# Electric field

1. Sep 29, 2005

### stunner5000pt

A large flat nonconducting surface carries a uniform surface charge density of sigma. A small circular hole of radius R has been cut out from this surface as in the figure. Ignore the fringing of the fierld lines around all the edges and calculate the electric field at point P located a distance z from the centre of the hole. (Hint says to refer to the electric field due to a charged disc and use superposition)

well im using the hint and thinking that the field would the electric field due to a nonconducting surface less the electric field due to this charged disc.

so would it be $$E = \frac{\sigma}{2\epsilon_{0}} - \frac{\sigma}{2\epsilon_{0}} (1- \frac{z}{\sqrt{z^2+r^2}}) = \frac{\sigma z}{2 \epsilon_{0} \sqrt{z^2+R^2}}$$

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2. Sep 29, 2005

### Staff: Mentor

Looks good to me.