Question: Q. 1. Define mutual inductance between a pair of coils. Derive an expression for the mutual inductance of two long coaxial solenoids of same length wound one over the other.

R [O.D. I, II, III 2017, 2014]

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Solution:

Ans. Definition of mutual inductance

1 Derivation of mutual inductance for two long solenoids

(i) Mutual inductance, is numerically equal to the induced emf in the secondary coil when the current in the primary coil changes by unity.

Alternatively,

Mutual inductance in numerically equal to the magnetic flux linked with one coil/secondary coil when unit current flows through the other coil/ primary coil.

(ii)

Let a current I2, flow in the secondary coil

B2=μ0N2i2l

Flux linked with the primary coil

=μ0N2N1A1i2l =M12i2

Hence,

M12=μ0N2N1A2l =μ0n2n1A1l (n1=N1l;n2=N2l)1/2

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