What is the Electric Field of a Uniformly Charged Disk at Different Z Positions?

[fallen186]

Homework Statement

A nonconducting disk of radius a lies in the z = 0 plane with its center at the origin. The disk is uniformly charged and has a total charge Q. Find Ez on the z axis at the following positions. (Assume that these distances are exact.)

Z = a
[__________]Q / (a2εo) - This is the format the answer should be in.

Homework Equations

E_z = \frac{kQz}{(z^2+a^2)^{3/2}}

k = 1/(2pi*εo)

The Attempt at a Solution

1. E_z = \frac{kQz}{(z^2+a^2)^{3/2}}

I filled in 'z' and 'k'

2. E_z = \frac{Qa}{(a^2+a^2)^{3/2}*2\pi*\epsilon}

3. E_z = \frac{Qa}{(2a^2)^{3/2}*2\pi*\epsilon}

4. E_z = \frac{Qa}{8a^{3}*2\pi*\epsilon}

5. E_z = \frac{Q}{(16\pi*a^{2}*\epsilon}

6. 1/16\pi

7. [_.1963495408_]Q / (a2εo) * Its wrong and I don't know what I did wrong. Please help

 

[nodkrot]

Hey man u have to try this formula, the one u used is for rings, here is the link
http://www.google.com/url?sa=t&sour...tTF80rVQ&sig2=4gSh-oAP_H6dmE-tIZPk0w&cad=rja"