Electric field exerted at P due to a non-conducting disk

[binbagsss]

A non-conducting disk has radius 1.25cm and charge 6.55nc.
What is the electric field (magnitude) experienced at a point P - where p = 2cm and is on the x-axis.
The disk's centre is placed at the origin.


I approached this problem using the general formula derived:

k/2Σσ [1 - (1/((R^2/X^2 +1)^1/2))]

However I have no idea what the surface linear charge density should be due to dQ and dR.

Thanks guys, really appreciated ... =]

 

[tiny-tim]

… I have no idea what the surface linear charge density should be due to dQ and dR.

ohh! you mean the linear charge density for the ring of radius r to r+dr ?

well, the charge on the ring is dQ = Q times area of ring / area of disc

so the linear charge density is that divided by the length (ie the circumference of the ring)

 

[binbagsss]

No worries - solved :)