Question

When a bar magnet of dipole moment $m$ is suspended in a uniform magnetic field $B$, then

• A. the force on it is $m \times B$
• B. its potential energy is $-m.B$
• C. the torque on it is zero
• D. both torque and force on it are zero
• E. it moves away from the field $B$

Hint:

Physics Tip: Uniform magnetic field gives torque only. Non-uniform magnetic field gives force + torque.

Answer 1

The Correct Option is B

Concept:
For a magnetic dipole in a uniform magnetic field: - Torque: $$\tau = mB\sin\theta$$ - Potential energy: $$U=-mB\cos\theta = -\vec m \cdot \vec B$$ - Net force in uniform field = 0
Step 1: Check option (A).
Force on a dipole in uniform field is zero, not $\vec m \times \vec B$. $\vec m \times \vec B$ gives torque direction idea, not force.
Step 2: Check option (B).
Potential energy of magnetic dipole: $$U=-\vec m \cdot \vec B$$ So this statement is correct.
Step 3: Check remaining options.
(C) Torque is zero only when $\theta=0^\circ$ or $180^\circ$, not always.
(D) Force is zero, but torque may exist.
(E) No translational motion due to uniform field.
Hence correct option is (B). :contentReference[oaicite:0]{index=0}