Chapter 8, Problem 1CQ

A dipole without angular momentum can simply rotateto align with the field (though it would oscillate unlessit could shed energy). One with angular momentumcannot. Why?

To determine

To Explain: The role of angular momentum in dipole in order to rotate to align with the field.

Explanation of Solution

Cross product between field vectorand dipole moment vector is known asTorque. The plane formed by the vectors is perpendicular to the direction of torque. Therefore,field(along $z$ axis)is perpendicular to the change in angular momentum. Assumingfield lies along $x$ axis and dipole moment vector ison $xy$ plane. If the dipole without angular momentum aligns, field along the $z$ axis will be perpendicular tochange in angular momentum. If it already has angular momentum along $xy$ plane then change in angular momentum will also be along the $xy$ plane.