The emf of a source is defined as the work done in moving a unit positive charge from low potential to high potential end while the potential difference between two terminals of a cell in a closed circuit is called terminal p.d. of the cell which is equal to the p.d. of a cross external resistance of the circuit. i.e.,
E = V + Ir -------- (i)
Where E is the electromotive force,
V is the potential difference between the terminals,
r is the internal resistance,
R is the external resistance, and
'I' is the current flowing in the circuit.
Hence from equation E = V + Ir, if the internal resistance 'r' of the cell is zero, then the emf and terminal potential differences are equal to each other.