A current loop is placed in a uniform magnetic field in four different orientations, I, II, III and IV. Arrange them in the decreasing order of potential energy.

Potential energy for magnetic dipole is

$\begin{array}{l}{\mathrm{u}}_{\mathrm{I}}=2\mathrm{MB} \left[\mathrm{\theta }={180}^0\right]\\ {\mathrm{u}}_{\mathrm{II}}=0\left[\mathrm{\theta }={90}^0\right]\end{array}$

$\begin{array}{l}{\mathrm{u}}_{\mathrm{III}}=-\mathrm{MB}\mathrm{cos\theta } \left[0<\theta <90\right]\\ {\mathrm{u}}_{I{V}_{ }}=-MB \cos \left( \theta \right) [90<\theta <180] =\mathrm{positive} \mathrm{but} \mathrm{less} \mathrm{than} \mathrm{MB} \end{array}$

$\therefore$   Decreasing order of potential energy is

I > IV$>$II$>III$