The word inductance comes from induce, meaning "to bring about" or "to give rise to." In electromagnetism, it refers to the ability of a changing current to give rise to a voltage.
In an electrical circuit, currents arise in conductors that connect points of different electrical potential, or voltage. The voltage is generally established by a device that converts another form of energy into electrical energy. A battery, for example, converts chemical energy into electrical energy; a solar cell converts light energy into electrical energy; and a generator converts motional energy into electrical energy by forcing a conductor to move through a magnetic field.
The moving charges that constitute an electric current set up a magnetic field in the space around them. As long as the field is unchanging it will have no effect on a second, nonmoving conductor placed in it. If, however, the second conductor is moving or the magnetic field around it is changing, a voltage is induced in it, and if it is part of a complete circuit, a current will flow in response. The magnitude of the induced electromotive force will be proportional to the rate at which the magnetic flux (field strength × area measured perpendicular to the field direction) is changing.
Inductance and changing current
Potential differences give rise to currents, which in turn create magnetic fields. Any change in the current will create a change in the field, which induces a voltage in any neighboring circuits and in the original circuit as well.
The induction of a voltage in one circuit by the changing current in another is termed mutual inductance. The additional voltage induced in a circuit by changes in its own magnetic field is termed self-inductance. According to a principle known as Lenz’s law, the voltage induced in any circuit by a changing magnetic flux will produce currents that resist the change in flux. Self- inductance is therefore a factor that resists any change in current.
Because the magnitude of induced voltage is proportional to the rate of change of current, high-frequency (fast-changing) current signals will produce larger induced voltages than low-frequency (slow-changing) signals. Furthermore, as these induced voltages always try to impede the current that produced them, they will impede higher frequencies more than lower frequencies. Steady currents are not affected by inductance.
In many situations self-inductance can be a nuisance. For example, when transmitting a signal along a wire, the self-inductance of the wire will impede the higher frequencies more than the lower ones. Inductors, on the other hand, are devices that are designed to have a specific self-inductance and are useful for their frequency-dependent properties. An inductor will allow DC to flow through unhindered but will progressively restrict higher and higher frequencies of alternating current (AC).
When a capacitor is combined with an inductor, they form tuned circuits that are of fundamental importance in oscillators. Capacitors have reverse frequency characteristics to inductors—that is, they impede low frequencies and DC more than high frequencies. Together, capacitors and inductors impede both the low and high frequencies. At a particular frequency, determined by the values of the inductance and capacitance, the impedences compensate each other exactly. A tuned circuit is just one example of a frequency filter, and inductors are generally used in such applications.
