Electromotive force, also referred to as EMF, is the voltage generated by a battery or by a magnetic force, which states that a time varying magnetic field induces an electric current. The EMF is not a force measured in newton’s, but a potential, or energy per unit of charge measured in volts. In nature, EMF is generated whenever magnetic field fluctuations occur through a surface.
In electromagnetic induction, EMF can be defined around a closed loop as the electromagnetic work that would be transferred to a unit of charge if it travels once around that loop. In time-varying magnetic flux affecting a loop, the electric potential scalar field is not defined due to circulating electric vector field. Nevertheless an EMF works so that it can be measured as a virtual electric potential around the loop.
In a two-terminal device, the EMF can be measured as voltage across the two open-circuited terminals. The created electrical potential difference drives current flow is a circuit is attached to the source of EMF. When current flows, the voltage across the terminals of the source of EMF is no longer the open-circuit value. This is due to the voltage drops inside the device due to its internal resistance.
In a battery, charge separation that gives rise to a voltage difference is accomplished by chemical reactions at the electrodes. Chemically, by separating positives and negative charges, an electric field can be produced, leading to an electric potential difference. A voltaic cell is thought of as having a “charge pump” of atomic dimensions of each electrode.