An electron in the ground state of hydrogen atom is revolving in anticlockwise direction in the circular orbit of radius R. The atom is placed in a uniform magnetic induction B such that the plane normal of electron orbit makes an angle `30^@` with B, as shown in figure. The torque experienced by electron will be
.

Options

A. `(ehB)/(8pim)`
B. `(eh)/(8pi Bm)`
C. `(eB)/(8 pi m h)`
D. `(hB)/(8 pi em)`

Answer

A

Explanation

`tau = M xx B and M = I A`
`M=(ev)/(2pi r) xx pi r^2= ((mvr)e)/(2m) =((nh)e)/(2pi (2m))`
`therefore M =(eh)/( 4pi m) tau = MB sin 30^@`