If a magnetic dipole of moment M situated in the direction of a magnetic field B is rotated by `180^(@)`, then the amount of work done is

Options

A. MB
B. 2MB
C. `(MB)/(sqrt2)`
D. zero

Answer

B

Explanation

The magnet rotates from `theta = 0 " to " theta = 180^@`
` W = int_(theta = 0)^(theta =180) tau d theta = int_(0)^(180) MB sin theta d theta `
` =MB [-cos theta]_(theta = 0)^(theta = 180)`
`MB [(-cos 180^@)(-cos 0^@) ]`
` = [ 1+ 0 = 2MB]= + 2MB`