A 250-turn rectangular coil, 2.1 cm long and 1.25 cm wide, carries current 85 μA in a magnetic field of 0.85 T. Calculate the work done in turning it through 180° against the magnetic torque.
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Question

A 250-turn rectangular coil, 2.1 cm long and 1.25 cm wide, carries current 85 μA in a magnetic field of 0.85 T. Calculate the work done in turning it through 180° against the magnetic torque.

G1 Physics · Accepted Answer

Step 1: Area of one rectangular turn A = (2.1×10⁻² m)(1.25×10⁻² m) = 2.625×10⁻⁴ m². Step 2: Magnetic dipole moment m = NIA = 250×85×10⁻⁶×2.625×10⁻⁴ = 5.578×10⁻⁶ A·m². Step 3: For reversal from θ=0 to 180°, energy change W = ΔU = 2mB. Step 4: W = 2×(5.578×10⁻⁶)×0.85 = 9.48×10⁻⁶ J ≈ 9.5 μJ. Final answer: (A) 9.5 μJ.