Which particles' spin determines the direction of a magnetic field?

Which particles' spin determines the direction of a magnetic field?

• A. Protons determine direction
• B. Electrons determine direction
• C. Neutrons determine direction
• D. Photons determine direction

Answer: (B)

The direction of a magnetic field is set by magnetic dipole moments, which for elementary particles come from spin. For an electron, the intrinsic spin creates a magnetic moment mu, and its magnitude is proportional to the spin, with mu pointing opposite to the spin direction because of the electron’s negative charge (mu = -g e/2m_e times S). This means the orientation of the electron’s spin directly fixes the direction of the magnetic moment, and thus the local magnetic field around that electron. In atoms, the net magnetic field direction is determined mainly by unpaired electron spins, since their magnetic moments are large compared with those of protons or neutrons. Protons and neutrons do have spins and magnetic moments too, but their effects are usually smaller or cancel out in many materials. Photons, while carrying angular momentum, aren’t responsible for a static magnetic field direction in the same way; they’re quanta of the electromagnetic field and relate to polarization rather than setting a fixed dipole direction in space. So the spin of electrons best establishes the direction of the magnetic field.

The direction of a magnetic field is set by magnetic dipole moments, which for elementary particles come from spin. For an electron, the intrinsic spin creates a magnetic moment mu, and its magnitude is proportional to the spin, with mu pointing opposite to the spin direction because of the electron’s negative charge (mu = -g e/2m_e times S). This means the orientation of the electron’s spin directly fixes the direction of the magnetic moment, and thus the local magnetic field around that electron.

In atoms, the net magnetic field direction is determined mainly by unpaired electron spins, since their magnetic moments are large compared with those of protons or neutrons. Protons and neutrons do have spins and magnetic moments too, but their effects are usually smaller or cancel out in many materials. Photons, while carrying angular momentum, aren’t responsible for a static magnetic field direction in the same way; they’re quanta of the electromagnetic field and relate to polarization rather than setting a fixed dipole direction in space.

So the spin of electrons best establishes the direction of the magnetic field.