Sciencemadness Discussion Board

Bloch / domain wall in bar magnet

D4RR3N - 7-6-2010 at 08:30

Well I thought I roughly understood what a magnet was until I started reading about bar magnets and the concept of there being a bloch wall at the centre of the bar magnet.

My understanding of a bar magnet is that the magnetism is caused by a combination of electron spin and electron orbit alignment within domains. Get the majority of these domains magnetic fields pointing in the same direction and you have a bar magnet.

What is confusing me is the talk about clockwise and anticlockwise spin in a magnet, surely all the electrons are spinning in the same direction, either clockwise or anticlockwise....why should the electrons decide to change direction of spin halfway through the bar magnet?

Look at the image below and note the arrow illustrating direction of spin.

Lets say we put a steel bar in a coil similar to the other image below and we magnetise the steel. Obviously the electrons (current) flowing in the coil are all rotating in the same direction so wouldn't you expect the spin of the electrons in would also all be rotating in the same direction. Why would the electrons halfway down the bar decide they are going to spin in the opposite direction to the rotation of electrons magnetising the bar via the coil?


bloch.gif - 3kBxxx.gif - 9kB

watson.fawkes - 7-6-2010 at 18:23

Quote: Originally posted by D4RR3N  
Well I thought I roughly understood what a magnet was until I started reading about bar magnets and the concept of there being a bloch wall at the centre of the bar magnet.
If there were a Bloch wall in the middle of a bar magnet, it would have two north ends (or two south ends), at least in the extreme case of exact anti-alignment. I read the Wikipedia article, and it's wrong on its example of bar magnets; there's no Bloch wall generically in the middle.

D4RR3N - 8-6-2010 at 05:44

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by D4RR3N  
Well I thought I roughly understood what a magnet was until I started reading about bar magnets and the concept of there being a bloch wall at the centre of the bar magnet.
If there were a Bloch wall in the middle of a bar magnet, it would have two north ends (or two south ends), at least in the extreme case of exact anti-alignment. I read the Wikipedia article, and it's wrong on its example of bar magnets; there's no Bloch wall generically in the middle.


Its not just Wikipedia which state that there is a bloch or domain wall at the centre of a bar magnet, I have read this several times which is why I'm now looking into this further.

blochw.gif - 58kB

hissingnoise - 8-6-2010 at 06:15

The illustration appears to show the Earth having the equivalent of two bar magnets with opposing poles in contact. . .

watson.fawkes - 8-6-2010 at 07:53

Quote: Originally posted by D4RR3N  
Its not just Wikipedia which state that there is a bloch or domain wall at the centre of a bar magnet, I have read this several times which is why I'm now looking into this further.
Where's the source of the illustration you posted? I'm curious because one of the references in the Wikipedia article is to an over-unity site, and that crowd is notorious for creatively (and confusingly) misunderstanding physics, usually just enough to really gouge things up.

The electromagnetic picture you posted is the correct one. Accelerated moving charges (as in a current-carrying coil) generate a magnetic field according to the right-hand rule, which is (roughly) put the fingers of your right hand in the direction of the moving charges, both direction and curvature, and the magnetic field is in the direction of your thumb. If you look and those double-bar magnet illustrations and get out your hand, you'll see that they illustrate two of the same magnetic ends.

D4RR3N - 8-6-2010 at 12:49

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by D4RR3N  
Its not just Wikipedia which state that there is a bloch or domain wall at the centre of a bar magnet, I have read this several times which is why I'm now looking into this further.
Where's the source of the illustration you posted? I'm curious because one of the references in the Wikipedia article is to an over-unity site, and that crowd is notorious for creatively (and confusingly) misunderstanding physics, usually just enough to really gouge things up.

The electromagnetic picture you posted is the correct one. Accelerated moving charges (as in a current-carrying coil) generate a magnetic field according to the right-hand rule, which is (roughly) put the fingers of your right hand in the direction of the moving charges, both direction and curvature, and the magnetic field is in the direction of your thumb. If you look and those double-bar magnet illustrations and get out your hand, you'll see that they illustrate two of the same magnetic ends.


I just did an google image search and those images came up, not sure what site that was from. The more I think about it though...it cant be correct.
The thing that really bugged me was the images and talk about reversal of electron spin as you approach the centre of a bar magnet. I just couldn't understand why the electrons should decide to spin in the other direction.

On the subject of magnets, anyone know what is the correct term for a bar magnet with two pairs of poles so that both ends of the bar is NN or SS

watson.fawkes - 9-6-2010 at 06:31

Quote: Originally posted by D4RR3N  
I just did an google image search and those images came up, not sure what site that was from. The more I think about it though...it cant be correct.
Best I can tell the origin of their misunderstanding is that they have an idea that the magnetic field can be described a pair of monopoles. "Everybody" knows there are no magnetic monopoles, but not everybody knows what that means. Moving charge generates the magnetic field, but that field always has (roughly) both north and south poles integrated into a single dipole; it starts out that way when the current starts and stays that way as the current flows. It seems that they're under the belief that opposite spins generate just one pole or another.

P.S. The page image is from a book titled "Anti-Gravity and the World Grid"; a search on one of the captions came up with an excerpt on Google Books. I can't help but type in the sentence immediately after the page, but I won't dignify it with a link:
Quote:
Two antithetical force fields, the magnetic and the electric (two vector force fields) meet and generate gravity, a neutral center force field and simple two-vector system (tensor).
Believe it or not, it gets worse from there.

[Edited on 9-6-2010 by watson.fawkes]

D4RR3N - 9-6-2010 at 12:45

Anti gravity would be a wonderful thing...................for sticking things to the ground!

If gravity and "anti gravity" are two polar opposites then an object surrounded with an anti-gravity field would be strongly attracted towards the Earth as it has a gravity field (opposites attract).

If we ever manage to negate gravity and apply it to space travel it will never be with "anti gravity" ;)

peach - 18-8-2010 at 04:43

Quote: Originally posted by watson.fawkes  
I'm curious because one of the references in the Wikipedia article is to an over-unity site, and that crowd is notorious for creatively (and confusingly) misunderstanding physics, usually just enough to really gouge things up.


No one listen to Watson! He works for the oil companies! :P

j/k

It is the standard reply by the over-unity nuts however.
Quote: Originally posted by D4RR3N  

If gravity and "anti gravity" are two polar opposites then an object surrounded with an anti-gravity field would be strongly attracted towards the Earth as it has a gravity field (opposites attract).


They're not, and it wouldn't. Gravity doesn't work like magnets, it has one polarity, with 'anti gravity' being the idea of negating the effect of a gravimetric field; in contrast to opposing it by repulsion (which would also result in the object floating off the ground). The only way two objects are pulled together by gravity is if they both have mass (or show the properties of doing so) and feel it's effect.