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Two circular coils can be arranged in any of the three situations shown in the figure. Their mutual inductance M will be ______.

Asked in RS Academy GUJCET booklet · Standard coil geometries

Figure: Standard coil geometries
Answer: (1) Maximum in situation (1)

Step-by-step solution

Idea: the mutual inductance M = (N₂Φ₂)/(I₁) measures how much of the flux produced by one coil passes through the other.

Situation (1): the coils are coaxial with parallel planes, so the field of one coil passes straight through the other and the linkage is largest.

Situation (2): the planes are perpendicular and the second coil is centred on the first coil's axis; the first coil's field there lies in the second coil's plane, so no flux threads it and M = 0.

Situation (3): the coils are side by side in one plane; only the weaker return field of one coil crosses the other, in the opposite direction, so the linkage is a fraction of that in (1).

So the mutual inductance is maximum in situation (1).

Why the other options are wrong

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