A 250-turn rectangular coil of length 2.1 cm and width 1.25

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A 250-turn rectangular coil of length 2.1 cm and width 1.25 cm carries a current of 85 µA and subjected to magnetic field of strength 0.85 T. Work done for rotating the coil by 180º against the torque is

1. 4.55 µJ
2. 2.3 µJ
3. 1.15 µJ
4. 9.1 µJ

Correct Option: D

Work done, W = MB(cosθ₁ – cosθ₂)
When it is rotated by angle 180° then W = MB (cos0° – cos 180°) = MB (1 + 1)
W = 2MB
W = 2 (NIA) B
= 2 × 250 × 85 × 10^–6[1.25 × 2.1 × 10^–4] × 85 × 10^–2 = 9.1 µJ