What is EMF current source?

If the poles of a charged capacitor are closed among themselves, then under the influence of the electrostatic field accumulated between its plates, in the external circuit of the capacitor in the direction from the positive to the negative pole the motion of the charge carriers-electrons begins.

However, during the discharge of the capacitor, the electric field acting on the moving charged particles rapidly weakens until it disappears completely. Therefore, the flow of the electric current that has arisen in the discharge circuit is of a short-duration nature and the process rapidly decays.

For long-term maintenance of current in the conducting circuit, devices that are inexactly called current sources of current (in a strictly physical sense this is not so) are used. Most often such sources are chemical batteries.

Due to the electrochemical processes taking place in them, an accumulation of unlike electric charges takes place at their terminals . Forces not of electrostatic nature, under the action of which such a distribution of charges is realized, is called external forces.

To understand the nature of the EMF concept of a current source, the following example will help.

Imagine a conductor in an electric field, as shown in the figure below, that is, in such a way that there is also an electric field 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 about 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 in the case of an open circuit, as a result of the influence of the electric field, the charges will accumulate at the ends of the conductor. In this connection, the electric current will not remain constant and the motion of electrons in the conductor will be very short, as shown in the figure below.

Thus, in order to maintain a constant flow of current in the conducting circuit, this circuit must be closed, i.e. Have the form of a loop. However, to maintain current, even this condition is not sufficient, since the charge always moves towards a lower potential, and the electric field always does positive work on the charge.

Now, after traveling along a closed circuit, when the charge returns to the starting point, where it began its journey, the potential at this point should be the same as it was at the beginning of the movement. However, the current flow is always associated with the loss of potential energy.

Therefore, we need some external source in the circuit, on the terminals of which a potential difference is maintained, which increases the energy of the motion of electric charges.

Such a source makes it possible to travel a charge from a lower potential to a higher one in a direction opposite to the motion of electrons under the action of an electrostatic force trying to push a charge from a higher potential to a lower one.

This force, which forces the charge to move from a lower to a higher potential, is usually called an electromotive force. EMF current source - a physical parameter that characterizes the work spent on moving charges inside the source by external forces.

As the devices providing 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 terminals, facilitating the continuous movement of electrons in the direction opposite to the action of the electrostatic force.

The EMF of the current source, the formula of which is given below, like the potential difference is expressed in volts:

E = A _st / Δq,

Where A _ст is the work of external forces, Δq is the charge moved inside the source.