Electromotive force is defined as energy per unit charge. Internal resistance provides an explanation for varying terminal potential difference under load.

Part of

An electrical cell is made from materials (metal or chemicals, for example). All materials have some resistance. Therefore, a cell must have resistance. This resistance is called the **internal resistance** of the cell.

A cell can be thought of as a source of electromotive force (EMF) with a resistor connected in series.

When a load resistance is connected, current flows through the cell and a voltage develops across the internal resistance. This voltage is not available to the circuit so it is called the lost volts, .

can also be calculated as using Ohm's Law.

The voltage across the ends of the cell is called the terminal potential difference, .

can also be calculated as where is the load resistance.

Voltage is a measure of energy, and energy is always conserved. So the EMF of a cell is equal to the sum of its terminal potential difference, , and the lost volts, .

This gives rise to the equation

This equation can be written in different forms, eg . To solve problems on internal resistance it should be remembered that such circuits involve using a series circuit with the internal resistance and the load.