For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter or Î½ (nu) (see e.g. Planck's formula).
An inductor, also called a coil or reactor, is a passive two-terminal electrical component that stores electrical energy in a magnetic field when electric current is flowing through it. An inductor typically consists of an electric conductor, such as a wire, that is wound into a coil. When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor, described by Faraday's law of induction. According to Lenz's law, the direction of induced electromotive force (e.m.f.) opposes the change in current that created it.
A coupled inductor, on the other hand, has two or more conductors wound on a single core. A coupled inductor is most often a four-terminal device, but common mode chokes may have six terminals for 3-phase applications, or more for multi-conductor applications. Coupled inductors provide high inductance in a small volume. Common mode chokes obtain high inductance by using a high permeability core. Their inductance is proportional to the number of turns squared, on each winding. Differential mode inductors achieve high inductance because their inductance is proportional to the square all winding turns on the core.