Work of forces of the electric field when moving the charge around a closed circuit is zero. This means, what if in a closed circuit the charges are only electric forces, we work through current can not be obtained. Therefore, in an electrical circuit must be at least one such site, in which mobile charge carriers, in addition to the strength of the electric field, would any other force, able to do work on the movement of these charges. Such forces are called third-party.

Imagine two charged conductors a and (rice. 16.3). Suppose, the potential of the conductor And more, than the potential of the conductor In. If you connect the conductor DIA, then a positive charge under the action of electric field F_E will go from A through a conductor DIA. However, this movement will stop very soon, since the potentials of conductors A and b will be equal. That did not happen and the movement of the carriers had gone on long enough, should the positive charges from the point In some way move back to point A, for example, Explorer ВDА. But this movement of charges is spontaneously happening, since the electric field strength acting on them in the opposite direction.

Therefore, in Windows Explorer ВDА the charges have to act a third-party force F_ST, directed against the forces of the electric field and exceeding their largest. Then the area of the DIA the carriers will move under the action of electric field from A, the site ВDА — under the action of the field third-party forces from B to A. In this case, in a closed circuit will continuously circulate the flow of charges, t. e. will go an electric current, and the potentials of conductors A and b will not be able to align.

Outside forces will do work on the movement of mobile charge carriers on the plot BOA, overcoming the opposition of the electric fields and particles, from which made the guide BOA. Due to the work, made by external forces against the forces of the electric field, the current performs work on the part of the circuit DIA.

Thus, on the site ВDА the electrical energy obtained at the expense of other forms of energy, and the area ACB, on the contrary, electrical energy will transform into other kinds of energy, for example, in the internal energy of the conductor. Therefore, the circuit, in which charges move in the direction of the action of external forces, call source electrical energy (plot ВDA), and the plot, in which charges move in the direction of the action of electric forces, call consumer electrical energy.

Note, what electrical sources of electrical energy are called generators. Those, in which chemical energy is converted into electrical, call electrochemical cells or batteries.

Size, characterizing the dependence of electrical energy, acquired by the charge in the generator, from the last internal device, called the electromotive force of the generator and designate e. d. with. or ξ. The electromotive force of the generator is measured by a third-party forces, implemented by moving a unit positive charge:

ξ= And_ST/q. (16.8)

So, continuous current can go in a closed circuit upon the presence of her with E. J.. If you remove the conductor DIA (rice. 16.3), the result of moving charges on a plot of WB And under the action of external forces at point a will accumulate positive charges, and at point b is negative. The voltage U between the conductors A and B will increase as long, while electric and third-party power, acting on the charges, equal. Then the accumulation of charges at points A and B will stop, and the voltage between A and b reaches the maximum value for this generator. (Explain, why is the voltage on the alternator with an open circuit is equal to its e. d. S.)

Thus, to measure e. d. with. generator, you need to attach to it the voltmeter for open circuit.