A magnet of magnetic moment M is situated with its axis along the direction of a magnetic-field of strength B. The work done in rotating it by an angle of 180° will be

Work done in rotating the magnet from angle ${\theta }_1$ to angle ${\theta }_2$ is given by

                                              $W=MB(\cos {\theta }_1-\cos {\theta }_2)$

Here,                                      ${\theta }_1=0^{\circ }$   and  ${\theta }_2={180}^{\circ }$

                                             $$\begin{gathered} W=MB(\cos 0^{\circ }-\cos {180}^{\circ }) \\ =2MB \end{gathered}$$