An inductor stores energy in a magnetic field associated with electric current. A coil of wire is a common implementation. Inductors are used in filters, impedance matching networks, oscillators and power converters.
Using the passive sign convention, the voltage across an ideal inductor is:
v = L di/dt E = ½ L I² X_L = 2 π f L
L is inductance in henries (H), di/dt is the rate of change of current in amperes per second, E is energy in joules, and X_L is inductive reactance in ohms. Common radio values use μH (10^-6 H) or nH (10^-9 H).
An ideal 10 μH inductor has approximately 62.8 Ω reactance at 1 MHz. At a steady 1 A, its stored energy is 5 μJ. A constant DC current produces no voltage across an ideal inductor; a real coil still has winding resistance.
For sinusoidal signals, an ideal inductor has impedance jX_L, so current lags voltage by 90 degrees. Rapidly interrupting current can produce a substantial voltage. This is why relay coils and switching circuits need an appropriate path for their stored energy.
Winding resistance, core losses and parasitic capacitance limit performance. A magnetic core can saturate, changing inductance and increasing distortion or loss. A coil also has a self-resonant frequency; above it, an ideal inductance model is unreliable.
Two nearby coils may couple magnetically. Their orientation and spacing therefore matter in an RF layout. Combining an inductor with a capacitor produces resonance, which can select or reject a frequency range.