# Homework Help: Couple of quick q's

1. Jan 28, 2009

### latentcorpse

ok if you could just tell em if this sounds alright.

(i) A large horizontal sheet of surface charge density $\sigma$ has a hole of radius b cout out of it. Find the electric field directly above the centre of the hole.

I already have expressions for the electric field due to the sheet and the disc of radius b on their own - can i just use the principle of superposition to subtract the field of the disc of radius b from the field of the sheet to get the total electric field?

(ii) By applying a Gaussian pillbox to an infintely large charged sheet of surface charge density $\sigma$, I obtained $\vec{E(z)} = \frac{\sigma}{2 \epsilon_0} \vec{z}$ for the electric field. I am now asked why this field would double if the same charge were placed on the surface of a conductor.

I have a feeling this is because in the conductor the charges inside arrange themselves to give a net internal field of 0 and then this arrangement of the charges would cause a field to be generated outside the conductor equal to the initial external field, hence doubling it as a result of the superposition principle.

How do these ideas sound?

2. Jan 28, 2009

### turin

Of course you can.