Magnets: how do they work?

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bluefoxicy
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Magnets: how do they work?

Postby bluefoxicy » Mon Dec 10, 2012 9:12 pm UTC

The physics of metal is all wrong. Iron is the worst offender.

Metallic bonds are explained by the "sea of electrons" model. Essentially, metal atoms all group together, sharing their electrons along the outside of the entire mass. This makes purified metals an extremely strong substance: rather than individual bonds, the entire mass is a single bond. Sharing valence electrons? Hell no, all the electrons go around the entire mass.

No explanation on why all this positively charged mass huddles together just fine, despite charge forces. Oh you can make one up, or quote the state-of-the-art explanation, but the fact remains that you have positive ions all grouped together and you've magically negated the effect of like-charged particles on each other. We'll just say this material is special and acts differently.

Then there's magnets.

Take a spring one day and compress it. That compacts potential energy into the spring; when you release it, kinetic energy returns the spring to its original shape. Leave that spring compacted for long enough, though, and it won't spring back when released. Why?

The answer is simple: All matter moves towards the lowest energy state. All everything moves toward the lowest energy state. Hot things flow their heat into cold areas. Things lifted off the ground fall. Take magnets away from each other and they attract... or repel, or whatever. When you squeeze that spring down and leave it that way, it slowly leaks heat--maybe not enough to elevate the temperature (detectably), but it'll release a very tiny amount of heat very slowly, until it's released as much heat as the force you used to compress it. After that, it won't return to its shape. The heat is lost as the structure changes, bits of mass move around, settling into a new form... and releasing energy that was supplied to move them around that way.

Magnets.

Magnets lose their power after, what, 400 years? Heat a magnet and it becomes a non-magnet.

We're told magnets are what happens when you line up magnetic domains. Magnetic domains are bits of material in a ferrous metal (iron) that point their magnetic field in a given direction. One points north, one points east, and so on.

Problem: a magnet is a stable state.

If you took all these jumbled, so-called random domains and organized them properly, they'd stay that way, non-magnetic. Give a little tap, a little external magnetic field, anything, and the domains start to experience pressure. Accordingly, they'll try to move to the lowest energy state--they'll shift. The delicate balancing act is broken, and now there are magnetic stresses between domains, which need to shift to relieve this stress.

You would think that the stress would be eliminated when all the domains finally pointed in the right direction. A magnet should stay perpetually a magnet; a non-magnet would have nearby domains rotate slightly until they're aligned north-south, applying more force to surrounding domains, making bigger and bigger domains that affect each other by twisting smaller domains at boundaries, propagating force through nearby domains, shifting each other bit by bit until they finally align. Then all the internal magnetic stresses are gone, and you have a magnet.

Nope.

They randomize into magnetic domains pointing all over the place, placing each other under magnetic stress, and stay that way. Permanent magnets form domains that, despite being permanently surrounded by a huge domain with a relatively powerful field going in one direction, go against the flow and don't bow to this stress. They are little sumo wrestlers, pushing and sweating against each other, as each joins in the fight until a mass orgy of rice-fueled competition occurs, all straining and grunting and pressing against each other.

Iron must be magic.

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Re: Magnets: how do they work?

Postby Qaanol » Mon Dec 10, 2012 10:10 pm UTC

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Re: Magnets: how do they work?

Postby Drowsy Turtle » Mon Dec 10, 2012 10:56 pm UTC

bluefoxicy wrote:The physics of metal is all wrong. Iron is the worst offender.

Metallic bonds are explained by the "sea of electrons" model. Essentially, metal atoms all group together, sharing their electrons along the outside of the entire mass. This makes purified metals an extremely strong substance: rather than individual bonds, the entire mass is a single bond. Sharing valence electrons? Hell no, all the electrons go around the entire mass.

No explanation on why all this positively charged mass huddles together just fine, despite charge forces. Oh you can make one up, or quote the state-of-the-art explanation, but the fact remains that you have positive ions all grouped together and you've magically negated the effect of like-charged particles on each other. We'll just say this material is special and acts differently.


What? No.

