a) Mutual inductance of two coils is equal to the e.m.f induced in one coil when rate of change of current through the other coil is unity.

SI unit of mutual inductance is henry.

b) Consider two long solenoids S1 and S2 of same length ‘l’ such that S2 surrounds S1 completely.



Let,

n1 = Number of turns per unit length of S1

n2 = Number of turns per unit length of S2

I1 = Current passing through solenoid S1

∅21 = Flux linked with S2 due to current flowing in S1



is the coefficient of mutual inductance of two solenoids.

When current is passed through S1, emf is induced in S2 .

Magnetic field inside solenoid S1 is given by,

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Magnetic flux linked with each turn of the solenoid =