1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Spin 1/2 particles and magnetic fields

  1. Apr 22, 2009 #1
    I was told that if you put a spin 1/2 particle in a magnetic field, it will align with or against the field. But some places it also says that it will precess around the field. Which one is right?
     
  2. jcsd
  3. Apr 22, 2009 #2

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    If the magnetic dipole of that particle is constant (as is in the classical case) then the dipole will allign with the magnetic field. If the magnetic dipole is proportional to the angular momentum, as is the case with actual particles \mu=\gamma*L Then the dipole (and therefore the spin) can no longer align with the magnetic field, and will precess around it (Larmer Precession).
     
  4. Apr 22, 2009 #3
    So the spin doesn't *really* align with the magnetic field, then, it just precesses around it. So what do spin 'up' and spin 'down' really refer to? To whether the magnetic moment is pointed towards the direction of the field or away from it, but not parallel or antiparallel? I guess that would make sense.
     
  5. Apr 22, 2009 #4

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    Spin up and spin down are not defined using magnetic moments. They usually mean the z-component of the spin vector (expectation) is +hbar/2 or -hbar/2 for up or down respectively.
     
  6. Apr 22, 2009 #5
    So I had it backwards? The spin is intrinsic, and the magnetic moment is defined using the spin - since spin is a type of angular momentum.

    So if I put a spin 1/2 particle in a magnetic field, whether or not the spin vector points towards the field or away from it determines whether or not the magnetic dipole moment precesses towards or away from the direction of the magnetic field?
     
  7. Apr 22, 2009 #6

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    The spin is an intrinsic property of a particle (electron is 1/2, photon is 1, etc). But think of this "spin" as the "total spin". I.e. it is like the magnitude of the spin vector. The spin's "direction" and what not is determined by external "forces" and the time evolution of the spinors.
     
  8. Apr 22, 2009 #7
    Now that makes sense! (-in so much as spin is sensible) Thanks!
     
  9. Apr 23, 2009 #8
    precessing particles emit radio waves until they stop precessing.
    en.wikipedia.org/wiki/Magnetic_resonance_imaging
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook