What is Inductance?

“An Inductor is a coil of wire which opposes the flow of current through itself in the form of a magnetic field

When an electrical current flows through a length of wire a magnetic field is built up around the wire conductor. The direction of this magnetic field can be thought in terms of the wood screw being screwed into the conductor in the direction of the flow of current, with the head of the wood screw being rotated in the direction of the lines of force.

If we now take this length of wire and form it into a coil of N turns, the magnetic flux surrounding the coil is increased many times over for a given coil of wire compared with the flux produced by a single straight length. Also, if the current which is flowing through the coils conductor is increased in magnitude, the magnetic flux produced around the coil will also increase in value.

However, as the strength of the magnetic flux increases, it induces a secondary voltage within the coil called a back emf (electro-motive force). Then for a coil of wire, a self-induced voltage is developed across the coil due to the change in current flowing through the coil. The polarity of this self-induced voltage produces a secondary current in the coil that generates another magnetic flux which opposes any changes to the original flux.

In other words, the instant the main current begins to increase (or decrease) in value, there will be an opposing effect trying to limit this change. But because the coil of wire is extremely long, the current through the coil cannot change instantaneously it takes a while for the current to change due mainly to the resistance of the wire and the self-induced effects of the wire coil.

Unit of Inductance

The ability of a coil to oppose any change in current is a result of the self-inductance, L of the coil. This self inductance, simply called inductance, value of an inductor is measured in Henries, (H).

The Henry is a large unit, so the milli-henry (mH) and the micro-henry (μH) are more commonly used instead. Then the greater the inductance value of the coil, the slower is the rate of change of current for a given source voltage.

 Inductance is the characteristic of an electrical conductor that opposes a change in current flow. An inductor is a device that stores energy within itself in the form of a magnetic field.

The Inductor also called a choke is another passive type electrical component which is just a coil of wire that is designed to take advantage of this relationship by inducing a magnetic field in itself or in the core as a result of the current passing through the coil. This results in a much stronger magnetic field than one that would be produced by a simple coil of wire. Inductors can also be fixed or variable.

Inductors are mainly designed to introduce specific amounts of inductance into a circuit. They are formed with wire tightly wrapped around a solid central core which can be either a straight cylindrical rod or a continuous loop or ring to concentrate their magnetic flux. The inductance of a coil varies directly with the magnetic properties of the central core. Ferrite and powdered iron materials are mainly used for the core to increase the inductance by increasing the flux linking the coil.

Inductance of the coil which has the uniform cross-sectional area, A length and coil turns N can be represented in mathematical form as:


Where: N is the number of coil turns, μ is the permeability of the central core, A is the area in m2 and is the mean length of the core in meters.

Inductor SymbolsInductor Symbols