Question

A short bar magnet of magnetic moment $0.4J{T}^{-1}$ is placed in a uniform magnetic field of $0.16T$. The magnet is in stable equilibrium when the potential energy is

• A. 0.064 J
• B. -0.064 J
• C. zero
• D. -0.082 J

Answer 1

Answer: (B)

Here magnetic moment, $M=0.4J{T}^{-1}$
Magnetic field, $B=0.16T$
When a bar magnet of magnetic moment $\vec{M}$ is placed in a uniform magnetic field $\vec{B}$, its potential energy is :
$\therefore U=-\vec{M}\cdot \vec{B}=-MB\cos \theta$
For stable equilibrium, $\theta =0^{\circ }$
$\therefore U=-MB$
$=-\left(0.4J{T}^{-1}\right)(0.16T)=-0.064J$