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.
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.