A torque acts on a current loop in a magnetic field. We use magnetic fields to trap elementary particles. It could be that the spin numbers we are observing is the product of the actual spins of the elementary particles and the spin induced by the incident magnetic field.
The spin of an object is invariant under rotations, that a magnetic field induces. The components of the spin can change, however and that's what happens.
The spin of an object is invariant under rotations, that a magnetic field induces. The components of the spin can change, however and that's what happens.
Indeed spin is an intrinsic property of elementary particles, so cannot change. There is a possibility of bound states of 2 or more particles, where the total spin is a result of coupling (like LS-coupling) in the atom. Such a coupling may be broken by an external magnetic field.
Static magnetic field doesn't change neither spin of state nor spin projection on a fixed direction. Static magnetic field may split spin degeneracy only.
Elementary particles with spin has an intrinsic magnetic dipole moment.
Spin angular momentum vector will display a precession along the axis of the magnetic field. For example if a proton is kept in a magnetic field B (say along Z direction), then the spin vector will precess in the so called Larmor frequency about the z direction while maintaining a constant angle with the z direction. The value of Larmor frequency is ωL= (eg/2mp) B, where g is the Lande g factor. The component (or the projection) of spin angular momentum along the z axis will not change, however.
See: http://users.fmrib.ox.ac.uk/~stuart/thesis/chapter_2/section2_2.html
Usually spin of an elementary particle is an internal property. It is not because of motion of charges. The spin of composite objects could be due to motion of constituents within the object and that may be affected by external magnetic fields. But if the system is a quantum system, change of the spin of the composite is associated with change in energy as well. Besides, the particles have orbital angular momentum because of their motion. If the particle is charged, it will interact with magnetic field and there orbital angular momentum will also be involved. In the original question, the author talks about charge currents. That is a composite system and is classical as well. One cannot apply results of those to elementary particles directly.
All material bodies can have a rotational motion, which implies a rotational momentum upon which some fields, like the magnetic field, can act. This hold for molecules, which have a definite geometric structure, and is assumed to hold also for atoms and elementary particles. The feature which is specific to atoms and elementary particles is that the rotational motion is quantized, roughly it is periodic with a fixed frequency, and this frequency can be characterized by a quantum number, usually called the spin.This quantum number does not change but it does not mean that there is no change in the rotational motion : the frequency of the motion is fixed, but the axis of rotation can change when a force is present. Moreover any rotational motion can be represented in two ways, according to the orientation of the axis and the speedof rotation : we have the same motion with the opposite axis and the opposite speed. Physically there is a difference between the two motions, and the orientation of the axis can be defined with respect to the velocity of the particle. So we have spin up or down and the two states are diffrent and a change in the orientation of the axis requires energy.
Spin of a particle will not change ,However its interaction in L~S coupling will change
The situation becomes more complicated when you deal with system of particles .
Some light reading on spectral splitting in Atomic and molecular spectra will be helpful.
Prof B.Rath
The spin of a particle (spin quantum number) cannot change under the effect of one external magnetic field. The best proof of this is that we cannot change fermions in bosons or vice versa by the application of a magnetic field.
Another very different thing, and which many people confuse, is the spin magnetic moment where the total spin is replaced by its eigenvalue using the spin quantum number. This magnitude change under a magnetic field using the Zeeman coupling between both. Energetically it is only important when there is spin-orbit coupling for the total spin which gives very different values for the spin magnetic moment of such system.
I hope that this can help
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