An inductor is a coil that's designed for use in electronic circuits. Inductors take advantage of an important characteristic of coils called self inductance, also called just inductance. Inductors are simple devices, consisting of nothing more than a coil of wire, often wrapped around an iron core. But their ability to exploit the idea of self-inductance is a stroke of genius.

Electromagnetic induction refers to the ability of a coil to generate a current when it moves across a magnetic field, self inductance refers to the ability of a coil to create the very magnetic field that then induces the voltage.

Inductance happens only when the current running through the coil changes. That's because only a moving magnetic field induces voltage in a coil. Whenever the current changes in a coil, the magnetic field created by the current grows or shrinks, depending on whether the current increases or decreases.

When the magnetic field grows or shrinks, it's effectively moving, so a voltage is inducted in the coil as a result of this movement. When the current stays steady, no inductance occurs.

Let's go over the idea point by point:

• When voltage is applied across a coil, the voltage causes current to flow through the coil. Remember, current always requires voltage, and voltage always results in a current when applied across a conductor.

• The current flowing through the coil creates a magnetic field around the coil. Keep in mind that the coil that creates the magnetic field is itself within the field and can therefore be influenced by it.

• If the current flowing through the coil changes, the magnetic field created by the current also changes. The magnetic field grows or shrinks, depending on whether the current increases or decreases. Either way, the changing magnetic field is in effect moving.

• Because the magnetic field is moving, voltage is induced in the coil. This is an additional voltage, on top of the voltage that's driving the main current through the coil.

• The amount of voltage induced by the changing magnetic field depends on the speed in which the current changes. The faster the current changes, the more the magnetic field moves and therefore the more voltage is induced.

• The polarity of the induced voltage depends on whether the current is increasing or decreasing. This is because the direction of movement in the magnetic field depends on whether the field is growing or shrinking, and the voltage induced by a moving magnetic field depends on which direction the field is moving, according to the following rules:

• When the current increases, the polarity of the induced voltage is opposite to the polarity of the voltage driving the coil. This inducted voltage is often called back voltage because it has the opposite polarity as the supply voltage.

• When the current decreases, the resulting self-induced voltage has the same polarity as the supply voltage.

• The induced voltage creates a current in the coil that flows either with or against the main coil current, depending on whether the coil current is decreasing or increasing, according to the following rules:

• When the coil current is increasing, the additional current flows against the main coil current. This has the effect of pushing back against the increasing main current, which effectively slows down the rate at which the current can change.

• When the coil current is decreasing, the additional current flows with the main coil current, thus counteracting the decrease in coil current.

• When the coil current stops changing, self-inductance stops. Thus, when current is steady, an inductor is simply a straight conductor. (It is also an electromagnet, as the current traveling through it produces a magnetic field.)

An inductor applies equal opposition to both increases and decreases in current.