Calculate the electrostatic potential a distance z from the center of a charged disk, charge
density p Coulombs per square meter, radius a.
I assume you mean a perpendicular distance. Lets say the disk lies in the $\displaystyle xy$-plane (centered at the origin) and we want the electrostatic potential along the $\displaystyle z$-axis. The the distance from the disk area element to point $\displaystyle (0,0,Z)$ is
$\displaystyle \delta = \sqrt{r^2+Z^2}$
The the electrostatic potential is given by
$\displaystyle \phi_E = \frac{\rho}{4\pi\epsilon_0}\int_0^{2\pi}\int_0^a\f rac{r\,dr\,d\theta}{\sqrt{r^2+Z^2}}$
This is a straightforward integral and so the potential is given by
$\displaystyle \phi_E = \frac{\rho}{2\epsilon_0}\left(\sqrt{a^2+Z^2}-|Z|\right)$
If you need to find the potential off the $\displaystyle z$-axis then the integral is a little tricker. It is given by
$\displaystyle \phi_E = \frac{\rho}{4\pi\epsilon_0}\int_0^{2\pi}\int_0^a\f rac{r\,dr\,d\theta}{\sqrt{(r-R)^2+Z^2}},$
where $\displaystyle R$ is the off-axis distance. If you need this in closed form it can be done but from the wording of the first post it sounds like you only need the on-axis solution
Hope that helps.