Charge On A Infinite Conductor Slab From An Infinite Sheet Of Charge

[Baumer8993]

Homework Statement

http://imgur.com/sg8czUR

An infinite sheet of charge has a charge density of σ= -2.1 uc/m². (uc is micro-coulombs). The inner edge of the infinite conductor slab is 2.6 cm away. The outer edge is 4.2 cm away. The conductor slab has a charge density of σ= 74 uc/m². What is the charge per unit area on the surface of the slab at 4.2 cm. (The outer edge)

Homework Equations

Gauss's Law!

The Attempt at a Solution

Well we know since the slab is a conductor the charges in it are going to move to make the electric field equal zero every inside of it. I have the inner being 2.1 uc/m². The total charge of the slab must be 74 uc/m². Therefore I thought the outer edge will have a charge of 71.9 uc/m². But this is not the right answer. Any idea of what I am doing wrong here?

 

[vela]

If you have 2.1 μC on the left surface of the conductor, the electric fields due to that charge and the sheet of charge will indeed cancel inside the conductor, but you would still have the rest on the charge on the conductor around, which would result in a non-zero field inside the conductor.

Look at it as three sheets of charge: the one through the origin, the one on the left surface of the conductor, and the one on the right surface of the conductor. The fields from the three sheets have to sum to zero inside the conductor.