Inductors

An inductor is a component whose action depends on the magnetic field that exists around any conductor when a current flows through that conductor. When the strength of such a magnetic field (or magnetic flux) changes, a voltage is induced between the ends of the conductor. This voltage is termed an induced EMF, using the old term of EMF ( electromotive force) to mean a voltage that has not been produced by a current flowing through a resistor.

At one time inductors were invariably fairly large components and were used in domestic radios as well as in a variety of other applications, but modern inductors for signal use are often SMD components and though used to a much lesser extent in domestic radio are extensively employed in other devices. Inductors intended for 50 Hz AC mains are invariably large components, but the extensive use of switch-mode power supplies has reduced the need for these items, though they are still made in large quantities.

If we confine our attention to static devices such as coils and transformers rather than moving devices such as electric motors, the change of magnetic field or flux can only be due to a change in the current through one conductor. The induced EMF is then in such a direction that it opposes this change of current, and the faster the rate of change of current the greater is the opposing EMF. Because of its direction, the induced EMF is called a