Electric Charge on a uniformly charged disk

[aquabug918]

Last question I promise,,,,

A uniformly charged disk has a radius of 2.5 cm and carries a total charge of 4*10^-12C.

Find the electric field on the x-axis at a distance of 20cm away.

I used the equation:

Ex = (sigma/ (2*Eo)) * (1 - ( 1/ sqrt( ( R^2/x^2) + 1))

this is what i did so far

Ex = (sigma/ (2*8.85*10^-12)*(1 - ( 1/ sqrt( ( 0.025m^2/0.002m^2)-1)

I am not sure how to calculate sigma in this case. I am guessing that it is the total charge dived by the area. But, i think i am missing something.

The answer is 0.89 N/C

Thanks!

 

[maverick280857]

No you are not missing anything. $\sigma$ is indeed the surface charge density and is equal to the total charge divided by the total area.

You could have verified this by dimensional analysis too. The expression really isn't as complicated as it seems.