Electric potential of dust

Homework Statement

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 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 sphereical, with radius R=1.0*10^-6m. How many electrons would one grain have to pick up to have a potential of -400V on its surface. (taking V=0 at infinity)?

V=U/q
V=-W/q

The Attempt at a Solution

not quite sure where to start. I think V=-400=U/q but I don't know how to get U. I'd like to understand why U is relevant in this question also. Thanks

SammyS
Staff Emeritus