# Electrical sources and internal resistance

An electrical cell is made from materials (metal or chemicals, for example). All materials have some resistance. Therefore, a cell must have . 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 connected in .

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

$$V_{L}$$ can also be calculated as $$I r$$ using .

The voltage across the ends of the cell is called the terminal potential difference, $$V_{tpd}$$.

$$V_{tpd}$$ can also be calculated as $$I R$$ where $$R$$ is the load resistance.

Voltage is a measure of energy, and energy is always conserved. So the EMF $$E$$ of a cell is equal to the sum of its terminal potential difference, $$V_{tpd}$$, and the lost volts, $${V_L}$$.

This gives rise to the equation $$E= {V_{tpd}} + {V_L}$$

This equation can be written in different forms, eg $$E= I(R + r)$$. 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.