The electromotive force is described as the energy converted from non- electrical forms to an electrical form, when one coulomb of positive charge passes through the cell. The e.m.f. of a supply is the work done in moving one coulomb of charge around a circuit.
Batteries
A battery is made of one or two more cells connected together in series , which gives a bigger e.m.f. so the current that flows will be greater. The e.m.f. in series add up. For three 1.5V cells in series.
Combined e.m.f. = 1.5 + 1.5 + 1.5 = 4.5 V
In parallel, three cells of 1.5 V will only provide a combined e.m.f. of 1.5 V . But the combination will provide the same current for a longer before it runs down than a single cell can do on its own.