Stupid question relating to electric induction

[atommo]

Hi,

I've been interested in the science behind electrons/magnetism for quite a while. I've been learning quite a bit from various sources online. However there is one thing that's really nagging me.

Magnetic fields result from moving electrons. That indicates that a permanent magnet has electrons inside it moving in a circular fashion to produce poles (essentially an electromagnet but the material itself retains that flow).

Now the thing that I'm wondering about is this: You can put an iron core inside a copper coil- run electricity through the coil and you induce a magnetic field in the iron (by causing the electrons in the iron to get dragged along by the current in the copper coil in that same direction).

Now, if you put a permanent magnet in the copper coil, would it cause any form of current to flow in the coil (even if only slightly)? I know that people say no movement = no energy, but a permanent magnet DOES contain energy (the electrons in it that are causing it to have a magnetic field in the first place).

This is all under the assumption that permanent magnets contain a type of internal electron flow in a circular fashion (I don't see how a field could be produced without moving electrons)

Something isn't adding up so hopefully someone could explain.

I know this is probably a stupid question but I'm really curious.

Thanks

 

[Merlin3189]

You're pretty near with this.
If you put a permanent magnet into a coil, you will get a pulse of current, but it will quickly die away when the magnet stays inside the coil.
You get another pulse of current when you remove the magnet.

Michael Faraday realized that you generate an emf and current only when the magnetic field is changing. The emf is proportional to the rate of change of magnetic flux linked to the coil.

With two adjacent coils one will induce current in the other only when its current is changing and producing a changing magnetic field. That's why transformers work only with AC.

 

[BvU]

Now, if you put a permanent magnet in the copper coil, would it cause any form of current to flow in the coil

No, in the same way you don't get current in an outer coil if there is an smaller coil inside that has DC current circulating. Only when the current is switched on (or off) you get a pulse. Try it !

 

[Merlin3189]

... You can put an iron core inside a copper coil- run electricity through the coil and you induce a magnetic field in the iron (by causing the electrons in the iron to get dragged along by the current in the copper coil in that same direction).

I think it is more like: the electrons in the iron are bound to the atoms, but already orbiting and therefore having a magnetic field, but the magnets are randomly oriented. When you apply the current to the coil, this creates a magnetic field and causes the iron atoms to align in the same direction. Then their effects add up to give a macroscopic magnetic field.

That is not an accurate description, because it is really a rough description of paramagnetism rather than ferromagnetism. Ferromagnetism is a much stronger effect involving large groups of atoms - domains - where the atomic fields are already aligned. When the random alignment of the domains becomes organised by the external field, their macroscopic field is much stronger and may also persist when the current is removed.

 

[atommo]

I kind of get that but I still don't fully understand why.

If the above is an electron moving towards the screen then the magnetic field it produces would be counterclockwise according to the right hand rule.

Using that logic, see below:

Green is copper wire, blue is electron movement in a permanent magnet and purple is the magnetic field.
I always picture a permanent magnet as having its own self-contained electric current as I don't know how they would produce a field otherwise, hence you see the electron flow inside the permanent magnet flowing counterclockwise. This produces the purple magnetic field. However this magnetic field would in theory envelop the copper coil surrounding it. If the magnetic field direction surrounding the electrons in the wire is stronger on one side, wouldn't it cause the electrons in said wire to also flow?

See picture above. If the big purple arrow (going up) of magnetic field is caused by something external, wouldn't it cause the other side of the electron's field to go down and cause the electron to move toward the screen?

That is what is confusing me a lot

 

[atommo]

The only (probably wrong) explanation I can think of as to why they only seem to interact when there is physical motion involved is when both are unchanging in state and position (even if such a state involves electrons flowing), an electron's own field will overcome any external field no matter how strong, a bit like if each electron became its own superconductor, and that state can only be broken by making the external field 'collide' with the electron to break that state. It just doesn't seem right.

