# Spin/angular magnetism Bohr magneton quantum thing

1. Aug 25, 2014

### Zondrina

So I want to get this whole spin/angular magnetism Bohr magneton quantum thing straight. Suppose a diamagnetic sphere lies at the origin. Lets talk in the x-y space for simplicity. Lets bring a bar magnet in along the x-axis approaching zero from the right.

If the north pole of a bar magnet is brought near the circle, the circle will be attracted to the bar magnet due to the induced magnetic dipole moment in the material, which wants to move away from a region of increasing magnetic field. These forces are relatively weak and would require a very strong external field to have any noticeable physical effect. This would also depend on the temperature of the material considering Curie's approximation $C = M \frac{B_{ext}}{T}$. If the temperature is too high, the magnetic domains inside the material would become agitated, and the dipoles in each respective domain won't line up to produce a strong net dipole.

If the south pole is brought near the circle instead, it should be expected that the circle be repelled due to the induced dipole.

If the material is now paramagnetic, it should be repelled by the N-Pole and attracted by the S-Pole. This is due to how the induced dipole lines up with the external field. The material needs to be kept quite cool and a decently strong external field must be applied to notice anything.

If the material is now ferromagnetic, a similar result to that of a paramagnetic material can be observed. A relatively weak external field can be applied due to the magnetic nature of the material. That is, the material already exerts its own magnetic field intrinsically and any external field would show obvious results.

Does this make sense?

2. Aug 26, 2014

### Simon Bridge

Diamagnetism is always repulsive.
You are thinking of Lenz's Law: the induced current in a conductor due to changing magnetic field - the current induced produces a magnetic field opposing the change in the applied field.

Note: I replaced "circle" with "sphere" - this was what you meant right?

Diamagnetism is usually a weak effect compared with paramagnetism - magnetization, in general, depends on temperature.

The diamagnetic repulsion does not depend on the pole.

Paramagnetism is always attractive.

Ferromagnetism is the property of being able to form into permanent magnets. In all magnetic materialls, the domains take a while to relax back to their jumbled configuration once the applied field is removed.

It's easy to get the bits mixed up.
See: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magpr.html

Later, when you start to study Maxwell's equations, the links between the different phenomena will become more clear.

3. Aug 26, 2014

### Zondrina

Yes a sphere would be the general case, but for visual simplicity I wanted to consider a 2-D case, which is why I said circle.

I also see why in the case of the diamagnetic, it is always going to be repelled. I dew an image to show this. The magnetic dipole set up in the material will always have a similar pole facing the bar magnet and so must be repelled in every case.

Similar drawings can be made for a paramagnetic material, except they will be attracted. Same deal with ferromagnetic materials.

I am familiar with Maxwell's equations, though I'm not sure how to explain how they relate to this.

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4. Aug 26, 2014

### Simon Bridge

It's just that a circle of material is a bit different from a sphere.
You just wanted to look at it in profile - which for a sphere is any single angle ;)

Ferro-, Para-, and, Dia-, magnetism are just names for emergent phenomena.
You seem to have it straight now.