Given

Radius of disk = R

Magnitude of charge distributed on the disk = Q

We have a disk of radius R, with uniformly distributed charge Q on its surface. For calculating the potential on the axis of this disk let us take a point Z on the axis of the disk at a distance x from the center of the disk, perpendicular to the disk. Now to determine the potential of this disk at Z, we start by considering the disk to be made of elemental rings of thickness dr and radius r. The potential of the disk at point Z can then be calculated by summing up the potential of these elemental rings.

Thus the charge on each ring will be

And the potential due to the ring at point Z is then,

Now R’ is the distance of a point on the ring to the point Z, which can be obtained by using the Pythagoras’s theorem:

Therefore, the total electric potential due to the disk is then obtained by summing or integrating the potentials due to all the elemental rings at point Z,

Now for the disk , therefore the potential on the axis of the disk is: