# Spin/angular magnetism Bohr magneton quantum thing

• STEMucator
In summary, the conversation discusses the concept of magnetism and its different forms - diamagnetism, paramagnetism, and ferromagnetism. It is mentioned that diamagnetic materials will always be repelled by a magnet due to the induced dipole moment, while paramagnetic materials will be attracted. Ferromagnetic materials, on the other hand, can form into permanent magnets and show obvious results even with a weak external magnetic field. The conversation also touches upon the importance of temperature in determining the strength of magnetization in different materials, and the links between these phenomena and Maxwell's equations.
STEMucator
Homework Helper
So I want to get this whole spin/angular magnetism Bohr magneton quantum thing straight. Suppose a diamagnetic sphere lies at the origin. Let's talk in the x-y space for simplicity. Let's 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?

Zondrina said:
So I want to get this whole spin/angular magnetism Bohr magneton quantum thing straight. Suppose a diamagnetic sphere lies at the origin. Let's talk in the x-y space for simplicity. Let's 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 [sphere], the [sphere] 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.
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?

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

If the south pole is brought near the [sphere] instead, it should be expected that the [sphere] be repelled due to the induced dipole.
The diamagnetic repulsion does not depend on the pole.

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.
Paramagnetism is always attractive.

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.
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.

Simon Bridge said:
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.

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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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.

1 person

## 1. What is a Bohr magneton?

A Bohr magneton is a unit of magnetic moment, or the strength of a magnetic field, used in quantum mechanics. It is equal to approximately 9.274 x 10^-24 joule per tesla, and is named after physicist Niels Bohr.

## 2. How does spin affect magnetism?

Spin is a quantum property of particles that can be thought of as their intrinsic angular momentum. This spin contributes to the magnetic moment of the particle, and can interact with external magnetic fields to affect the overall magnetic properties of a material.

## 3. What is the relationship between angular momentum and magnetism?

Angular momentum and magnetism are closely related through the concept of spin. In quantum mechanics, particles with spin have an associated magnetic moment, which is proportional to their angular momentum. This allows for the prediction and understanding of magnetic properties in materials.

## 4. What does the term "quantum" refer to in this context?

In this context, the term "quantum" refers to the principles of quantum mechanics, which is the branch of physics that studies the behavior of particles on a very small scale. These principles are necessary to understand and explain the properties of spin, magnetism, and the Bohr magneton.

## 5. How is the Bohr magneton used in research and industry?

The Bohr magneton is an important constant used in many calculations and models in research and industry, particularly in the fields of quantum mechanics and magnetism. It helps scientists and engineers understand and predict magnetic behavior in materials, which has applications in technologies such as MRI machines, computer hard drives, and magnetic data storage devices.

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