Electromotive force

Students, when dealing with electric circuits, often have difficulty in understanding the distinction between potential difference (PD) and electromotive force (EMF). Yet it is important to understand this distinction, which occurs in several situations: the potential difference at the terminals of a battery may be less than its EMF. In the author's experience, any difficulty usually arises from a misunderstanding of the nature of electromotive force rather than a failure to understand the nature of potential difference. The distinction between PD and EMF in a circuit is explained by looking at an analogous fluid circuit.