Selvinduktans

Selvinduktans, or self-inductance, is a property of an electrical circuit whereby a changing current induces an electromotive force in the same circuit. The induced emf e is proportional to the time rate of change of current i: e = -L di/dt, where L is the inductance. The negative sign reflects Lenz's law, which states that the induced emf opposes changes in current.

The unit of inductance is the henry (H). A circuit with inductance L resists changes in current because the self-induced emf grows with di/dt. The magnitude of L depends on the circuit’s geometry and materials, such as the number of turns, cross-sectional area, length, and the magnetic permeability of any core. For a long, thin solenoid, L is approximately μ N^2 A / l, where μ is the core permeability, N the number of turns, A the cross-sectional area, and l the length.

Self-inductance also relates to energy storage in a magnetic field: the energy stored is E = 1/2 L I^2. At higher frequencies, parasitic capacitances, core losses, and magnetic saturation can affect the effective inductance. A practical inductor may exhibit a self-resonant frequency, above which its behavior becomes less inductive due to the interaction with parasitic elements.

Applications include energy storage and filtering in power supplies, chokes in switched-mode converters, and impedance shaping in RF circuits. Self-inductance is distinct from mutual inductance, which involves two circuits influencing each other’s currents; self-inductance concerns only the circuit’s own current.