Time derivative of the angular momentum as a cross product

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Discussion Overview

The discussion revolves around the time derivative of angular momentum in a system involving a particle with a magnetic moment in a magnetic field. Participants explore the relationship between the time evolution of angular momentum and angular velocity, particularly in the context of Hamiltonian mechanics.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant presents the Hamiltonian for a system with a particle of mass and magnetic moment, leading to the equation for the time evolution of angular momentum.
  • Another participant suggests that the term related to the magnetic field can be defined as a new parameter, specifically identifying it as angular velocity.
  • A question is raised regarding the significance of the equation relating the time derivative of angular momentum to angular velocity, indicating some uncertainty about its implications.
  • A link to an external resource on Larmor precession is provided, possibly as a reference for further understanding.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the justification for identifying the term with angular velocity, and there remains uncertainty about the implications of the derived equation.

Contextual Notes

The discussion does not clarify the assumptions behind the identification of parameters or the specific conditions under which the equations apply.

AndersF
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I am trying to find the equations of motion of the angular momentum ##\boldsymbol L## for a system consisting of a particle of mass ##m## and magnetic moment ##\boldsymbol{\mu} \equiv \gamma \boldsymbol{L}## in a magnetic field ##\boldsymbol B##. The Hamiltonian of the system is therefore

##H=\frac{p^{2}}{2 m}-\gamma \boldsymbol{L} \cdot \boldsymbol{B}##

I have found that the time evolution equation for the angular momentum is

##\frac{d \boldsymbol{L}}{d t}=-\gamma \boldsymbol{B} \times \boldsymbol{L}##

However, the solution identifies the term ##-\gamma \boldsymbol{B}## with the angular velocity ##\boldsymbol{\Omega}##:

##\frac{d \boldsymbol{L}}{d t}=\boldsymbol{\Omega} \times \boldsymbol{L}##

I do not understand what the justification is for making this identification. This last equation looks familiar to me, but I'm not sure where I've seen it... Could someone give me some guidance on this?
 
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I think he introduced a new parameter defined as
\Omega:=-\gamma B.
 
Okay, but does the formula ##\frac{d \boldsymbol{L}}{d t}=\boldsymbol{\Omega} \times \boldsymbol{L}## have any special meaning, being ##\boldsymbol{\Omega}## the angular velocity?
 

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