 

[sophiecentaur]

@atommo You may be trying to understand this by using your own terms alone. They may not be appropriate for you to get the right answers.
Am emf is induced by a varying current or Magnetic field.No change, no emf.
If you have googled electromagnetic induction you will have seen many diagrams along the lines of yours but the details may be different. Look at those diagrams and read the words that accompany them. (Several times and in detail, without skimming). I could give you exactly the same story and the same diagram but why - when it's available all over the Internet?
Do not rely on just one source - it could actually be wrong. Several sources can be used to check each other. The Hyperphysics is a good source for all this basic stuff. Try it.

 

[atommo]

I have seen diagrams like what I've shown and they say that if electricity ran through the coil it would enhance the effect of the magnet, but didn't say if any current would flow in the coil when external power was not applied. I think I'll have to try and set up an experiment if I can acquire the parts.

Thanks for the responses!

 

[sophiecentaur]

but didn't say if any current would flow in the coil when external power was not applied.

I don't know what you read but any information about electromagnetic induction will include the fact that the Field has to be changing. The magnetic field will be there for all non-zero currents, whether changing or not- so the situations are not reciprocal.

I repeat my comment that you should read more and more. and it is essential not to stop half way through what you are reading because you think you have taken it all on board. This is a problem that we can all have - we start 'arguing' in our heads with what we are reading and don't read to the end. First lesson is that it has all been sorted out (to that level of knowledge at any rate) and that the standard explanations are 1. Complete and 2. Consistent with what we can observe.

 

[atommo]

That is very true but at the same time I feel like I need to see/experience it first-hand to fully accept it

 

[Merlin3189]

I think your diagrams have an error: the current flows in the opposite direction to the electrons. So your "towards" diagram has the field reversed. This follows on in your main diagram.

 

[anorlunda]

This is all under the assumption that permanent magnets contain a type of internal electron flow in a circular fashion (I don't see how a field could be produced without moving electrons)

I always picture a permanent magnet as having its own self-contained electric current as I don't know how they would produce a field otherwise,

That's wrong, and it may lie at the root of your misunderstandings. There are no currents in a permanent magnet just sitting there. We require members to do some of their own study before posting questions here. In this case, I'll give you a start. Read and understand this: https://en.wikipedia.org/wiki/Magnetism#Sources_of_magnetism

 

[atommo]

I think your diagrams have an error: the current flows in the opposite direction to the electrons. So your "towards" diagram has the field reversed.

Good spot- the green positive and negative symbols should have been swapped around. I think everything else was as I intended it though. Sorry if that led to any confusion

That's wrong, and it may lie at the root of your misunderstandings. There are no currents in a permanent magnet just sitting there. We require members to do some of their own study before posting questions here.

Savage but fair enough.

This was what gave me the idea of self-conatined electron flow within a magnet: https://www.falstad.com/vector3dm/

The way it shows it makes me imagine electrons going round in circles within the magnet

 

[sophiecentaur]

That is very true but at the same time I feel like I need to see/experience it first-hand to fully accept it

How many really interesting and important phenomena would satisfy that requirement? You cannot 'see' electrons, you cannot identify a single atomic energy transition by looking, you cannot feel individual molecules moving through a gas. I think you are confused between familiarity with an idea and with personal experiences.
Reading a book / website is about the closest we can hope for in most of our experiences of Science. If we are lucky, we may do a few experiments on our own but, people who try random experiments without some good direction tend to walk away with nothing.
Science is referred to as a 'discipline' with good reason.

 

[BvU]

This was what gave me the idea of self-conatined electron flow within a magnet: https://www.falstad.com/vector3dm/

The way it shows it makes me imagine electrons going round in circles within the magnet

That is definitely not what the accompanying text says !
You got carried away by your imagination and forgot to check the basics.

 

[gmax137]

Reading a book / website is about the closest we can hope for in most of our experiences of Science. If we are lucky, we may do a few experiments on our own but, people who try random experiments without some good direction tend to walk away with nothing.

I think I understand what you're getting at here, but still there are lots of "experiments" that any of us can do that are very instructive. In this case, the OP could wire up a coil and a voltmeter; then see for himself that pushing a magnet into the coil causes the needle to jump, and also that the needle returns to zero when he stops moving the magnet. Seeing is believing, etc.

