Voltage is defined between two points A, B in an electric field or in an electric circuit as the work done by the electric field when a unit positive charge moves from A to B. The potential at a point A is defined as the work done by the field when a unite positive charge moves from A to the place of zero potential. From these definitions follows that the voltage UAB = UA - UB, that is the negative of the potential difference which is UB-UA. Electromotive force (emf) is defined for voltage sources, and it is equal to the voltage across the terminals of the unloaded source (when nothing is connected to it). ehild