Electric Potential and Energy

We know that a body raised above the ground level has a certain amount of mechanical potential energy which, by definition, is given by the amount of work done in raising it to that height. If, for example, a body of 5 kg is raised against gravity through 10 m, then the potential energy of the body is 5 × 10 = 50 m-kg.

wt. = 50 × 9.8 = 490 joules.

The body falls because there is attraction due to gravity and always proceeds from a place of higher potential energy to one of lower potential energy. So, we speak of gravitational potential energy or briefly ‘potential’ at different points in the earth’s gravitational field. Now, consider an electric field. Imagine an isolated  positive charge Q placed in air (Figure given below). Like earth’s gravitational field, it has its own electrostatic field which theoretically extends upto infinity. If the charge X is very far away from Q, say, at infinity, then force on it is practically.  zero. As X is brought nearer to Q, a force of repulsion acts on it (as similar charges repel each other), hence work or energy is required to bring it to a point like A in the electric field. Hence, when at point A, charge X has some amount of electric potential energy. Similar other points in the field will also have some potential energy. In the gravitational field, usually ‘sea level’ is chosen as the place of ‘zero’ potential. In electric field infinity is chosen as the theoretical place of ‘zero’ potential although, in practice, earth is chosen as ‘zero’ potential, because earth is such a large conductor that its potential remains practically constant although it keeps on losing and gaining electric charge every day.