A 250-turn rectangular coil of length 2.1 cm and width 1.25 cm… | NEET 2017

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Field of a Magnet & Magnet in a Uniform Field

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° against the torque is

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Answer: (d)

Rotating through $180^\circ$ from stable position: $W = 2NIAB$ .
$A = 2.1 \times 10^{-2} \times 1.25 \times 10^{-2} = 2.625 \times 10^{-4}\ \text{m}^2$
$W = 2 \times 250 \times 85 \times 10^{-6} \times 2.625 \times 10^{-4} \times 0.85 \approx 9.5 \times 10^{-6}\ \text{J}$
The nearest option is 9.1 $\mu$ J, so the answer is (d).

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