Inductance: understand the basics

Understanding the basics of inductance enables inductors & transformers to be used more effectively.


Inductance and Transformer Tutorial Includes:
Inductance     Symbols     Lenz's law     Self inductance     Inductive reactance calculations     Inductive reactance theory     Inductance of wire & coils     Transformers    


Inductance is a key parameter in electrical and electronic circuits. Like resistance and capacitance it is a basic electrical measurement that affects all circuits to some degree.

Inductance is used in many areas of electrical and electronic systems and circuits. Components can be in a variety of forms and may be called by a variety of names: coils, inductors, chokes, transformers, . . . Each of these may also have a variety of different variants: with and without cores and the core materials may be of different types.

Understanding inductance and the different forms and formats for inductors and transformers helps provide an understanding of what is happening within the electrical and electronic circuits.

An inductor

The term inductance was coined by Oliver Heaviside in 1886. It is customary to use the symbol L for inductors shown on circuit diagrams and inductance in equations after the physicist Heinrich Lenz.

Inductance basics

Inductance is the ability of an inductor to store energy and it does this in the magnetic field that is created by the flow of electrical current.

Energy is required to set up the magnetic field and this energy needs to be released when the field falls.

As a result of the magnetic field associated with the current flow, inductors generate an opposing voltage proportional to the rate of change in current in a circuit.

Inductance is caused by the magnetic field generated by electric currents flowing within an electrical circuit. Typically coils of wire are used as a coil increases the coupling of the magnetic field and increases the effect.

There are two ways in which inductance is used:

  • Self-inductance:   Self-inductance is the property of a circuit, often a coil, whereby a change in current causes a change in voltage in that circuit due to the magnetic effect of caused by the current flow. It can be seen that self-inductance applies to a single circuit - in other words it is an inductance, typically within a single coil. This effect is used in single coils or chokes.
    Read more about . . . . self inductance.

  • Mutual-inductance:   Mutual inductance is an inductive effect where a change in current in one circuit causes a change in voltage across a second circuit as a result of a magnetic field that links both circuits. This effect is used in transformers.

Inductance unit definition

When indicating an inductor on a circuit diagram or within an equation, generally the symbol "L" is used. On circuit diagrams, inductors are generally numbered, L1, L2, etc.

The SI unit of inductance is the henry, H which can be defined in terms of rate of change of current and voltage.

Definition of the henry:

The inductance of a circuit is one henry if the rate of change of current in a circuit is one ampere per second and this results in an electromotive force of one volt.

One henry is equal to 1 Wb/A.

Inductance - what happens

When a current flows within a conductor, whether it be straight or in the form of a coil, a magnetic field builds up around it and this affects the way in which the current builds up after the circuit is made.

In terms of how inductance affects and electrical circuit, it helps to look at the way the circuit operates, first for a direct current, and then for an alternating current. Although they follow the same laws and the same effects result, it helps the explanation, the direct current example is simpler, and then this explanation can be used as the basis for the alternating current case.

  • Direct current:   As the circuit is made the current starts to flow. As the current increases to its steady value the magnetic field it produces builds up to its final shape. As this occurs, the magnetic field is changing, so this induces a voltage back into the coil itself, as would be expected according to Lenz's Law.
    An inductor in a circuit with battery and resistor
    An inductor in a circuit with battery and resistor
    The time constant T in seconds of the circuit which will include the inductor value L Henries and the associated circuit resistance, R Ohms can be calculated as L/R. T is the time for the current I amps to rise to 0.63 of its final steady state value of V/R. The energy stored in the magnetic field is 1/2 L I2.
    The rise in current in an inductor after DC us switched on
    The rise in current when a steady voltage is applied to an inductor
    When the current is switched off this means that effectively the resistance of the circuit rises suddenly to infinity. This means that the ratio L / R becomes very small and the magnetic field falls very rapidly. This represents a large change in magnetic field and accordingly the inductance tries to keep the current flowing and a back EMF is set up to oppose this arising from the energy stored in the magnetic field. The voltages mean that sparks can appear across the switch contact, especially just as the contact is broken. This leads to pitted contacts and wear on any mechanical switches. In electronic circuits this back EMF can destroy semiconductor devices and therefore ways of reducing this back EMF are often employed.
  • Alternating current:   For the case of the alternating current passing through an inductor, the same basic principles are used, but as the waveform is repetitive, we tend to look at the way the inductor responds in a slightly different way as it is more convenient.

    By its very nature, an alternating waveform is changing all of the time. This means that the resulting magnetic field will always be changing, and there will always be an induced back EMF produced. The result of this is that the inductor impedes the flow of the alternating current through it as a result of the inductance. This is in addition to the resistance caused but he Ohmic resistance of the wire.

    It means that if the Ohmic resistance of the inductor is low, it will pass direct current, DC with little loss, but it can present a high impedance to any high frequency signal. This characteristic of an inductor can be used in ensuring that any high frequency signals do not pass though the inductor.
Read more about . . . . inductive reactance calculations.

A further aspect of inductance is that the reactance of an inductor and that of a capacitor can act together in a circuit to cancel each other out. This is known as resonance, and it is widely used in bandpass filters.

Inductance of wires and coils

Straight wires and coils have an inductance. Normally coils are used for inductors because the linking of the magnetic field between the different turns of the coil increases the inductance and enables the wire to be contained within a smaller volume.

For most low frequency applications, the inductance of a straight wire can be ignored, but as the frequency increases into the VHF region and beyond, the inductance of the wire itself can become significant, and interconnections need to be kept short o minimise the effects.

calculations are available to enable the inductance of wires to be calculated quite accurately, but the inductance of coils is a little more complicated and depends upon a variety of factors including the shape of the coil and the constant of the material in and around the coil.

Read more about . . . . inductance of straight wire & coils.


Inductance is a key aspect of wires and coils. Inductance is an indispensable characteristic that can be used to great effect in many circuits.



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