The potential energy of circular loop in the magnetic field = - M B cos θ, where θ is the angle between normal to plane of coil and magnetic field B. Initial potential energy, Ui=−NiABcos0°=−NiAB When coil is turned through 180°, therefore final potential energy, Uf=−NiABcos180°=NiAB Required work, W = gain in potential energy =Uf−Ui=NiAB−(−NiAB)=2NiAB Here, N =100, i = 0.1A, B = 1.5 Wb/m 2 and radius r = 0.05 m ∴A=πr2=3.14×(0.05)2=7.85×10−3m2 ∴Work, W=2×100×0.1×7.85×10−3×1.5 ⇒W=0.2355J