emf

Emf, short for electromotive force, is the energy provided per unit charge by an electrical source as charges complete a circuit. It is not a mechanical force but a convention for energy transfer in circuits. The SI unit is the volt (V). In circuit theory, the electromotive force of a source is treated as the potential difference between its terminals when no current flows, but in operation it is reduced by the source’s internal resistance: V_terminal = emf − I R_internal.

In a closed loop, emf can be defined as the line integral of the electric field: emf = ∮ E · dl. This integral is zero in purely electrostatic situations with conservative fields, but it is nonzero when time-varying magnetic fields induce currents (Faraday’s law). The induced emf is given by emf = − dΦ_B/dt, where Φ_B is the magnetic flux through the loop, and the sign is determined by Lenz’s law.

EMF is produced by various energy conversion processes. Batteries and fuel cells convert chemical energy into electrical energy; dynamos and generators convert mechanical energy; solar cells convert light energy. The term “electromotive force” reflects the source’s ability to do work on charges around the circuit, not a mechanical push.

Understanding emf is essential for analyzing real sources with finite internal resistance, as well as ideal voltage sources used in circuit diagrams.