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The magnetic potential energy, when a magnetic bar of magnetic moment $\vec{m}$ is placed perpendicular to the magnetic field $\vec{B}$ is
A
$-\dfrac{mB}{2}$
B
Zero
C
$-mB$
D
$mB$
Detailed Solution
Potential energy of a magnetic dipole is $U=-\vec{M}\cdot\vec{B}=-MB\cos\theta$. Since the bar is placed perpendicular to $\vec{B}$, $\theta=90^\circ$, so $U=-MB\cos 90^\circ=0$.