A 250-turns recantagular coil of length 2.1 cm and width 1.25 cm carries a current of `85 muA` and subjected to magnetic field of strength `0.85 T`. Work done for rotating the coil by `180^(@)` against the torque is

Options

A. `9*1muJ`
B. `4*55muJ`
C. `2*3muJ`
D. `1*15muJ`

Explanation

Here `N=250`, `l=2*1xx10^-2m`,
`b=1*25xx10^-2m`,
`I=85xx10^-6A`,
`B=0*85T`
Magnetic moment of coil due to current
`M=NIA=NIlxxb`
`=250xx(85xx10^-6)xx(2*1xx1*25)xx10^-4`
Work done, `W=MB(cos theta_1-cos theta_2)`
`=MB(cos 0^@-cos 180^@)=2MB`
`=2xx250xx(85xx10^-6)xx(2*1xx1*25)xx10^-4xx0*85`
`=9*1xx10^-6J=9*1muJ`