Time derivative of the angular momentum as a cross product

In summary, the conversation discusses the equations of motion and Hamiltonian for a system consisting of a particle with mass and magnetic moment in a magnetic field. The time evolution equation for the angular momentum is found to be equal to the cross product of the angular velocity and the angular momentum. The justification for this identification is not clear, but it is mentioned that the formula is similar to the Larmor precession equation.
  • #1
AndersF
27
4
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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  • #2
I think he introduced a new parameter defined as
[tex]\Omega:=-\gamma B[/tex].
 
  • #3
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?
 

FAQ: Time derivative of the angular momentum as a cross product

What is the formula for calculating the time derivative of angular momentum as a cross product?

The formula for calculating the time derivative of angular momentum as a cross product is dL/dt = r x F, where dL/dt represents the time derivative of angular momentum, r is the position vector, and F is the net force acting on the object.

How is the time derivative of angular momentum related to rotational motion?

The time derivative of angular momentum is directly related to rotational motion, as it represents the change in angular momentum over time. This change is caused by the net torque acting on an object, which causes it to accelerate or decelerate in its rotational motion.

What is the significance of using a cross product in the calculation of the time derivative of angular momentum?

The cross product takes into account both the magnitude and direction of the force and position vectors, resulting in a more accurate calculation of the time derivative of angular momentum. It also allows for the incorporation of the right-hand rule, which determines the direction of the resulting vector.

Can the time derivative of angular momentum be negative?

Yes, the time derivative of angular momentum can be negative. This indicates that the angular momentum is decreasing over time, which can occur when there is a net torque acting in the opposite direction of the initial angular momentum.

How is the time derivative of angular momentum used in real-world applications?

The time derivative of angular momentum is used in many real-world applications, such as spacecraft navigation, gyroscopic stabilization systems, and the design of rotating machinery. It is also an important concept in understanding the motion of objects in rotational motion, such as planets and satellites.

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