I had this same thought (simple experiment / demonstrations) when reading a recent thread on momentum & energy in collisions. When I was in school, we did all these things on air tables to bring the words home. I think trying to learn basic physics by reading books, without demonstrations, is a tough way to go.

 

[sophiecentaur]

“Seeing is believing” but not when it’s Penn and Teller showing you.
There are plenty of examples where seeing can lead you to the wrong conclusion.
You have to be so careful about the validity of evidence.

 

[gmax137]

You are quite right, this part of your post is very important to keep in mind:

random experiments without some good direction

Careful experiments, with good notes. And no jumping to conclusions.

 

[Charles Link]

For magnetic surface currents in a permanent magnet see: http://farside.ph.utexas.edu/teaching/302l/lectures/node77.html $\\$ See also a Physics Forums Insights article that I authored: https://www.physicsforums.com/insights/permanent-magnets-ferromagnetism-magnetic-surface-currents/ $\\$ Some very detailed calculations have shown these magnetic surface currents to be a theory having considerable merit. The magnetic surface currents, along with Biot-Savart's law give results for the magnetic field that are completely consistent with the magnetic pole model for the magnetic field that is computed. $\\$ Griffiths' E&M textbook also presents them in chapter 6. His derivation is rather advanced and is easily overlooked by physics students who study his book, because he doesn't emphasize in great detail the results of his derivation, which I think are quite important in explaining the magnetic field of a permanent magnet.

 

[atommo]

I just have some very strange things I want to try and I learn best by visualising!

For example I know that the electromagnetic force is caused by (virtual) photons emitted by Fermions (normally electrons in examples) which then interact with other Fermions in a way which either repels or attracts based on the wave-type (atleast that's the basic gist of what I get). I just find it hard to imagine the electrons are not really traveling inside a permanent magnet. All my understanding points to magnetic force happening as a result of the movement of electrons (or any Fermion for that matter).

From what has been explained here there is a fundamental difference between an electromagnet and a permanent magnet (electromagnets use current whereas permanent magnets use magnetic moment alignment..?)

Whereas I was under the impression permanent magnets had a self-contained current of their own which was inside the magnetic material itself- I thought the material acted in such a way as to allow for such a condition for a long period of time (and the act of taking a hammer or something of the like to it caused the material to go out of alignment which is what caused it to lose the magnetism/current).

I even visualise a magnet attracting some unmagnetised iron as the magnet causing the iron electrons to align and circulate at the same speed as the magnet, creating a magnetic field of equal strength and then attracting. See below for how I thought the magnetic fields and electron movement interlinked

 

[Charles Link]

I just have some very strange things I want to try and I learn best by visualising!

For example I know that the electromagnetic force is caused by (virtual) photons emitted by Fermions (normally electrons in examples) which then interact with other Fermions in a way which either repels or attracts based on the wave-type (atleast that's the basic gist of what I get). I just find it hard to imagine the electrons are not really traveling inside a permanent magnet. All my understanding points to magnetic force happening as a result of the movement of electrons (or any Fermion for that matter).

From what has been explained here there is a fundamental difference between an electromagnet and a permanent magnet (electromagnets use current whereas permanent magnets use magnetic moment alignment..?)

Whereas I was under the impression permanent magnets had a self-contained current of their own which was inside the magnetic material itself- I thought the material acted in such a way as to allow for such a condition for a long period of time (and the act of taking a hammer or something of the like to it caused the material to go out of alignment which is what caused it to lose the magnetism/current).

I even visualise a magnet attracting some unmagnetised iron as the magnet causing the iron electrons to align and circulate at the same speed as the magnet, creating a magnetic field of equal strength and then attracting. See below for how I thought the magnetic fields and electron movement interlinked

View attachment 236038

Please see my post 19.

