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X and Y coils are joined in a circuit in such a way that when the change of current in coil X is 2 A, the change in the magnetic flux linked with coil Y is 0.4 Wb. The mutual inductance of the system of two coils is ______ H.

Asked in GSEB Board July 2015 · Mutual inductance and induced EMF

• (1) 0.8
• (2) 0.4
• (3) 0.2
• (4) 5

Step-by-step solution

Why the other options are wrong

• (1) Comes from multiplying instead of dividing: 0.4×2=0.8. Mutual inductance is flux linkage per unit current, so the flux change is divided by the current change.
• (2) Takes the flux change itself as M. That 0.4 Wb goes with a 2 A change; per ampere it is half as much.
• (4) The ratio taken upside down: (Δ I)/(ΔΦ)=2/(0.4)=5. That has the unit A Wb⁻¹, the reciprocal of a henry.

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