[UPSC CSE 1991] Electrostatics and Magnetostatics (Q271, 60M)

Show that the electric field intensity due to any distribution of charges at rest can be expressed as the gradient of a potential. What is the relation between potential and potential energy? A thin disc of radius R is uniformly charged, \sigma being the charge per unit area. Find the potential and the electric intensity at points on the axis of the disc. How do these change as one crosses the disc? Explain the changes physically.