Consider two points 'a' and 'b' in an electrostatic field of a single isolated point charge +q.
If a unit positive charge 'q' moves from 'a' to 'b' without acceleration, then the potential difference between 'a' and 'b' is given as
= - Edl
But dl = - dr
[This is because when we move a distance 'dl' towards the source, we move in the direction of decreasing of 'r']
From equation (1), we have
If the point 'a' is at infinity, then
From the above, it is evident that for a given charge 'q', potential depends only on 'r'. Therefore, if the charge is positive, potential is positive and if the charge is negative, potential is negative.
