Walker, Halliday & Resnick — Electric Potential: Problem 9

9 A plastic rod has been bent into a circle of radius $R = 8.20\text{ cm}$. It has a charge $Q_1 = +7.07\text{ pC}$ uniformly distributed along one quarter of its circumference and a charge $Q_2 = -6Q_1$ uniformly distributed along the rest of the circumference (Fig. 24-27). With $V = 0$ at infinity, what is the electric potential at (a) the center $C$ of the circle and (b) point $P$, on the central axis of the circle at distance $D = 2.05\text{ cm}$ from the center?