By definition, the potential in the case of a surface charge distribution is defined by integral 
In order to simplify integration, we shall choose the area element dS in the form of a part of the ring of radius r and width dr in (Fig.). Then dS - 2θ r dr, r = 2R cos θ and dr = - 2R sin θ d θ. After substituting these expressions into integral 
we obtain the expression for φ at the point O:
