If you close the poles of a charged capacitor together, 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 one.
However, in the process of discharging a capacitor, the electric field acting on moving charged particles quickly weakens until it disappears completely. Therefore, the flow of electric current that has arisen in the discharge circuit is of a short-term nature and the process quickly decays.
To maintain current in a conducting circuit for a long time, 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.
As a result of the electrochemical processes occurring in them, opposite electrical charges accumulate on their terminals. Forces of a non-electrostatic nature, under the action of which such a distribution of charges is carried out, are called external forces.
The following example will help to understand the nature of the concept of EMF of a current source.
Imagine a conductor in an electric field, as shown in the figure below.figure, 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 is what happens to the charge carriers when they reach the end of the conductor, and whether this current will remain the same over time.
We can easily conclude that in 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 a conducting circuit, this circuit must be closed, i.e. be in the shape of a loop. However, even this condition is not sufficient to maintain the current, since the charge always moves towards a lower potential, and the electric field always does positive work on the charge.
Now after traveling through a closed circuit, when the charge returns to the starting point where it started its journey, the potential at this point should be the same as it was at the beginning of the movement. However, the flow of current is always associated with a loss of potential energy.
Consequently, we need some external source in the circuit, on the terminals of which a potential difference is maintained, which increases the energy of movementelectric charges.
Such a source allows the charge to travel from a lower potential to a higher one in the opposite direction to the movement of electrons under the action of an electrostatic force trying to push the charge from a higher potential to a lower one.
This force, which causes the charge to move from a lower to a higher potential, is called the electromotive force. The EMF of a current source is a physical parameter that characterizes the work expended on moving charges inside the source by external forces.
As devices that provide the EMF of the current source, as already mentioned, batteries are used, as well as generators, thermoelements, etc.
Now we know that the battery, due to its internal EMF, provides a potential difference between the source leads, contributing to the continuous movement of electrons in the opposite direction to the electrostatic force.
EMF of the current source, the formula of which is given below, as well as the potential difference is expressed in volts:
E=Ast/Δq,
where Astis the work of external forces, Δq is the charge moved inside the source.
