Mutual Inductance — Definition
Definition
Imagine you have two separate coils of wire, let's call them Coil 1 and Coil 2, placed close to each other. They are not electrically connected, but their magnetic fields can interact. When you pass an electric current through Coil 1, it generates a magnetic field around itself.
This magnetic field doesn't just stay around Coil 1; it spreads out, and some of its magnetic field lines will pass through Coil 2. This 'passing through' of magnetic field lines is what we call magnetic flux linkage with Coil 2.
Now, here's where the magic of mutual inductance comes in: If you change the current flowing through Coil 1 (either by increasing it, decreasing it, or switching it on/off), the magnetic field it produces will also change.
Consequently, the magnetic flux linked with Coil 2 will also change. According to a fundamental principle in electromagnetism known as Faraday's Law of Electromagnetic Induction, any time there's a change in magnetic flux through a coil, an electromotive force (EMF) – essentially a voltage – is induced in that coil.
So, a changing current in Coil 1 induces an EMF in Coil 2, even though there's no direct electrical connection.
The 'mutual inductance' (represented by the symbol 'M') is a measure of how effectively a change in current in one coil induces an EMF in the other. A higher mutual inductance means that a small change in current in one coil will induce a larger EMF in the other.
It quantifies the magnetic coupling between the two coils. The value of mutual inductance depends on several factors: the number of turns in each coil, their geometric shapes and sizes, their relative orientation (how they are positioned with respect to each other), and the permeability of the medium or core material between them.
For instance, if the coils are wound on a common soft iron core, the mutual inductance will be much higher than if they are in air, because soft iron is a ferromagnetic material that can concentrate magnetic field lines much more effectively.
This phenomenon is the operating principle behind devices like transformers, which are crucial for changing AC voltages.