NOT feel a force, otherwise it would move
[7]How can one feel a force but the other cant is the contradiction
[8]The fact that the metallic shell "shields" the charge and get polarized by both charges allows the inner charge to feel no force

I feel like I get the first 3 parts, but 4-8 are just my intuition and was wondering if it was correct train of thought. If not just give me a hint in the right directions please![/QUOTE]