Consider a magnetic dipole which on switching ON external magnetic field orient only in two possible was i.e., one along the direction of the magnetic field (parallel to the field ) and another anti-parallel to magnetic field. Compute the energy for the possible orientation . Sketch the graph.

Explanation

Let `vec(p_(m)) ` be the dipole and before switching ON the external magnetic field. There is no orientation. Therefore, the energy U = 0
As soon as external magnetic field is switched ON, the magnetic dipole orient parallel `(theta = 0)^(@)` to the magnetic field with energy .
`U_("parallel") = U_("minimum") = - p_(m) B cos 0 `
`U_("parallel") = - p_(m) B " since cos " 0^(@)` = 1
otherwise, the magnetic dipole orients anti-parallel `(theta = 180^(@))` to the magnetic field with energy.
`U_("anti-parallel") = U_("minimum") = - p_(m) B cos ` 180
` rArr U_("anti-parallel") = - p_(m) B " since cos " 180^(@)` = -1