Each current source has its own internal resistance. An electrical circuit is a closed circuit with consumers to which voltage is applied. Each such circuit has external resistance and internal.
External is the resistance of the entire circuit with consumers and conductors, and internal resistance comes from the source itself.
If an electric machine is used as a current source, then its internal resistance is divided into active, inductive and capacitive. Active depends on the length of the conductor and its thickness, as well as the material from which the conductor is made, and its condition. The inductive depends on the inductance of the coil (the value of its back-EMF), and the capacitive occurs between the turns of the winding. It is quite small. If an ordinary battery is used as a source, then resistance is also created in it due to the electrolyte.
Current is the directional movement of particles, and resistance is the obstacle created in the way of its movement. Such obstacles are found both in the electrolyte and in the lead plates of batteries, in a word,wherever there is current.
Due to the fact that there is internal resistance in the source, it cannot be assumed that the voltage in the circuit is the total electromotive force of the source. Of course, the voltage drop in the source itself can be neglected, but only if it is negligible.
If large currents are created in the source circuit, then the voltage at the terminals cannot be considered a true electromotive force. The current in the source is a sign of a voltage drop in it. In this case, Kirchhoff's law applies, which states that the true EMF of the circuit is the sum of the voltage drops in all sections, including the source itself. And the formula is written like this:
E=∑U + Ir r
Where:
E is the total electromotive force of the circuit;
U is the voltage drop in the circuit sections;
Ir is the internal current generated in the source;r is the internal resistance of the source.
To understand the physical meaning of the internal resistance of the source, you should conduct a little experiment. Initially, the electromotive force of the source is measured. This is done by connecting a voltmeter to a battery that is not under load. After that, you need to connect a small resistance and install an ammeter in series. Thus, the current will be known, while the voltage under load must also be measured.
By writing down all the values, it is easy to determine the internal resistance. To do this, the voltage drop in the battery is first determined. Using the formula
Ur=E-U
calculate.
In this formula:
Ur – voltage drop of the internal resistance of the source;
E – voltage (EMF) measured at the source without consumer;U – voltage measured directly across the resistance.
Thus, the internal resistance is calculated by the following formula:
r=Ur/I
Some experts neglect this value, believing that it can be ignored due to its small value. However, practice shows that with complex calculations, internal resistance greatly affects the final result.
