A positively charged disk has a uniform charge per unit area s.

http://www.webassign.net/serpse8/23-figure-17.gif

The total electric field at P is given by the following.
Ex = 2 p ke s (1 – x/(R^2 + x^2)^1/2)
Sketch the electric field lines in a plane perpendicular to the plane of the disk passing through its center.