Self-inductance
The property of a coil that makes it oppose changes in its own current, characterised by L = Φ/I, where Φ is the flux the coil produces through itself. Units of henry (H = V·s/A).
Definition
Self-inductance is the electromagnetic analogue of inertia. When current flows through a coil, it produces a magnetic field, and that field threads flux through the coil's own turns. If the current changes, the flux changes, and Faraday's law demands that an EMF appear in the coil opposing the change — a back-EMF. The proportionality constant between current and self-flux is the self-inductance: L = Φ_self / I, or equivalently V_L = −L dI/dt. The minus sign is Lenz's law: the back-EMF opposes the change in current.
L depends only on the coil's geometry and the magnetic properties of its environment, not on the current flowing through it (at least for linear materials). For a long solenoid of length ℓ, cross-section A, and n turns per metre, L = μ₀ n² ℓ A — scaling linearly with the core volume and quadratically with turn density. Inserting a ferromagnetic core multiplies L by the relative permeability μ_r, which can easily be 10³ or 10⁴ — which is why power-frequency inductors are almost always wound around iron laminations, and why RF chokes that need a well-defined inductance often omit the core (better linearity, lower dissipation). In SI units, L is measured in henries: 1 H = 1 V·s/A — the inductance of a coil in which a current changing at 1 A/s induces an EMF of 1 V.
The current in an inductive circuit doesn't change instantaneously. Switch on a battery in series with an inductor L and resistor R, and the current rises exponentially with time constant τ = L/R: I(t) = (V/R)(1 − e^(−t/τ)). Switch it off and the current dies exponentially on the same timescale — except that if the circuit is abruptly opened, the back-EMF required to bring the current to zero in zero time is formally infinite, which is why opening an inductive circuit produces a spark. Energy storage: the inductor stores U = ½LI² joules in its magnetic field, and that energy must go somewhere when the current is interrupted — through an arc, through a snubber resistor, or (in a switching power supply) transferred deliberately into a capacitor for reuse.