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

Work done in a coil W=mBcosθ1-cosθ2When it is rotated by angle 180° thenW=2 m B=2(N I A) B             . . . .(i) Given: N=250,I=85μA=85×10-6 AA=1.25×2.1×10-4 m2≈2.5×10-4 m2B=0.85 TPutting these values in eqn. (i), we get W=2×250×85×10-6×2.5×10-4×0.85≈9.1×10-6 J=9.1μJ