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A 250-Turn rectangular coil of length 2.1 cm and width 1.25 cm carries a current of 85 $\mu A$ and subjected to a magnetic field of strength 0.85 T. Work done for rotating the coil by $180^\circ$ against the torque is
Explanation
Rotating from $0^\circ$ to $180^\circ$ requires $W = 2NIAB$.
Detailed Solution
$W = MB(\cos\theta_1 - \cos\theta_2)$
When it is rotated by $180^\circ$, $W = 2MB$
$W = 2(NIA)B$
$= 2\times250\times85\times10^{-6}\times[1.25\times2.1\times10^{-4}]\times85\times10^{-2}$
$= 9.1\ \mu J$