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