# Why objects with magnetic dipole moment sometimes orient towards external B and other times they do precession?

Given the magnetic dipole moment in external uniform magnetic field $\vec{B}$, I am trying to understand why sometimes $\vec{\mu}$ simply aligns with the $\vec{B}$ and stays that way, while other times $\vec{\mu}$ starts precessing around $\vec{B}$?

I am interested in both the classical and the quantum concepts of this phenomenon. When I read quantum physics books it seems to me that in quantum world we always have precession and never simple alignment (interaction between say electron spin and external field):

while in classical world we sometimes get one and other times the other.

Is there some condition for precession? I am looking for an intuitive answer just to get some picture/concept in my head, not strict mathematical explanation.

• The dipole does indeed precise. In classical mechanics, we only approximate its allignment for simplification. This is only valid if the dipole turns quasistatically whrn B is switched on(hypothetical situation) Jul 5, 2016 at 0:12

Classically, the magnetic potential energy is $$U=-\boldsymbol{\mu}\cdot\textbf{B}$$. Energy is conserved. Therefore if there is no mechanism for dissipation or for exchanging potential energy for kinetic energy, the angle between the two vectors has to stay the same. If you take a bar magnet and hang it from a string, then it can precess in the earth's field, or just keep its original orientation, or oscillate, depending on the initial state of rotation. If it's oscillating, then there is an interchange of kinetic and potential energy. In a hiking compass, it will oscillate, but there is strong damping because it's water filled, so the energy is dissipated rapidly into heat, and the needle ends up aligned with the field.

Quantum-mechanically, there really isn't any concept of precession for a microscopic body. For example, suppose an electron is in a magnetic field that's in the $$z$$ direction. Say we put the electron in a state of definite energy. Since there is no dissipative mechanism and no way to exchange PE with KE, the electron must have constant $$\boldsymbol{\mu}\cdot\textbf{B}$$, i.e., $$\mu_z$$ stays constant. But in this situation $$\mu_x$$ and $$\mu_y$$ are completely uncertain, so it doesn't make sense to imagine the moment as a vector that's precessing.

I want to give an answer for electrons which have both an intrinsic spin and a magnetic dipole moment. The key for the understanding why electrons in rest in relation to an external magnetic field get aligned and do not precess while moving non-paralle to the external magnetic field electrons undergo a precession is their kinetic energy in relation to the external field.

Electron in rest

An electron in rest when under the influence of a static magnetic field will get aligned with its magnetic dipole moment. For this case nothing more happens, there is no acceleration of the electron nor emission of EM radiation.

Electron in motion in relation to an external magnetic field

There is a well known macroscopic effect of the precession of a rotating wheel, called the gyroscopic effect. On the electrons level this effect can be explained as follows. Underthe influence of the external magnetic field the electron gets aligned with its magnetic dipole moment and by this a small amount of the kinetic energy gets converted into EM radiation. This is what we observe. Emitting photons the electron gets disaligned again. The photons momentum has to be compensated by the movement of the electron and by this the electron gets deflected from its straight trajectory. Once disaligned the game starts again and again. The resulting trajectory is not only a spiral path lasting until the electron is in rest with the external magnetic field and has exhausted its kinetic energy, but the trajectory in detail is made of tangerine slices.

• so if I get it right, if electron is at rest then it will only align to the external field, but if electron is moving then we have precession? In other words, precession happens only for moving charges? Oct 3, 2016 at 15:06
• @matori82 Exact. You get it. Oct 3, 2016 at 15:29
• This just sounds wrong. This has nothing to do with whether the electron is moving or not, unless you mean something about Thomas precession, and what you're saying doesn't sound like a correct description of at at all.
– user4552
Oct 30, 2019 at 20:02

I don't think the interpretation of HolgerFiedler is correct, or I really don't understand why. I feel there is a confusion between "motion", angular momentum and magnetic moment...