<Velocity> <differentiation> gives $\boxed{p_r=m\dot r,\ p_\theta=mr^2\dot\theta,\ p_z=m\dot z+qA(r)}$. The coordinates $\theta$ and $z$ are cyclic, so $p_\theta,p_z$ are conserved. The radial <momentum> generally changes. In particular the conserved axial <canonical momentum> differs from the axial <mechanical momentum> by $qA$.

Solution

Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S7 / 1 / b / Solution

 Solution (source code)