What is internal resistance?

Internal resistance refers specifically to the opposition to current flow within a power supply, such as a battery. This resistance arises from the materials and chemical processes inside the power supply that cause energy to be lost as heat when current flows. As a result, the internal resistance affects the output voltage and efficiency of the power supply, particularly when it is delivering current to an external load.

In practical terms, when a battery provides power, it can only deliver a maximum output voltage that is lower than its open-circuit voltage due to this internal resistance. Therefore, understanding internal resistance is crucial in evaluating how effectively a power supply can perform under load conditions.

This is why the option that describes it as the resistance of a power supply is the correct choice. The concept is vital in scenarios involving batteries, where knowing the internal resistance helps predict how the battery will behave when supplying power to an external circuit.