Electromagnetic Force—

The emf of a source is equal to the maximum potential difference between its terminals when it is in open circuit that is when it is not sending any current in the circuit.

The emf of a source is defined as the energy supplied by the source in taking a unit positive charge once round the complete circuit.   

If a circuit has R external circuit and r is the internal resistance of cell that is resistance of electrodes and electrolyte respectively and ‘i’ be the current flowing in the circuit. Then,