When an atom has a net magnetic moment that is not aligned with external magnetic field, it is said that the atom rotates because of the torque and the magnetic moment gets aligned with external field.

If I put a compass needle in uniform magnetic field in a way it is acted on a net torque, the needle should start oscillating and continue like that if there is no friction.

My question: What keeps the atom from oscillating about the line where it experiences no torque? Is it some kind of atomic friction due to the presence of other atoms?


Oscillating magnetic dipole will lead to emission of radiation. That's the route for loosing energy. Same goes for the needle, but there the friction usually wins.

You start with atomic magnetic dipole in some arbitrary orientation. The atom is in the ground-state. You switch on the magnetic field. The ground state of the atom splits into a new ground state and an excited state. If the atomic moment is not aligned with the field, it will be, at least partially, in excited state. In this case, over time, it will decay into ground state, by (spontaneously) emitting light (assuming no other decay channels are available).


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