If we close the poles of the chargedcondenser, 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 capacitorThe electric field acting on moving charged particles is rapidly weakened until it disappears completely. Therefore, the flow of an electric current that has arisen in the discharge circuit has a short-term character and the process rapidly decays.
For long-term maintenance of the current in the conductingcircuits are used devices, inaccurately called in everyday life by current sources (in a strictly physical sense this is not so). Most often such sources are chemical batteries.
Due to the electrochemicalprocesses on their terminals there is an accumulation of unlike electric charges. Forces of non-electrostatic nature, under the influence of which a similar 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 inconductor begins to flow electrical current. 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 when thechains as a result of the influence of the electric field 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.
Now after traveling along a closed circuit, whenthe 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 connected with the loss of potential energy.
Consequently, we need some external source in the circuit, at the terminals of which a potential difference is maintained, which increases the energy of the motion of electric charges.
Such a source allows for a journeycharge 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 the charge from a higher potential to a lower one.
This force, which causes the charge to move from morelow to a higher potential, it is customary to call 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 is powered byits 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 = Aart./ Δq,
where Aart. - the work of external forces, Δq - the charge moved inside the source.