 

[atommo]

For magnetic surface currents in a permanent magnet see: http://farside.ph.utexas.edu/teaching/302l/lectures/node77.html $\\$ See also a Physics Forums Insights article that I authored: https://www.physicsforums.com/insights/permanent-magnets-ferromagnetism-magnetic-surface-currents/ $\\$ Some very detailed calculations have shown these magnetic surface currents to be a theory having considerable merit. The magnetic surface currents, along with Biot-Savart's law give results for the magnetic field that are completely consistent with the magnetic pole model for the magnetic field that is computed. $\\$ Griffiths' E&M textbook also presents them in chapter 6. His derivation is rather advanced and is easily overlooked by physics students who study his book, because he doesn't emphasize in great detail the results of his derivation, which I think are quite important in explaining the magnetic field of a permanent magnet.

I think that was exactly what I was wanting! Explains in an easy-to-visualise way and I think I get it now. That makes a lot more sense. So I wasn't entirely wrong; there is an electric current. Its just exclusively on the very outside of the material.

Thank you!

 

[Charles Link]

I think that was exactly what I was wanting! Explains in an easy-to-visualise way and I think I get it now. That makes a lot more sense. So I wasn't entirely wrong; there is an electric current. Its just exclusively on the very outside of the material.

Thank you!

It's actually a different kind of current in that there is no actual electrical charge transport occurring. It is impossible to measure this current with a current meter, but all the calculations that are done regarding this "bound current" are extremely consistent. $\\$ On another note, this is actually extremely useful in this sense. In a transformer, plastic laminations are usually included to block unwanted "eddy currents" that occur. The "bound" magnetic surface currents are not blocked by these laminations, so that the magnetic field of the transformer does just what it is supposed to. If the laminations were to block the magnetic surface currents, the magnetic field of the transformer would be greatly reduced by the laminations.

 

[atommo]

That's really interesting. The next nagging question on my mind is what if you physically rotate the magnet in the direction its 'virtual current' is traveling in? Would that turn it into an actual current since you would be doing the act of physically rotating the magnet, thus the electrons too?

This is what I'm tempted to set up an experiment for. I get a coil, put a rod magnet inside it, attach both ends of the coil to a voltmeter, attach one pole of the magnet to the end of a drill and then turn the drill on. The magnet will rotate and I wonder if the voltmeter will read a current...

 

[sophiecentaur]

For example I know that the electromagnetic force is caused by (virtual) photons emitted by Fermions (normally electrons in examples) which then interact with other Fermions in a way which either repels or attracts based on the wave-type (atleast that's the basic gist of what I get). I just find it hard to imagine the electrons are not really traveling inside a permanent magnet. All my understanding points to magnetic force happening as a result of the movement of electrons (or any Fermion for that matter).

One cannot be selective about this sort of thing. You cannot 'see' any virtual photons and they are a very sophisticated concept - way beyond the level of this thread and yet you feel you can visualise electrons buzzing around in circles?? Physics is not a subject that takes well to the intuitive approach. Many invalid experiments and demonstrations have been used in the past. They convinced great minds at the time but have been proved wrong later. Finding something "hard to imagine" is not a good argument one way or another. At PF we. at least try to follow the protocol that's used to advance the subject.

 

[Charles Link]

Rotating the magnet has virtually no effect. The speed of the electrons, if you do a classical description, is quite high. Not at the speed of light, but perhaps 1/100 the speed of light. $\\$ Meanwhile, if you rotate the magnet on the other axis, you can get EMF's (voltages) from the Faraday effect. Suggest you read about the Faraday effect.

 

[sophiecentaur]

So I wasn't entirely wrong; there is an electric current. Its just exclusively on the very outside of the material.

If there were a current in the sense that you imply, it would be affected by the Resistance of the material and the Energy in the Permanent Field would fade. A 'permanent magnet' does not fade.

 

[sophiecentaur]

Rotating the magnet has virtually no effect. The speed of the electrons, if you do a classical description, is quite high. Not at the speed of light, but perhaps 1/100 the speed of light.

Typical electron drift speeds are around 1mm/s. Any model that's based on currents will involve some very different concept of current than conventional current.

 

[Charles Link]

Typical electron drift speeds are around 1mm/s. Any model that's based on currents will involve some very different concept of current than conventional current.

I'm referring to bound electron currents=essentially a Bohr atom approach where the electron orbits the atom. (Of course, in the case of electron spin, this model is not realistic, but the answer to the OP's manual rotation question is still the same).

