Electromotive Force

The term electromotive force is used for historical reasons, coined by Alessandro Volta, the inventor of the battery. Despite its name, emf is not a force but rather a potential difference between the two terminals of a battery. It represents the maximum voltage a battery can deliver in an open circuit, measured in volts.

Terminal voltage is the voltage measured across a battery's terminals when current flows through it, and it is always less than the emf. This difference occurs because the battery has internal resistance. Terminal voltage equals the emf minus the voltage drop across the internal resistance, which increases as the battery depletes.

An emf source functions as a charge pump, moving negative charges from the positive terminal to the negative terminal to maintain the potential difference. Work must be done on these charges against the electric field, requiring energy from chemical reactions in the battery. This process raises the charges' potential energy and maintains the high potential at the positive terminal.

Examples of emf sources include batteries, electric generators, solar cells, thermocouples, and fuel cells. All these sources transform energy of some kind—mechanical, chemical, thermal, or other forms—into electric potential energy and send it to the circuit. This energy transformation is essential for maintaining current flow in any circuit.

Electromotive force is defined as the amount of work done to move a unit charge across the terminals of a battery. When there is no current flowing, the emf equals the work done on the charge per unit charge. Its unit is the volt, derived from joules of work divided by coulombs of charge.

Internal resistance generally increases as a battery is depleted due to oxidation of the plates or reduction of the acidity of the electrolyte. This increase in internal resistance causes a greater voltage drop across the battery's internal resistance, further reducing the terminal voltage available to the external circuit.

When a wire is connected between the two terminals, the emf pushes electrons from the negative terminal to the positive terminal through the external circuit. Simultaneously, the battery's internal charge pump moves negative charges from the positive terminal to the negative terminal internally, maintaining the potential difference needed for continuous current flow.