Diamagnetism and orbital magnetic moment

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Discussion Overview

The discussion centers on the behavior of diamagnetic materials in the presence of an external magnetic field, particularly focusing on the role of the orbital magnetic moment of electrons. Participants explore the classical and quantum mechanical interpretations of this phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that in diamagnetic materials, electrons with orbital magnetic moments speed up in response to an external magnetic field, proposing that this occurs to conserve the initial magnetic moment.
  • Another participant argues that electrons do not experience their own magnetic field in a relevant way, implying that the magnetic field exerts an additional force on the electrons.
  • A different viewpoint indicates that the application of an external magnetic field affects the centripetal force acting on the electrons, which in turn influences their velocity and magnetic moment.
  • One participant challenges the classical interpretation, stating that electrons cannot be treated as classically orbiting particles and that the effects of magnetic fields on atoms are better described by the Zeeman effect, particularly in the context of filled orbitals in diamagnetic materials.
  • This participant further distinguishes between diamagnetic and paramagnetic materials, noting that the coupling between orbital and spin magnetic moments complicates the classical picture.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of classical physics to atomic behavior in magnetic fields, with some advocating for classical interpretations and others emphasizing quantum mechanical effects. No consensus is reached on the correct explanation of the phenomena discussed.

Contextual Notes

The discussion reflects a mix of classical and quantum mechanical perspectives, with participants highlighting the limitations of applying classical physics to atomic systems. There are unresolved assumptions regarding the treatment of magnetic moments and the effects of external fields.

AdityaDev
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For diamagnetic materials, when an external magnetic field is applied, the electrons having orbital magnetic moment in opposite directions spped up.
how does this happen?
The direction of magnetic momwnt is given by right hand thum rule. If there is an electron revolving in anticlockwise direction, magnetic moment is upwards and magnetic field is upwards. So when external magnetic field is applied in downward direction, the magnetic field through the orbit decreases because the two magnetic fields cancel each other. So the electron has to revolve faster to conserve the initial magnetic moment. Is this the reason?
 
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The electron does not feel its own field (at least not in ways relevant here). The magnetic field just leads to an additional inwards(?) force for those electrons.
 
I got it. When an external magnetic field is applied, the force evB will either increase or decrease the centripetal force depending on the direction of rotation of the electron. If centripetal force decreases, the velocity of electron decreases and so the magnetic moment decreases.
 
It sounds like you are trying to apply classical physics to atoms, which are quantum objects. You can't treat the electron as classically orbiting around the nucleus. The effect of the magnetic field on an atom is better described using the Zeeman effect. For the simpler cases of diamagnetic materials where the spin is 0, you have the normal Zeeman effect, where you have coupling between the orbital motion and the field. The energy levels are split so some are lower energy and some are higher energy than the no field case, but when the orbital is filled, then the total energy is slightly higher, so the field repels the atom.

In a paramagnetic atom, spin is nonzero, in which case you have coupling between the orbital magnetic moment and spin magnetic moment, and the picture of an electron orbiting the nucleus is even more untenable. Only the lower energy levels in some orbital gets filled, so Zeeman splitting causes a decrease in total energy and the atom is attracted to the field.
 
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