When a current carrying coil is placed in a uniform external magnetic field with its magnetic moment antiparallel to the external magnetic field, then
(A) its potential energy is minimum
(B) its potential energy is maximum
(C) torque on it is maximum
(D) torque on it is zero
(E) coil is in unstable equilibrium
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B), (D) and (E) only
Potential energy of a magnetic dipole in uniform field:
$U = -\vec{M} \cdot \vec{B} = -MB\cos\theta$
Case: $\vec{M}$ antiparallel to $\vec{B}$, i.e. $\theta = 180^\circ$
$U = -MB\cos(180^\circ) = -MB(-1) = +MB$
This is the maximum potential energy.
Torque: $\tau = MB\sin\theta = MB\sin(180^\circ) = 0$
At $\theta = 180^\circ$, torque is zero and system is in unstable equilibrium.
Correct options: (B), (D), (E)