Inductance

An electric current produces a magnetic field. When the current changes, the associated magnetic flux changes and an electromotive force (emf) is induced. Inductance quantifies the relationship between that induced emf and the rate of current change. In a simple fixed-geometry circuit, the magnitude of the induced emf is proportional to the rate at which the current changes; the proportionality constant is its inductance. The induced emf acts in a direction that opposes the change that produced it, as described by Lenz's law.

Self-inductance, conventionally written L, is the induction of an emf in a circuit by a changing current in that same circuit. A coil that exhibits appreciable self-inductance is called an inductor. Mutual inductance, conventionally written M, occurs when a changing current in one circuit changes magnetic flux through another circuit and induces an emf there. The extent of mutual coupling depends on the circuits' shapes, relative positions and magnetic environment. Transformers use mutual inductance to transfer electrical energy between windings.

The SI unit of inductance is the henry (H), named for the American scientist Joseph Henry. A circuit has an inductance of one henry when a current changing at one ampere per second induces an emf of one volt, in the idealized definition. For a coil, inductance depends on factors such as its dimensions, number of turns, winding arrangement and the magnetic properties of nearby materials or a core. The inductance of a given fixed arrangement is a property of its geometry and materials, rather than a measure of the current itself.

An inductor resists rapid changes in current because energy is stored in its magnetic field. For an ideal linear inductor carrying current I, the stored energy is U = ½LI². Inductive effects are used deliberately in transformers, filters, motors, generators and energy-storage circuits. They can also produce unwanted voltage spikes when current is interrupted quickly, so practical circuits may include components that limit or redirect such transients.