Self Inductance — Definition

Definition

Imagine you have a coil of wire, like a spring. When you pass an electric current through this coil, it creates a magnetic field around itself. This magnetic field isn't just static; its lines of force pass through the coil itself, a phenomenon we call magnetic flux.

Now, here's where it gets interesting: if you try to change the current flowing through the coil – either by increasing it or decreasing it – the magnetic field it produces also changes. And according to Faraday's Law of Electromagnetic Induction, a changing magnetic flux through a coil will induce an electromotive force (EMF) or voltage across that same coil.

This induced EMF is often called a 'back EMF' because, crucially, it always acts in a direction that opposes the change in current that caused it. This opposition is a manifestation of Lenz's Law, which states that the induced current will flow in a direction that creates a magnetic field opposing the original change in flux.

So, if you try to increase the current, the induced EMF will try to push current in the opposite direction, resisting the increase. If you try to decrease the current, the induced EMF will try to push current in the same direction, resisting the decrease.

This inherent property of a coil to resist changes in the current flowing through it is what we call self-inductance. It's like the electrical inertia of the coil. The greater the self-inductance of a coil, the more it will oppose any attempt to change the current passing through it.

This property is quantified by a value 'L', measured in Henries (H). A coil designed to have significant self-inductance is called an inductor. Inductors are essential components in many electronic circuits, used for filtering, energy storage, and tuning circuits, all thanks to this 'self-resistance' to current changes.