Inductance is the property of a circuit element, usually a coil, that resists rapid changes in current. When current changes, the magnetic field around the coil changes too, and that changing field induces a voltage that opposes the change.

What an inductor stores

An inductor stores energy in a magnetic field: E = 1/2·L·I², where L is inductance (henry) and I is current. Larger L means current changes more slowly for a given applied voltage.

Behavior with AC signals

In alternating current, inductive reactance is XL = 2pi·f·L. As frequency rises, reactance rises, so inductors pass low-frequency changes more easily than high-frequency noise. This is why they are central in filters and switching power supplies.

  • DC-DC converters: transfer and smooth energy in buck/boost stages.
  • EMI filters: reduce unwanted high-frequency components.
  • Motors and relays: shape current in magnetic loads.

A useful memory rule: capacitors resist voltage change; inductors resist current change.