EMF and voltage
To electricity passed through the circuit for a long time, it is necessary to continuously maintain the potential difference at the poles of the voltage source. Similarly, if you connect two vessels with different water levels with a pipe, the water will move from one vessel to another until the levels in the vessels become equal. By adding water to one vessel and diverting it from another, it is possible to ensure that the movement of water through the tube between the vessels will continue continuously.
When the source of electrical energy is operated, electrons from the anode transfer to the cathode.
From this we can conclude that inside the source of electrical energy there is a force that must continuously maintain the current in the circuit, that is, in other words, should ensure the operation of this source.
The cause, which establishes and maintains the potential difference, causes a current in the circuit, overcoming its external and internal resistance, is called the electromotive force (abbreviated ED), and is denoted by the letter E.
The electromotive force of the sources of electrical energy arises under the influence of the causes specific to each of them.
In chemical sources of electrical energy (galvanic cells, batteries) e. d. is obtained as a result of chemical reactions in the generators e. d. arises from electromagnetic induction , in thermoelements - due to thermal energy.
The potential difference that causes the passage of current through the resistance of a section of an electric circuit is called the voltage between the ends of this section. Electromotive force and voltage are measured in volts. To measure e. d. and voltages serve as instruments - voltmeters (Figure 1).
Thousands of volts - millivolts - are measured in millivoltmeters, thousands of volts - kilovolts - kilovolts.
To measure uh. d. The source of electrical energy must be connected to the voltmeter terminals of this source with an open external circuit (Figure 2). To measure the voltage on any part of the electrical circuit, a voltmeter must be connected to the ends of this section (Figure 3).
Figure 2. Voltmeter measurement of the electromotive force of an element. Voltage meter measurement of voltages in various parts of an electrical circuit.
Video 1. What is an electromotive force (ed. S.)
Source: Kuznetsov M.I., "Fundamentals of Electrical Engineering" - 9th edition, revised - Moscow: High School, 1964 - 560s.