An electron is the ground state of hydrogen atom is revolving in anticlockwise direction in a circular orbit of radius $'r'$. The atom is placed is a unifom magnetic field $B$ in such a way magnetic moment of orbital electron makes an angle $30^o$ with the magnetic field. The torque experienced by orbital electon is
A$\frac {ehB}{4\pi m}$
B$\frac {ehB}{8\pi m}$
C$\frac {ehB}{2\pi m}$
D$\frac {\sqrt 3 ehB}{8\pi m}$
Diffcult
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B$\frac {ehB}{8\pi m}$
b $\vec{\tau}=\vec{M} \times \vec{B}$
$|\vec{\tau}|=M B \sin \theta$
where $\theta$ is the angle between $\vec{M}$ and $\vec{B} \theta=30^{\circ}$
The direction of $\vec{\tau}$ is perpendicular to both $\vec{M}$ and $\vec{B}$.
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