Internal Resistance of Circuit:
As mentioned earlier in the section on batteries, every source of electricity, such as a cell or generator has resistance to current flow called internal resistance.
TERMINAL VOLTAGE = EMF - LOST VOLTS
Loss of potential only occurs when current flows from the source. If therefore the external circuit is open, no current flows and the terminal voltage is equal to the emf.
TERMINAL VOLTAGE = EMF
The Size of the 'lost voltage' is determined by the internal resistance and the current flowing (Ir). For a given emf the larger the external resistance, the smaller the current and the smaller the 'lost volts'. Thus if the internal resistance is much smaller than the external resistance the 'lost volts' is very small and the terminal voltage is almost equal to the source emf.