A solid cylinder whose radius is $R$, rotates with a constant angular velocity $\omega$. The potential difference between surface of cylinder and the axis is:
(a) $\frac{m R^{4} \omega^{2}}{2 e}$
(b) $\frac{m \omega^{2} R^{2}}{2 e}$
(c) $\frac{m \omega^{2} R^{2}}{e}$
(d) none of these