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Spinny
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We have a particle with electric charge [tex]e[/tex] that moves in a strong magnetic field [tex]B[/tex]. The particle is constrained to the (x,y)-plane, and the magnetic field is orthogonal to the plane, and constant with [tex]B[/tex] as the [tex]z[/tex]-component of [tex]\mathbf{B}[/tex].
Furthermore we have the rotationally symmetric form of the vector potential,
[tex]\mathbf{A} = -\frac{1}{2}\mathbf{r}\times \mathbf{B}[/tex]
the relation between between velovity and momentum
[tex]\mathbf{v} = \frac{1}{m}\left(\mathbf{p}-\frac{e}{c}\mathbf{A} \right)[/tex]
and the Hamiltonian
[tex]H = \frac{1}{2m}\left(\mathbf{p}-\frac{e}{c}\mathbf{A} \right)^2[/tex]
We now have to show by use of the equation of motion that, generally, the mechanical angular momentum
[tex]L_{mek} = m(xv_y-yv_x)[/tex]
is not a constant ot motion, whereas
[tex]L_{mek}+\left(\frac{eB}{2c} \right)r^2[/tex]
is conserved.
I'm really not sure where or how to start. Do I have to use the Lagrangian and the Euler-Lagrange equation? If so, how do I find out wether or not any of these expressions are constants of motion or not? Or am I way off?
Edit: minor correction to equations.
Furthermore we have the rotationally symmetric form of the vector potential,
[tex]\mathbf{A} = -\frac{1}{2}\mathbf{r}\times \mathbf{B}[/tex]
the relation between between velovity and momentum
[tex]\mathbf{v} = \frac{1}{m}\left(\mathbf{p}-\frac{e}{c}\mathbf{A} \right)[/tex]
and the Hamiltonian
[tex]H = \frac{1}{2m}\left(\mathbf{p}-\frac{e}{c}\mathbf{A} \right)^2[/tex]
We now have to show by use of the equation of motion that, generally, the mechanical angular momentum
[tex]L_{mek} = m(xv_y-yv_x)[/tex]
is not a constant ot motion, whereas
[tex]L_{mek}+\left(\frac{eB}{2c} \right)r^2[/tex]
is conserved.
I'm really not sure where or how to start. Do I have to use the Lagrangian and the Euler-Lagrange equation? If so, how do I find out wether or not any of these expressions are constants of motion or not? Or am I way off?
Edit: minor correction to equations.
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