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## Homework Statement

I came across an expression in the following pdf at the bottom of page two:

http://iweb.tntech.edu/murdock/books/v4chap2.pdf

## Homework Equations

The electric field for a disk:

[itex]\vec{E} = \frac{\sigma}{2{\epsilon}_{0}} ( 1 - \frac{z}{\sqrt{{z}^{2} + {r}^{2}}})[/itex]

Now logically It will be via gauss's law:

[itex] {\phi}_{E} = \iint \vec{E} \cdot d\vec{S} = \frac{{Q}_{tot}}{{\epsilon}_{0}} [/itex]

## The Attempt at a Solution

I'm not best sure how to approach this, it is in the z direction which suggests the Ex and Ey fields are zero so this disk would lie in the z plane - which is fine. But i can't quite put my finger on the trick here.

Thank you,

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