We consider a quantum spinless nonrelativistic charged particle moving in the xy plane under the action of a time-dependent magnetic field, described by means of the linear vector potential A=B(t)-y(1+α),x(1-α)/2, with two fixed values of the gauge parameter α: α=0 (the circular gauge) and α=1 (the Landau gauge). While the magnetic field is the same in all the cases, the systems with different values of the gauge parameter are not equivalent for nonstationary magnetic fields due to different structures of induced electric fields, whose lines of force are circles for α=0 and straight lines for α=1. We derive general formulas for the time-dependent mean values of the energy and magnetic moment, as well as for their variances, for an arbitrary function B(t). They are expressed in terms of solutions to the classical equation of motion ε¨+ωα2(t)ε=0, with ω1=2ω0. Explicit results are found in the cases of the sudden jump of magnetic field, the parametric resonance, the adiabatic evolution, and for several specific functions B(t), when solutions can be expressed in terms of elementary or hypergeometric functions. These examples show that the evolution of the mentioned mean values can be rather different for the two gauges, if the evolution is not adiabatic. It appears that the adiabatic approximation fails when the magnetic field goes to zero. Moreover, the sudden jump approximation can fail in this case as well. The case of a slowly varying field changing its sign seems especially interesting. In all the cases, fluctuations of the magnetic moment are very strong, frequently exceeding the square of the mean value.
We consider a quantum spinless nonrelativistic charged particle moving in the
xy plane under the action of a time-dependent magnetic field, described by means of the linear vector potential ${\bf A}=B(t)\PG{-y(1+\alpha),x(1-\alpha)}/2$, with two fixed values of the gauge parameter
α:
α=0 (the circular gauge) and
α=1 (the Landau gauge). Although the magnetic field is the same in all the cases, the systems with different values of the gauge parameter are not equivalent for nonstationary magnetic fields due to different structures of induced electric fields, whose lines of force are circles for
α=0 and straight lines for
α=1. We derive general formulas for the time-dependent mean values of the energy and magnetic moment, as well as for their variances, for an arbitrary function
B(t). They are expressed in terms of solutions to the classical equation of motion $\ddot\vep +\omega_{\alpha}^2(t) \vep=0$, with
ω1=2ω0. Explicit results are found in the cases of the sudden jump of magnetic field, the parametric resonance, the adiabatic evolution, and for several specific functions
B(t), when solutions can be expressed in terms of elementary or hypergeometric functions. These examples show that the evolution of the mentioned mean values can be rather different for the two gauges, if the evolution is not adiabatic. It appears that the adiabatic approximation fails when the magnetic field goes to zero. Moreover, the sudden jump approximation can fail in this case, as well. The case of slowly varying field changing its sign seems especially interesting. In all the cases, fluctuations of the magnetic moment are very strong, frequently exceeding the square of the mean value.
Abstract I consider a quantum spinless charged particle moving in the xy plane under the action of a time-dependent magnetic field described by means of the linear vector potential in the ‘circular’ gauge A = B ( t )(− y , x )/2 and in the presence of the confining potential m g 2 x 2 + y 2 / 2 . This potential guarantees finite values of all second-order moments in the initial equilibrium (thermal) state. Time-dependent mean values of the energy and magnetic moment are expressed in terms of solutions to the classical equation of motion ε ̈ + Ω 2 ( t ) ε = 0 . Simple approximate results are found in the cases of the sudden jump of magnetic field, the parametric resonance, and the adiabatic evolution. They are compared with exact solutions in terms of the hypergeometric function for the specific function B ( t ) of the Epstein–Eckart type. A great amplification of the magnetic moment is discovered for the initial high-temperature states, both for the fast and slow monotonous variations of the magnetic field. Classical adiabatic invariants do not exist if the magnetic field slowly goes to zero or changes its sign.
Abstract Because the relativistic transformation of time involves spatial coordinates, multipole moments can take on very different values in relatively-moving inertial frames. As a related aspect, an infinite solenoid of finite cross-section will have non-zero time-varying electric fields outside its winding arising from the discrete charges carrying the solenoid currents. In the rest frame of the solenoid, the time averages of these time variations vanish and may be ignored. However, these time-varying electric fields have non-zero time averages in an inertial frame where the solenoid is moving perpendicular to its axis, and this non-zero time average plays an important role in the classical electromagnetic interpretation of the Aharonov–Bohm phase shift, which was treated in earlier work. Here we illustrate these ideas for a circular current loop, and then note their applicability to solenoids and toroids. The ‘ideal magnetic moment’ with no charges and no spatial extent is completely unphysical, and is seen to lead, in a moving inertial frame, to a different and incorrect average electric field from that where discrete charges are used and the radius of the magnetic moment is finite.
In the context of the Aharonov-Bohm effect, it is well-known that the local action of the vector potential or the non-local action of the magnetic field are properties attributable to quantum physics, which can explain the observed Aharonov-Bohm phase. Here we point out that, surprisingly, these properties are also attributable to classical physics and that this result follows from examining the electromagnetic angular momentum of a static electric charge outside an infinitely-long solenoid of finite radius carrying a uniform magnetic flux. This angular momentum may be expressed in three equivalent forms. The first form shows that there is an electromagnetic angular momentum even in regions for which the magnetic field vanishes but the potential does not, which stresses the physical nature of the vector potential in classical physics. The second form shows that there is an electromagnetic angular momentum produced by the non-local action of the magnetic field. The third form shows that the Aharonov-Bohm phase is proportional to the magnitude of the electromagnetic angular momentum, which sheds light on the physical meaning of this phase.
The classical electromagnetic interaction for a charged particle with a long, constant-current solenoid is investigated in the context of classical electromagnetism. The conservation laws connected with energy, linear momentum, and angular momentum are first verified. Then the changes in the electromagnetic field quantities are evaluated explicitly, and the values are seen to have interesting connections with the solenoid vector potential. The calculations are of interest in connection with the Aharonov-Bohm effect involving the passage of electrons past a solenoid, and the classical expressions for energy and momentum are reminiscent of terms in the quantum-mechanical calculations.