This effect can be understood as follows: the current produces a magnetic field; a change in current gives a change of this magnetic field; a changing magnetic field causes an electromotive force in the conductor.
The behaviour of the device is described by the formula:
where V is the voltage generated, dI/dt is the rate of change of current, and L is a property of the device called inductance. The SI unit of inductance is the henry (H).
Thus an inductor resists changes in current. A pure inductor does not offer any resistance to direct current (an actual one does slightly), except when the current is switched on and off, then it makes the change more gradual.
When a sinusoidal alternating current flows through an inductor, a sinusoidal alternating voltage (or electromotive force, abbr. emf) is induced. The amplitude of the emf is related to the amplitude of the current and to the frequency of the sinusoid by the following equation.
where ω is the angular frequency of the sinusoid defined in terms of the frequency f as
The term ωL is known as inductive reactance, which is denoted by the symbol X_{L} and is the positive imaginary component of impedance.
An induction coil is closely related to electromagnets in structure, but used for a different purpose—to store a magnetic field energy. Some of its applications include coil guns[?] and Tesla coils. It is a basic part of an electric transformer.
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