A magnetic dipole in a constant magnetic field has: (a) maximum potential energy when the torque is maximum. (b) zero potential energy when the torque is minimum. (c) zero potential energy when the torque is maximum. (d) minimum potential energy when the torque is maximum.

Answer: (c). U = -mBcos θ and τ = mBsin θ. Torque is maximum at θ = 90°, where cos 90° = 0, so U = 0. Torque is zero (minimum) at θ = 0° or 180°, where U = mB, not zero. So the answer is (c): zero potential energy when the torque is maximum.

A magnetic dipole in a constant magnetic field has: | JEE Main 2017 (8 Apr, Shift 1)

Q78JEE Main 2017 · 8 APR · SHIFT 1Easy

Current Loop as a Magnetic Dipole

A magnetic dipole in a constant magnetic field has:

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Answer: (c)

$U = -mB\cos\theta$ and $\tau = mB\sin\theta$ .
Torque is maximum at $\theta = 90^\circ$ , where $\cos 90^\circ = 0$ , so $U = 0$ .
Torque is zero (minimum) at $\theta = 0^\circ$ or $180^\circ$ , where $U = \mp mB$ , not zero.
So the answer is (c): zero potential energy when the torque is maximum.

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Current Loop as a Magnetic Dipole question from JEE Main 2017 (8 Apr, Shift 1)

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