 

[Charles Link]

One cannot be selective about this sort of thing. You cannot 'see' any virtual photons and they are a very sophisticated concept - way beyond the level of this thread and yet you feel you can visualise electrons buzzing around in circles?? Physics is not a subject that takes well to the intuitive approach. Many invalid experiments and demonstrations have been used in the past. They convinced great minds at the time but have been proved wrong later. Finding something "hard to imagine" is not a good argument one way or another. At PF we. at least try to follow the protocol that's used to advance the subject.

In general, most of the time when someone posts their "guesses" or what their "instincts" tell them, they have it almost all completely wrong. In this case, I think the OP actually had a couple of reasonably good guesses. I would encourage the OP @atommo to do some more reading on the subject, such as the "links" that were provided to him, but for a couple of his "guesses", he was on the right track.

 

[sophiecentaur]

; there is an electric current. Its just exclusively on the very outside of the material.

In this case the currents are essentially macroscopic effects (no?) around the outside of the material

I'm referring to bound electron currents=essentially a Bohr atom approach where the electron orbits the atom.

Here, the suggested currents are around the atom. But in a bound state round an atom, where can you say the electron 'is' to be moving in a loop?
The two ideas seem to be totally different and neither seems to be equivalent to electron spin, which is how ferromagnetism is usually explained.
I see no point in trying to bend the accepted theory to fit someones personal intuition. Where could that take @atommo further in understanding the mechanisms of magnetism if you start so far off course?

 

[Charles Link]

In this case the currents are essentially macroscopic effects (no?) around the outside of the material

Here, the suggested currents are around the atom. But in a bound state round an atom, where can you say the electron 'is' to be moving in a loop?
The two ideas seem to be totally different and neither seems to be equivalent to electron spin, which is how ferromagnetism is usually explained.
I see no point in trying to bend the accepted theory to fit someones personal intuition. Where could that take @atommo further in understanding the mechanisms of magnetism if you start so far off course?

The electron spin is different classically from orbital electron current, but in Griffiths derivation in chapter 6 of his book, he treats the case of a magnetic moment $\vec{\mu}$ of either form (spin or orbital angular momentum). (Note: $\vec{\mu}_s=\frac{g_s \mu_B \vec{S}}{\hbar}$, and $\vec{ \mu}_L=\frac{g_L \mu_B \vec{L}}{\hbar }$,(c.g.s. units), where the Bohr magneton $\mu_B=\frac{e \hbar}{2 mc}$, and where $g_s=2.0023...$ and $g_L=1.0$ ).$\\$ He considers an arbitrary distribution of such magnetic moments and derives the equation for the vector potential $\vec{A}$. This vector potential takes the form (c.g.s. units) of $\vec{A}(x)=\int \frac{\vec{J}(x')}{c|x-x'|} \, d^3x'$, where $\vec{J}$ is any arbitrary current density distribution. With a couple of vector identities, he shows that the potential from his arbitrary distribution of magnetic moments has two terms that are of the following: $\vec{A}(x)=\int \frac{\vec{M}(x') \times \hat{n}'}{|x-x'|} \, dA' +\int \frac{\nabla' \times \vec{ M}(x')}{|x-x'| } \, d^3x'$, where $\vec{M}$ is the density of magnetic moments $\vec{\mu}$ per unit volume. By looking at this result, one can conclude that the magnetic surface current density per unit length $\vec{K}_m=\vec{M} \times \hat{n}$, etc. $\\$ The first integral is the magnetic surface currents with magnetic surface current per unit length $\vec{K}_m=\vec{M} \times \hat{n}$, and the second integral involves gradients in the magnetization with bulk magnetic current density $\vec{J}_m=\nabla \times \vec{M}$, where I may have left off $\mu_o$ and/or $c$, (or a $4 \pi$ ), in a couple of places. (Switching between c.g.s. and various SI units requires a little extra work. Griffiths uses a form of SI units). $\\$ (It is left open for discussion whether these currents are real or not. There is no actual charge transport here, but these currents can be used with Biot-Savart to compute the magnetic field $\vec{B}$. Alternatively, the magnetic field is given by $\vec{B}=\nabla \times \vec{A}$ ). $\\$ Many physics students who have studied Griffiths textbook seem to overlook this very important derivation that he does. The magnetic surface currents that arise here can be used to readily explain the magnetic field of a permanent magnet of cylindrical shape that has uniform magnetization $\vec{M}$ along its axis. The magnetic surface currents can be used to compute the magnetic field that exists both inside and outside the permanent magnet.

