If we close the poles of a charged capacitor between themselves, then under the influence of the electrostatic field accumulated between its plates, the movement of charge carriers - electrons begins in the external circuit of the capacitor in the direction from the positive pole to the negative.
However, during the discharge of the capacitor, the electric field acting on the moving charged particles rapidly weakens until it disappears completely. Therefore, the occurrence of an electric current in the discharge circuit is of a short-term nature and the process decays quickly.
For long-term maintenance of current in a conducting circuit, devices are used that are inaccurately called current sources in everyday life (in a strictly physical sense, this is not so). Most often, these sources are chemical batteries.
Due to the electrochemical processes occurring in them, the accumulation of opposite electric charges occurs at their terminals . Forces of non-electrostatic nature, under the influence of which a similar charge distribution is carried out, are called external forces.
To clarify the nature of the concept of EMF of a current source, consideration of the following example will help.
Imagine a conductor located in an electric field, as shown in the figure below, that is, in such a way that an electric field also exists inside it.
It is known that under the influence of this field an electric current begins to flow in the conductor. Now the question arises of what happens to charge carriers when they reach the end of the conductor, and whether this current will remain unchanged over time.
We can easily conclude that with an open circuit, as a result of the influence of an electric field, charges will accumulate at the ends of the conductor. In this regard, the electric current will not remain constant and the movement of electrons in the conductor will be very short-lived, as shown in the figure below.
Thus, in order to maintain a constant current flow in the conductive circuit, this circuit must be closed, i.e. have a loop shape. However, even this condition is not sufficient to maintain current, since the charge always moves toward a lower potential, and the electric field always does a positive job of the charge.
Now, after traveling in a closed circuit, when the charge returns to the starting point, where he began his journey, the potential at this point should be the same as he was at the beginning of the movement. However, the flow of current is always associated with the loss of potential energy.
Therefore, we need some external source in the circuit, at the terminals of which a potential difference is maintained, which increases the energy of movement of electric charges.
Such a source allows the charge to travel from a lower potential to a higher one in the opposite direction to the motion of electrons under the action of an electrostatic force trying to push the charge from a higher potential to a lower one.
This force, causing the charge to move from a lower to a higher potential, is called an electromotive force. EMF of a current source is a physical parameter that characterizes the work spent on moving charges inside the source by external forces.
As devices that provide EMF of a current source, as already mentioned, batteries are used, as well as generators, thermocouples, etc.
Now we know that the battery due to its internal EMF provides a potential difference between the terminals of the source, contributing to the continuous movement of electrons in the direction opposite to the action of electrostatic force.
The EMF of the current source, the formula of which is given below, as well as the potential difference is expressed in volts:
E = A st / Δq,
where A st is the work of external forces, Δq is the charge displaced inside the source.