Question

what is the difference between emf and potential difference?

Answer 1

Electromotive force, or emf, is better described as the total voltage in an electric circuit generated by the source or battery. Emf is not a physical force. Potential difference is the work done per unit charge to move a charge between the negative and the positive terminal of the battery..;)

Answer 2

When you measure the potential difference across (or between) the terminals of a battery that is not in a complete circuit you are measuring the emf (electromotive force) of the battery. This is the maximum amount of work per coulomb of charge the battery can do to drive charge from one terminal, through the circuit, to the other terminal.