In Fig. 33.1 the conductor A has a positive charge and a positive potential, while conductor B has a negative charge, and hence a negative potential. If A and Bare connected to earth electrons will flow from earth to A and from B to earth until both
A and B are at earth potential. Since the potential of A is above earth potential, while that of B is below earth potential, it is clear that electrons tend to move from one place to another where the potential level is higher. In other words, electrons tend to move up the potential gradient. On the other hand, if positive charges were able to move they would tend to move down the potential gradient. In using the word gradient we are treating potential as a kind of electric level, just as we refer to the gradient on a road when it goes uphill or downhill.
In spite of what we have just said about the potentials of A and B, it must not be assumed that positive and negative charges always have positive and negative potentials respectively. A charged conductor not only has a potential due to its own charge but also a potential due to the presence of other charges which may be in its neighbourhood. A charge raises (or lowers, as the case may be) the potential of all points in its neighbourhood by an amount which decreases with distance from the conductor. It is therefore possible for a positively charged conductor to have a negative potential if another conductor with a large enough negative charge happens to be near by.