physics notes

Batteries, just like other components in an electric circuit, have a resistance. This resistance is known as internal resistance. This means that applying Ohm's law (V = IR) to circuits is more complex than simply feeding the correct values for V, I or R into the formula.
The existence of internal resistance is indicated by measuring the potential difference across a battery. This is always less than the EMF of the battery. This is because of the internal resistance of the battery. This idea gives us the following formula:

PD across battery = EMF of battery - voltage to be accounted for
Let us replace these values with letters to give the simpler formula:
V_external = E - V_internal
Since V = IR:
V_external = E - IR_internal
You may also need to use the following formula to work out the external potential difference, if you are not given it:
V_external = IΣR_external
You should also remember the effects of using resistors in both series and parallel circuits.