Electric Potential

 

Electric Potential and Potential Difference

We know that when a body is lifted through certain height above the ground level, work is done against the force of gravity and is stored as a potential energy in the body. Hence such a body which is raised above the ground level can be said to have a gravitational potential. Obviously, the total potential energy and hence, the gravitational potential of a body is a function of its position with respect to earth. Here the example is very much analogous to the Electric Field around a charge.

Now consider that a small isolated positive charge q is placed at a far distance let’s say infinity with respect another positive charge Q. Theoretically the electric field of the positive charge reaches upto infinity but in the practical sense it’s influence on positive charge q which is placed at infinity is very negligible.

Fig. 1 : Two isolated charges 

If this charge q is shifted near to Q and brought at the same point X some amount of work is done  in bringing the charge q at X against the force of repulsion ( as both charges are positively charged when brought near or together they will repel each other because like charges repel each other). So while pushing the charge q against the force of repulsion the charge q will attain some potential energy which is potential.

As the charge reaches to point X it acquires energy than it had at the previous position or location where it was before so this creates a potential difference between the point X and the point where the charge q was rested before.

So as per above explanation we can conclude that potential is the characteristic of a location of a point which has potential. So potential is nothing but a possibility or a milestone for a charge. If the charge crosses that milestone it will acquire that amount of potential energy with respect to which it had before. But this potential energy is not stored at particular place or a point. That potential energy is acquired by the charge when work is done by that charge by moving from rest position to the milestone or to that potential point.

To elaborate this discussion further let us take above example when the charge q is shifted to the milestone or a potential point X it acquired some amount of potential energy with respect to which it had before. So this potential energy was not stored at that point X but in fact when the charge q was shifting towards the point X against the electric field some work was being done by that charge q while moving. Due to this work done the charge was acquiring the energy till it reached the potential point X.

So we can define the Electric Potential at any point in an electric field as equal to the work done in joules in moving a unit positive charge from infinity (which is considered as the point of zero potential) to that point against the electric field.

We have seen previously in Electric Field and Electric Charge that when we keep any certain charge into electric field the charge experiences a force on it. The force changes according to the intensity of electric field. So the force or energy experienced by the Electric charge when kept in the electric field is called as Electric Potential Energy.

The unit of electric potential is volt (V)

In practice we are more concerned with the potential difference (p.d) between two points in an electric field rather than the potential at one point with respect to infinity.

The potential difference between two points in the electric field is the work done, in joules in moving a positive charge from the point of lower potential to the point of higher potential. The unit of Potential difference is also volt (V).

Difference between Electromotive Force and Potential Difference: 

It is well known that heat flows from a body at a high temperature to a body at a lower temperature similarly like water flows from high head to low head. In this study of electrical engineering, electric potential is somewhat analogous to temperature and water level as per given in above cases. In the same way the current always flows from higher potential to lower potential in the electric circuits. It is that way similar to the pressure head of the water pump connected in the hydraulic system.

The difference between the electrical potentials or we can say pressures at any two given points in the electric circuit is known as potential difference (p.d).

To explain electromotive force we can state that, A potential difference generated by a source of electrical energy across its terminals which tends to produce an electric current in a circuit is called an Electromotive Force also abbreviated as e.m.f. The principal sources of e.m.f are primary and secondary cells, generators, thermo-junctions etc.

As the volt is unit for electromotive force as well as potential difference, both are often indiscriminately referred to as voltage. The identity of the unit quite often causes confusion between these two terms or quantities.

To understand the fundamental difference between these two terms let us consider one simple closed circuit which is made by shorting the two terminals of the battery or a cell.

Fig. 2 : Short circuited battery

Since the conductor is connected between the terminals of a battery, current flows from its positive terminal to the negative terminal (conventional direction of electric current). There are two energy transformations going on concurrently. Chemical energy is being converted into electrical energy by the battery and electrical energy is been converted into heat energy into the conductor (H = I2Rt).

 Whenever some form of energy capable of being converted into electrical energy which is introduced in any part of an electrical circuit, we say that an electromotive force is acting in the circuit. On the other hand, if between any two points in a circuit, electrical energy is being converted into any other form of energy, we say that potential energy is established between two points. Thus in the simple circuit above, we can say that the battery converting chemical energy into electrical energy is supplying an electromotive force and there exists a potential difference between any two points on the conductor where electrical energy is being dissipated in the form of heat.