Self and Mutual Inductance — Definition

Definition

Imagine you have a coil of wire. When you pass an electric current through this coil, it creates a magnetic field around itself. If this current changes, the magnetic field it produces also changes. According to Faraday's Law of Electromagnetic Induction, a changing magnetic field (or more precisely, a changing magnetic flux) through a coil will induce an electromotive force (EMF) in that coil.

Now, let's consider two scenarios:

Self-Inductance: When the current *in a single coil* changes, the magnetic field it generates changes, and this changing magnetic field passes through the *same coil* that created it. This induces an EMF *within that very coil*.

This induced EMF, by Lenz's Law, always opposes the change in current that caused it. If the current is increasing, the induced EMF tries to reduce it; if the current is decreasing, the induced EMF tries to maintain it.

This property of a coil to oppose any change in the current flowing through it is called self-inductance. It's like the coil has an inherent 'inertia' against current changes. The measure of this property is the self-inductance (symbol $L$), and its unit is the Henry (H).

A larger self-inductance means a greater opposition to current changes.

Mutual Inductance: Now, imagine you have *two coils* placed close to each other. If you change the current in the *first coil*, its magnetic field changes. This changing magnetic field doesn't just pass through the first coil; it also extends outwards and can pass through the *second coil*.

When this changing magnetic flux passes through the second coil, it induces an EMF in the *second coil*, even though there's no direct electrical connection between them. Similarly, if you change the current in the second coil, it can induce an EMF in the first coil.

This phenomenon, where a changing current in one coil induces an EMF in a nearby coil, is called mutual inductance. It represents the magnetic coupling between two coils. The measure of this coupling is the mutual inductance (symbol $M$), also measured in Henrys (H).

A larger mutual inductance means a stronger magnetic coupling, leading to a larger induced EMF in the secondary coil for a given change in current in the primary coil.

In essence, both self and mutual inductance are about how changing magnetic fields, generated by changing currents, induce EMFs. Self-inductance is an internal property of a single coil, while mutual inductance describes the interaction between two separate coils.