# Electrostatics BVP with Poissons

## Homework Statement

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http://shup.com/Shup/45816/Image-25.png [Broken]

## The Attempt at a Solution

As you can see, this is an example problem from my text. I'm looking for some clarification in their decision to assume potential exists only in the x direction. Specifically, I dont understand why they assumed the potential only varies in the x direction when the dialectric encloses a volume charge. I know when the charge is a surface charge on parallel plates with surface Area >> than the distance between them, we can assume the plates are of infinite size and so the electric field between them is normal to the surface. But here, the plates are BOTH at a potential of 0 V and the enclosed charged is a volume... So I'm hoping someone can elaborate on this.

I was also hoping someone can help me understand how they are using Gauss's law in excercise 5.1 to solve for the electric field. I tried many ways to solve it and I don't always get the answer they do. I'm thinking that you use the E-field found in the example to find E(x) at x=0 and x=d?
Thanks.

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