# Diamagnetism and orbital magnetic moment

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1. Feb 25, 2015

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?

2. Feb 27, 2015

### Staff: Mentor

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.

3. Feb 28, 2015

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.

4. Feb 28, 2015

### Khashishi

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.