Internal resistance and its physical meaning

Each current source has its own internal resistance. The 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 to consumers and conductors, and the internal resistance comes from the source itself.

If an electrical machine is used as a current source , its internal resistance is divided into active, inductive and capacitive. The active depends on the length of the conductor and its thickness, as well as the material from which the conductor is made and its state. Inductive depends on the inductor of the coil (the magnitude of its counter-electromotive force), and the capacitive appears between windings of the winding. It is rather small. If the source is a conventional battery, then it also creates resistance due to the electrolyte.

A current is a directed movement of particles, and resistance is a created obstacle in the path of its movement. Such obstacles are found both in the electrolyte and in the lead plates of storage batteries, in a word, wherever there is a current.

Because there is an internal resistance in the source, one can not assume 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 ignored, but only if it is negligible.

If large currents are created in the source circuit, then the voltage at the terminals can not be considered a true electromotive force. The current in the source is a sign of voltage drop in it. In this case, the Kirchhoff law operates, which states that the true EMF of the chain is the sum of the voltage drops at all sites, including the source itself. And the formula is written like this:

E = ΣU + Ir r

Where:

E is the total electromotive force of the chain;
U - voltage drop on the sections of the circuit;
Ir is the internal current generated at the source;
R is the internal resistance of the source.

To understand the physical meaning of the internal resistance of the source, a little experience should be made. 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 this, 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 written down all the values of the quantities, it is easy to determine the internal resistance. To do this, the voltage drop in the battery is first determined. Using formula

Ur = EU

We perform the calculation.

In this formula:

Ur - voltage drop of the source internal resistance;
E is the voltage (EMF) measured at the source without the consumer;
U is the voltage measured directly on the resistance.

Thus, the internal resistance is calculated by the following formula:

R = Ur / I

Some specialists neglect this value, considering that it can be ignored because of the small value. However, practice shows that in complex calculations, the internal resistance strongly affects the final result.