Mutual induction is the phenomenon in which a changing current in one coil (the primary coil) induces an electromotive force (emf) in a neighbouring coil (the secondary coil) due to a change in magnetic flux linkage.
The induced emf in the secondary coil is given by:
where:
The negative sign reflects Lenz's Law — the induced emf opposes the change that caused it.
The mutual inductance between two coils depends on:
Practical Application: The transformer operates on the principle of mutual induction. A changing current in the primary winding creates a changing magnetic flux that induces an emf in the secondary winding.
Self-inductance is the property of a coil by which it opposes any change in the current flowing through itself. When the current in a coil changes, the changing magnetic flux through its own turns induces an emf in the same coil.
This self-induced emf (also called back emf) is given by:
Rearranging to define :
Alternatively, in terms of flux linkage:
where is the number of turns and is the magnetic flux per turn.
By Lenz's Law, the self-induced emf always acts in a direction that opposes the change in current that produced it. This opposition is reflected by the negative sign in the formula. When current increases, the back emf opposes the increase; when current decreases, it opposes the decrease.
The SI unit of both self-inductance and mutual inductance is the henry (H).
From :
A coil has a self-inductance of 1 henry if a current changing at the rate of 1 A/s induces a back emf of 1 V.
An inductor (also called a choke) is a coil of wire, often wound around a soft iron core, that exploits self-inductance to oppose changes in current.
Because an inductor passes DC but blocks (chokes) AC, it is used as a choke in:
Key advantage of a choke over a resistor: A choke limits AC current by reactance (no power dissipated as heat), whereas a resistor wastes energy. This makes chokes energy-efficient current limiters.