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Mutual inductance $M$ between two coils satisfies:
A$M_{12} \ne M_{21}$ always
B$M_{12} = M_{21}$ by symmetry
C$M_{12} = M_{21}^2$
D$M_{12} = -M_{21}$
Answer & Solution
Correct answer: B. $M_{12} = M_{21}$ by symmetry
By the symmetry of the mutual-inductance integral (Neumann formula), $M_{12} = M_{21} \equiv M$. So you can compute it whichever way is easier — flux in coil 2 per unit current in coil 1, or vice versa.
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