Question

A current carrying loop is placed in a uniform magnetic field in four different orientations as shown in figure. Arrange them in the decreasing order of potential energy.

• A. 4, 2, 3, 1
• B. 1, 4, 2, 3
• C. 4, 3, 2, 1
• D. 1, 2, 3, 4

Answer 1

Answer: (B)

Potential energy,
$ U = - \vec{M} \cdot \vec{B} = -MB \,cos\theta$
$\left(1\right) U=- MBcos180^{\circ} = MB$
$\left(2\right) U = - MBcos90^{\circ} = 0$
$\left(3\right) U = - MBcos\theta$
$\left(4\right) U= + MBcos\theta$