Much of the material making up Saturn's rings is in the variety of tiny dust grains having radii on the order of 10–6 m. Such grains are placed in a region having a dilute ionized gas, and they pick up surplus electrons. As an approximation, assume each grain is spherical, with radius R = 1.3 x 10–6 m. Determine how many electrons would one grain have to pick up to encompass a potential of –420 V on its surface (taking V = 0 at infinity)?