Why do you assume electrons are only found at the edge of a metal object? Are you confusing de-localised electrons with the electrons that convey charge in circuits?

bluefoxicy wrote:Then there's magnets.

Take a spring one day and compress it. That compacts potential energy into the spring; when you release it, kinetic energy returns the spring to its original shape. Leave that spring compacted for long enough, though, and it won't spring back when released. Why?

The answer is simple: All matter moves towards the lowest energy state. All everything moves toward the lowest energy state. Hot things flow their heat into cold areas. Things lifted off the ground fall. Take magnets away from each other and they attract... or repel, or whatever. When you squeeze that spring down and leave it that way, it slowly leaks heat--maybe not enough to elevate the temperature (detectably), but it'll release a very tiny amount of heat very slowly, until it's released as much heat as the force you used to compress it. After that, it won't return to its shape. The heat is lost as the structure changes, bits of mass move around, settling into a new form... and releasing energy that was supplied to move them around that way.

Magnets.

Magnets lose their power after, what, 400 years? Heat a magnet and it becomes a non-magnet.

We're told magnets are what happens when you line up magnetic domains. Magnetic domains are bits of material in a ferrous metal (iron) that point their magnetic field in a given direction. One points north, one points east, and so on.

Problem: a magnet is a stable state.

If you took all these jumbled, so-called random domains and organized them properly, they'd stay that way, non-magnetic. Give a little tap, a little external magnetic field, anything, and the domains start to experience pressure. Accordingly, they'll try to move to the lowest energy state--they'll shift. The delicate balancing act is broken, and now there are magnetic stresses between domains, which need to shift to relieve this stress.

You would think that the stress would be eliminated when all the domains finally pointed in the right direction. A magnet should stay perpetually a magnet; a non-magnet would have nearby domains rotate slightly until they're aligned north-south, applying more force to surrounding domains, making bigger and bigger domains that affect each other by twisting smaller domains at boundaries, propagating force through nearby domains, shifting each other bit by bit until they finally align. Then all the internal magnetic stresses are gone, and you have a magnet.

Nope.

They randomize into magnetic domains pointing all over the place, placing each other under magnetic stress, and stay that way. Permanent magnets form domains that, despite being permanently surrounded by a huge domain with a relatively powerful field going in one direction, go against the flow and don't bow to this stress. They are little sumo wrestlers, pushing and sweating against each other, as each joins in the fight until a mass orgy of rice-fueled competition occurs, all straining and grunting and pressing against each other.

Iron must be magic.


"Permanant" magnets are in what you would call metastable states. They're not in their lowest energy state, but cannot reach it without first passing over an "energy barrier".

http://en.wikipedia.org/wiki/Metastability
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Re: Magnets: how do they work?

Postby FancyHat » Tue Dec 11, 2012 4:14 am UTC

I would also like to bite.

bluefoxicy wrote:Metallic bonds are explained by the "sea of electrons" model. Essentially, metal atoms all group together, sharing their electrons along the outside of the entire mass.

No. The delocalised electrons are spread out throughout the metal, not just on the outside. I think Drowsy Turtle has probably spotted where you've gone wrong there. Yes, if you give a conductor a net charge, that charge will be distributed over its surface. But within the metal, you've still got the electron sea and ions, equal and opposite charges attracting and balancing each other.

No explanation on why all this positively charged mass huddles together just fine, despite charge forces.

The electrons in the electron sea, spread throughout the metal, are attracted to the positive ions. The positive ions are attracted to the electrons in the electron sea. It's a little bit like how positive sodium ions and negative chloride ions are attracted to each other in sodium chloride (table salt), resulting in stable, solid crystals. Except instead of negative chloride ions, you've got delocalised electrons in an electron sea. So, what you've got in a solid metal are solid crystals of positive ions attracted to a negatively charged fluid consisting of delocalised electrons that permeates those crystals.

Then there's magnets.

Shake a bag of magnets. What do the magnets do? They don't neatly arrange themselves into one, big, bar magnet, do they? They try to reach the lowest energy state, where N poles and S poles of different magnets are together, and as much of their magnetic fields pass through magnets rather than air or bag as possible.

