Mutual Inductance

When current changes in a coil, a changing magnetic flux develops around it, which may induce emf across an adjoining coil. The magnetic flux linking each turn of coil B is due to the magnetic field of the current in coil A.

Therefore, a changing current in each coil induces an emf in the other coil.

φ₂ ∝ φ₁ ∝ I₁

⇒ φ₂ = MI₁

where M is called the mutual inductance of the pair of coils. Also back emf induced across the second coil.

e₂ = – dφ/dt

= – M dI/dt = – M(I₂ - I₁)/t

where the current in coil A changes from I₁ to I₂ in t seconds.

The mutual inductance depends only on the geometry of the two coils, if no magnetic materials are nearby. The SI unit of mutual inductance is henry (H), the same as the unit of self-inductance.

One of the most important applicances based on the phenomenon of mutual inductance is transformer. Some commonly used devices based on self-inductance are the choke coil and the ignition coil.