Much of the material making up Saturn's rings is in the form of tiny dust grains having radii on the order of 10-6 m. These grains are located in a region containing a dilute ionized gas, and they pick up excess electrons. As an approximation, suppose each grain is spherical, with radius R = l .0 x 10-6 m. 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)?