Much of the material making up Saturn’s rings is in the form of tiny dust grains having radii on the order of 10-6m. These grains are located in a region containing a dilute ionized gas, andthey pick up excess electrons. As an approximation, suppose each grain is spherical, with radius R = 1.0 × 10-6m. How many electrons would one grain have to pick up to have a potential of -400 V on its surface (taking V = 0 at infinity)?