Each current source has its own internal resistance. An electrical circuit is a closed loop with consumers to which voltage is applied. Each such circuit has an external resistance and an internal one.
External is called 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 of which the conductor is made, and its condition. Inductive depends on the inductance of the coil (the magnitude of its counter-emf), and capacitive arises between the turns of the winding. It is quite small. If a conventional 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 the batteries, in a word, wherever the current occurs.
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 areas, including in 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 sections of the circuit;
Ir is the internal current generated in the source;
r is the internal resistance of the source.
To understand the physical meaning of the source’s internal resistance, a little experiment should be done. 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, it is necessary to connect a small resistance and install an ammeter in series. Thus, the current will be known, and the voltage under load must also be measured.
Having recorded all the values of the quantities, it is easy to determine the internal resistance. For this, the voltage drop in the battery is primarily determined. Using formula
Ur = EU
we make calculation.
In this formula:
Ur is the voltage drop of the internal resistance of the source;
E is the voltage (EMF) measured at the source without a consumer;
U is the voltage measured directly on 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 the small value. However, practice shows that in complex calculations, internal resistance greatly affects the final result.