electromotive force

Electromotive force is the 'push' a battery, generator, or solar cell gives to keep charge circulating around a circuit. Think of it as an electrical pump: just as a water pump lifts water back up so it can flow downhill again, an EMF source raises charge back up in energy so it can drive current around the loop once more. It answers the question at the heart of every power source: how much energy does this source hand to each unit of charge that passes through it?

Precisely, the electromotive force, written EMF or the symbol script e, is the energy supplied by a source per unit charge, EMF = energy given / charge, measured in volts. Despite the word 'force', it is not a force at all and is not measured in newtons; it is an energy-per-charge, the same units as voltage. A source of EMF does work on the charges (chemical work in a battery, magnetic work in a generator) to carry them from low to high potential inside the source, against the electric field.

The EMF is the maximum voltage a source can offer, seen across its terminals only when no current is being drawn (an open circuit). As soon as current flows, some of that energy is lost inside the source itself, across its internal resistance, so the useful terminal voltage drops below the EMF. This is why a car's headlights dim for a moment when the starter motor draws a huge current: the battery's EMF is unchanged, but the heavy current makes the internal losses large. EMF is the source's promise; terminal voltage is what it actually delivers under load.