# Homework Help: Electric field in an empty cavity

1. Dec 28, 2007

### hisashiburi

1. The problem statement, all variables and given/known data
The electric field inside an empty cavity in a conductor is zero. Is this statement true no matter what the shape of the cavity? Why or why not?

2. Relevant equations
Electric field equation. E = kQ / R^2

3. The attempt at a solution
I can't seem to figure out if this is true no matter what the shape.

2. Dec 28, 2007

### jacobrhcp

no, consider a charge in a cavity in a conductor.

it's only in the meat of the conductor, because when you would put a conductor in a field, the electrons would move and create a field with the protons on their own. The two fields tend to cancel. But in a hole, you don't have this problem. (The edges of the conductor will be charged however, in that case.)

3. Dec 28, 2007

### Dick

That's correct, but it's stated that the cavity is empty, so it doesn't contain a charge. There's no net charge density in the conductor and the cavity is empty, so the only place a charge could be is on the surface of the cavity. Now consider a gaussian surface around the cavity but inside the conductor.

4. Dec 28, 2007

### jacobrhcp

oh I'm sorry, ah well... I guess Dick gave you quite a hint there.