explain the difference between electromotive force and potential difference
Potential difference is the result. Potential difference is always less than electromotive force. Potential difference takes place between any two points in the circuit. Potential difference of two points depends on the resistance of those points. It does not remain constant.
Electromotive force, abbreviated as E.M.F and denoted by $\varepsilon$, is not a force. It is defined as the energy utilized in assembling a charge on the electrode of a battery when the circuit is open.Simply, it is the work done per unit charge which is the potential difference between the electrodes of the battery measured in volts.
The difference between the electric potential at two points, A and B, is measured in volts and called the Potential Difference. However if you connected a resistor between points A and B, no current would flow. Now consider you battery, with the same voltage across its terminals as the PD between A and B above.