Both levels (potentials) are positive, but the mountain is more positive than the mine.
Let us now revert to our positively charged sphere. At infinite distance the potential is zero, and as we approach the sphere it becomes positive and grows in value inversely as the distance diminishes. Its greatest possible value is at the least possible distance, which is on its surface. The maximum value, the potential of the sphere, is on its own surface, and is numerically given by
where denotes the radius of the sphere in cm. For a negatively charged sphere the potential at infinite distance is also zero, and on its surface it is
A unit charge free to move will therefore fly from infinity towards the sphere and right on to it. If the unit charge is carried on some conductor having ponderable mass, this conductor would strike the surface of the sphere with a certain velocity. It is easy to determine this, since we know the total energy (namely, the potential difference between in-
