Choose the wrong statement

The correct answer is Option (3) → The potential energy is maximum when magnetic dipole moment \( \vec{M} \) is parallel to magnetic field \( \vec{B} \)

A bar magnet of moment \( \vec{M} \) in a field \( \vec{B} \) experiences the torque \( \vec{\tau} = \vec{M}\times\vec{B} \), and its potential energy is

    \( \displaystyle U = -\vec{M}\cdot\vec{B} = -MB\cos\theta \)

Option (1) A bar magnet placed in a uniform magnetic field experience no force but experience torque — True statement. In a uniform field the forces on the two poles are equal and opposite, so the net force is zero, but they form a couple, giving the torque \( MB\sin\theta \).

Option (2) In a non-uniform magnetic field a bar magnet experiences a force as well as a torque — True statement. In a non-uniform field the forces on the two poles are unequal, so there is a net force as well as a torque.

Option (3) The potential energy is maximum when magnetic dipole moment \( \vec{M} \) is parallel to magnetic field \( \vec{B} \) — Wrong statement. When \( \vec{M} \) is parallel to \( \vec{B} \), \( \theta = 0^\circ \) and \( U = -MB \), the minimum potential energy (stable equilibrium). The maximum, \( +MB \), occurs when \( \vec{M} \) is antiparallel to \( \vec{B} \). This is the statement asked for.

Option (4) The work done in rotating a magnet in a uniform magnetic field from \( \theta_1 \) to \( \theta_2 \) is \( W = MB(\cos\theta_1 - \cos\theta_2) \) — True statement. The work done equals the gain in potential energy: \( W = U_2 - U_1 = -MB\cos\theta_2 + MB\cos\theta_1 = MB(\cos\theta_1 - \cos\theta_2) \).

NCERT (2026-27):
The Dipole in a Uniform Magnetic Field – “… Equation (5.3) shows that potential energy is minimum (= –mB) at θ = 0° (most stable position) and maximum (= +mB) at θ = 180° (most unstable position).”