Potential field created by a disc at a point on its axis of revolution - electrostatic potential-Corrected exercises electromagnetism

Consider a disk its center O and its radius R.

This system is loaded on the surface with the surface density:
  a is a constant homogeneous to a length and r is the distance of a point P on the surface of the disc to its center .

  1. Determine the electrostatic potential at a point M of the axis  of revolution Oz of this disc.
    For the calculation is carried out the change of variable r = z u sh.
  2. Deduce the expression of the electrostatic field in point M.