when the terminal potential difference of a real cell is less than its emf
A source of emf also has some resistance which is due to electrolytes and electrodes. Therefore some useful energy of emf-source is used in passing the current through the source. Let the internal resistance of the source is “r” Potential drop across emf source = Vr Since V = E - Vr, so V = E - Ir. Where V = terminal potential difference When no current is drawn from the source then emf and terminal potential difference are equal. If the current is passing through the circuit then the terminal potential difference is less than that of the emf of the source.