Walker, Halliday & Resnick — Electric Potential: Problem 44

44 In Fig. 24-43a, we move an electron from an infinite distance to a point at distance $R = 8.00 \text{ cm}$ from a tiny charged ball. The move requires work $W = 5.32 \times 10^{-13} \text{ J}$ by us. (a) What is the charge $Q$ on the ball? In Fig. 24-43b, the ball has been sliced up and the slices spread out so that an equal amount of charge is at the hour positions on a circular clock face of radius $R = 8.00 \text{ cm}$. Now the electron is brought from an infinite distance to the center of the circle. (b) With that addition of the electron to the system of 12 charged particles, what is the change in the electric potential energy of the system?