Question

Electrostatics: You have a hollow sphere with a charge -2Q it has an inner radius R1 and outer radius R2 there is nothing inside the hollow cavity. Find the charge density at R1 and R2.

Answer 1

My instructor told us this in lecture. I have written: "We can argue that the charge density on the inner surface at R1 is =0. The reasoning is that E lines must originate from + and terminate at - charges.
If it is minus charged in the inner space there needs to be a positive charge to originate from but E=0 so that cannot happen"

Answer 2

Is what I have drawn. I do not understand what he meant at all. I don't know if he meant that the -2Q was on the inside or if that was just a hypothetical and the -2Q is still residing elsewhere.

Answer 3

Charges will always lie only on the outer surface.. assuming its a conductor ofcourse.. and therefore.. what he is trying to say is that inside the conductor, charge must be zero and hence the charge density is also zero!

Answer 4

Could you explain that to me? I'm trying to reason through it. I'm stuck.

Answer 5

Is it because they would repel each other until they could get furthest away?

Answer 6

ys exactly.. charges in a conductor will always try to move away from each other as much as possible.. and in doing so they would always end up being on the surface .. :)

Answer 7

I would look into shell theory! If I remember correctly, that might be helpful!

Answer 8

Thanks! I'll look into shell theory too!