A dipole of magnetic moment `vec(m)=30hatjA m^(2)` is placed along the y-axis in a uniform magnetic field `vec(B)= (2hat i +5hatj)T`. The torque acting on it is

Options

A. `-40hatk Nm`
B. `-50hatk Nm`
C. `-60hatk Nm`
D. `-70hatk Nm`

Answer

C

Explanation

`"Here", vec(m)=30hatjAm^(2) and vec(B)=(2hati+5hatj)T`
`"Since", vec(tau)=vec(m)xxvec(B)`
`=30hatjxx(2hati+5hatj)=60hatjxxhati+150hatjxx 60(-hatk)+150xx0`
`=-60hatkNm [therefore hatj xxhatj=-hatkand hatjxxhatj=0]`