Much of the material comprising Saturn’s rings is in the form of tiny dust particles having radii on the order of . These grains are in a region containing a dilute ionised gas, and they pick up excess electrons. Assuming that the electric potential at the surface of a grain is , we have to estimate the number of excess electrons it has picked up.

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Potential on the surface of a charged sphere, charge q, radius, r, is

.

It is given that the typical radius of a grain in the Saturn’s ring is

,

and that the potential on the surface of a grain is

.

Electron charge is .

Therefore, the number of excess electrons on a grain will be

.