Chapter 26, Problem 73PQ

A Start with $V=2\pi k\sigma \left[\left(\sqrt{R^2+x^2}\right)-x\right]$ for the electric potential of a disk of radius R and excess surface charge density σ at a position x from the center of a disk on its axis, and derive an expression for the electric field at this position. Hint: See Example 24.6 (page 732) to check your answer.