- #1
Jansen
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Homework Statement
Show that the Lagrangian
$$ L = \frac{m}{2}(\dot{r}^2 + r^2\dot{\theta}^2 + r^2\dot{\phi}^2sin^2\theta) - qg\dot{\phi}cos\theta $$
describes the motion of a charged particle (mass ##m## charge ##q##) in the magnetic field ## \vec{B} = g\vec{r}/r^3## (##g## is a constant). Find the first integrals of the equations of motion.
Homework Equations
##L=T-V##, ##T=\frac{m}{2} \dot{\vec{x}}^2## and for non conservative ##\vec{F}## we may still use an effective 'potential energy' provided it satisfies $$F_\alpha=\frac{d}{dt}\left(\frac{\partial V}{\partial \dot{q}_\alpha} \right) - \frac{\partial V}{\partial q_\alpha}$$
The Attempt at a Solution
First thing that grabs me is that this is a diverging magnetic field. g represents some sort of magnetic charge, I suppose, maybe there is a prefactor in g too. I do not know. So, I guess that means there is no vector potential. I mean, ## \vec{\nabla} \times \vec{B} = \vec{0} \Rightarrow \exists \Phi_m \ni \vec{B} = -\vec{\nabla}\Phi_m##
Here, ##\Phi_m=-\frac{g}{r}##. But the Lorentz force that results from such a field is not conservative.
I figure there could be 2 approaches:
1) The lazy way. We see from the given L that the first part of the sum, ## \frac{m}{2}(\dot{r}^2 + r^2\dot{\theta}^2 + r^2\dot{\phi}^2sin^2\theta)=T##. Then we already have, ##V=qg\dot{\phi}cos\theta## We need simply show that it satisfies ##F_\alpha=\frac{d}{dt}\left(\frac{\partial V}{\partial \dot{q}_\alpha} \right) - \frac{\partial V}{\partial q_\alpha}##. But I get: $$\vec{F}=q\dot{\vec{x}} \times \vec{B} = q(\dot{r} \hat r + r\dot{\theta} \hat \theta + r\dot{\phi}sin\theta \hat \phi) \times \frac{g}{r^2} \hat r = q(\frac{g \dot{\phi}}{r} sin \theta \hat \theta - \frac{g \dot{\theta}}{r} \hat \phi)$$
But $$ \frac{d}{dt}\frac{\partial}{\partial \dot \theta}(gq\dot \phi cos\theta)= 0$$ and $$\frac{\partial}{\partial \theta}(gq\dot \phi cos\theta) = -gqsin\theta \dot \phi \neq F_\theta$$
Am I doing the calculus wrong? I don't understand if I have a 1/r factor in the force, this should appear somehow in the derivatives of the given potential. But how? Am I assuming an incorrect expression for the force in the first place? Do forces on charged particles due to magnetic fields from magnetic monopoles behave in some strange way I am not thinking about?
2) Derive the potential function and end up with the suggested expression. This is how I would prefer to do it, but I don't know quite where to start. Taking the line integral over some arbitrary curve will not help, I think, because the Lorentz force is not conservative. Yes, so if anyone has a hint on this method I would really appreciate it. The examples of how to set up the equation of motion of a particle due to an electromagnetic force in my classical mechanics book just give a potential function and don't describe the way to obtain the potential.
I feel obliged to say this is my first post and I hope that my formatting isn't too painful and that I didn't break any major rules. Thanks!