In which orientation of the magnetic dipole with the external uniform magnetic field is the system most stable?

θ = 180 θ = 45 θ = 90 θ = 0

θ = 0

The system will be most stable when the potential energy is the least. The potential energy of a magnetic dipole in a uniform magnetic field be the minimum when θ = 0. Um = -m.B = -mB Cos θ Umin = -mB ⇒ Cos θ = 1 ⇒ θ = 0