Start with two bar magnets. What do they do? What do they seem to prefer? Do they prefer to be side by side in the same direction, N pole beside N pole and S pole beside S pole? Or do they prefer to be end on end, one N pole against the other's S pole? Or do they instead prefer to be side by side in opposite directions, each N pole beside an S pole? I bet it's the latter. What about three magnets? What about four magnets?

Magnetic domains behave similarly, in that each tries to align itself in the most energetically favourable way, which tends to be N pole to S pole.
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bluefoxicy
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Re: Magnets: how do they work?

Postby bluefoxicy » Tue Dec 11, 2012 1:53 pm UTC

Drowsy Turtle wrote:What? No.

Why do you assume electrons are only found at the edge of a metal object? Are you confusing de-localised electrons with the electrons that convey charge in circuits?


No, the "Sea of Electrons" things was the explanation given in advanced chemistry. They said that's how metal works.

Maybe my fault for paying attention to high school chemistry, but I didn't want to do a physical sciences degree. They made us take two physical sciences in college, two semesters... 8 credits of Chemistry didn't even catch up to the first 3 months of high school chemistry, they never touched organic chemistry or electron configurations or orbital shapes or nuclear fission in radioactive heavy elements. Felt like fifth grade. Well okay, they didn't teach us how chemical batteries worked in fifth grade.

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Re: Magnets: how do they work?

Postby bluefoxicy » Tue Dec 11, 2012 1:55 pm UTC

FancyHat wrote:blahblahblah


Probably the best explanation I've ever heard.

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Re: Magnets: how do they work?

Postby eSOANEM » Tue Dec 11, 2012 3:18 pm UTC

bluefoxicy wrote:
Drowsy Turtle wrote:What? No.

Why do you assume electrons are only found at the edge of a metal object? Are you confusing de-localised electrons with the electrons that convey charge in circuits?


No, the "Sea of Electrons" things was the explanation given in advanced chemistry. They said that's how metal works.


The "sea of electrons" thing isn't far wrong and certainly the best explanation most people will be able to deal with at that level; that's not what we're asking about. We're asking whether you're misremembering it slightly because usually, the "sea of electrons" is described as filling all the gaps between the +ve ions rather than sitting on the edge. When described like this it's pretty accurate, if it was described as you say then your teacher was pretty terrible.
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Re: Magnets: how do they work?

Postby tooyoo » Wed Dec 12, 2012 7:45 am UTC

As far as I know, it's impossible to get a decent understanding of magnetism without some quantum mechanics. Which is why all explanations given in high school or undergraduate courses tend to be flawed. To be fully honest, I never bothered studying the glorious details, but magnetism is actually a fairly subtle effect. You have to take into account the Pauli exclusion principle as well as the geometry of the crystal lattice and in the end get things such as ferro- and paramagnetism.

My point is that in order to understand these things you need to have a fairly decent understanding of what's going on microscopically. That doesn't make the things mentioned about Weiss domains wrong, yet shows that it's insufficient to think about domains and classical potentials.

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Re: Magnets: how do they work?

Postby doogly » Wed Dec 12, 2012 9:44 pm UTC

Gonna give a shoutout to my homies Aschroft and Mermin.
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Re: Magnets: how do they work?

Postby LaserGuy » Thu Dec 13, 2012 6:35 pm UTC

bluefoxicy wrote:They randomize into magnetic domains pointing all over the place, placing each other under magnetic stress, and stay that way. Permanent magnets form domains that, despite being permanently surrounded by a huge domain with a relatively powerful field going in one direction, go against the flow and don't bow to this stress. They are little sumo wrestlers, pushing and sweating against each other, as each joins in the fight until a mass orgy of rice-fueled competition occurs, all straining and grunting and pressing against each other.

Iron must be magic.


The problem here is you're only looking at the energy from one source here. You need to minimize the total energy, not just the energy of the domains. If all of the domains were perfectly aligned, you would have a very, very strong magnet indeed. The problem is, this state is energetically unfavourable, because a large amount of energy is stored in the magnetic field created by the aligned domains. The system can reduce its free energy by having some of the domains orient in other directions to the prevailing one. This reduces the strength of the overall magnetic field, and lowers the free energy of the system.


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