Further consider the two points A and B as shown in the Fig. 2.9. Considering the movement of a unit positive test charge from B to A , we can write an expression for the potential difference as:

..................................(2.26)

It is customary to choose the potential to be zero at infinity. Thus potential at any point ( r_A = r) due to a point charge Q can be written as the amount of work done in bringing a unit positive charge from infinity to that point (i.e. r_B = 0).

..................................(2.27)

Or, in other words,

..................................(2.28)