A circular coil having N turns and radius r carries a current I. It is held in the XZ plane in a magnetic field B i. The torque on the coil due to the magnetic field is: (a) Br²Iπ N (b) B π r2IN (c) Zero (d) Bπ r²IN

Answer: (b). A coil in the XZ plane has m = NIπ r² along j. B is along i, so the angle is 90° and τ = mB. So the answer is (b) Bπ r² IN.

A circular coil having N turns and radius r carries a current I. It… | JEE Main 2019 (8 Apr, Shift 1)

Q73JEE Main 2019 · 8 APR · SHIFT 1Medium

Current Loop as a Magnetic Dipole

A circular coil having N turns and radius r carries a current I. It is held in the XZ plane in a magnetic field B $\hat{i}$ . The torque on the coil due to the magnetic field is:

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Answer: (b)

A coil in the XZ plane has $\vec m = NI\pi r^2$ along $\hat j$ .
$\vec B$ is along $\hat i$ , so the angle is $90^\circ$ and $\tau = mB$ .
So the answer is (b) $B\pi r^2 IN$ .

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