Final answer:
Electromotive force (EMF) refers to the potential difference produced by a source when no current is flowing and is measured in volts. It is not an actual force, but rather indicates the energy per unit charge that the source can provide, with internal resistance influencing the actual output under load.
Step-by-step explanation:
The term electromotive force (EMF) refers to the potential difference of a source when no current is flowing. Rather than being an actual force, EMF is better understood as a kind of potential difference that indicates the work done per unit charge to keep the source's potential difference constant. Despite its name, EMF is not a force; it is a measure of the energy per unit charge that a source can provide, typically measured in volts. For instance, in the context of a battery, EMF refers to the maximum potential difference that the battery can produce when it is not supplying any current.
Faraday's insights into electromagnetic induction highlighted the concept of induced EMF, where a changing magnetic field can result in the generation of a current in a coil - indicative of an EMF being present. All voltage sources, such as batteries and generators, have an EMF that can supply current to an external circuit when connected. The internal resistance of a device affects the actual voltage output when the current is flowing, typically resulting in a lower voltage than the EMF under load.