 

[atommo]

You cannot 'see' any virtual photons and they are a very sophisticated concept - way beyond the level of this thread and yet you feel you can visualise electrons buzzing around in circles?? Physics is not a subject that takes well to the intuitive approach. Many invalid experiments and demonstrations have been used in the past. They convinced great minds at the time but have been proved wrong later. Finding something "hard to imagine" is not a good argument one way or another. At PF we. at least try to follow the protocol that's used to advance the subject.

Well not quite buzzing in circles... More like I can visualise electron clouds (since due to quantum mechanics electrons sort of appear and disappear all over a certain area rather than 'orbit' an atom). I can work out on a rather simple level the exchange of energy between these electrons. I know that electrons do not really 'spin'- its just a term used to identify the energy (virtual photon) type they give out.

I would say so long as a good base understanding is there, you can simulate quite a lot in your head (and if it gets crazy you can start writing stuff down/drawing it).

Of course you can get things wrong- science is about testing theories, getting them wrong and then getting closer to a proper understanding. And sometimes simulations don't work out either such as the case here where I needed input from others to help clear up my misunderstanding.

I'm not a maths or science guru for that matter- I do it on the side as a passion because I enjoy it (but sadly I don't know many of the formulas for that reason). But even so, that doesn't stop me from learning about for example the quanta of the electromagnetic force. I just have to try and convert the information I read into something I can understand (which may mean it is not 100% mathematically accurate, but it is usually still generally on the right lines).

Ultimately I want to make something based on what I've learned but I want to have as thorough understanding of electromagnetism as possible before that. I may be back with more crazy questions later on but for now I've got the answer I was looking for so thanks to all of you! Also in future I'm not too particular on mathematical accuracy in answers so long as it gives me a good idea because of the reason above [unless I've stated otherwise in that question] (obviously if so please say it is not completely accurate)

 

[BvU]

Oh boy ...

 

[sophiecentaur]

I want to have as thorough understanding of electromagnetism as possible before that.

Read a serious book on EM theory. There is no other way to achieve what you need.

 

[ZapperZ]

Does anyone other than me thinks that this thread is going in a million different directions, that it lacks focus, the original issue has been buried, and that someone is trying to do the sprint in the Olympics before he learned even how to crawl?

Zz.

 

[atommo]

Does anyone other than me thinks that this thread is going in a million different directions, that it lacks focus, the original issue has been buried, and that someone is trying to do the sprint in the Olympics before he learned even how to crawl?

Zz.

Honestly, the OP question has been answered but I don't see a 'mark post as best answer' button anywhere or anything of the sort...

 

[ZapperZ]

Honestly, the OP question has been answered ...

I'm not sure that it has, at least based on your continued misunderstanding in Post #33.

You are attempting to understand the origin of magnetism in matter, without first understanding the origin of magnetic moment in atoms. Magnetism in matter is a many-body phenomenon, and significantly more complex than magnetic moment in an atom. Thus, my comment about attempting the sprint before you learned how to crawl. But not only that, it also appears that you haven't understood classical E&M either.

Please note that "visualization" and "modeling" are meaningless if they are not accompanied by quantitative analysis. If what you visualize does not produce numbers that match experiment, your visual model is incorrect, no matter how beautiful, or how useful it is to you. Just because you are happy with it does not make it correct.

Physics just doesn't say what goes up must come down. It must also say when and where it comes down. This is the component of physics that many non-experts trivialize.

Zz.

 

[atommo]

Post #19 is what I considered to be the answer I was looking for- here's how I interpreted it:

- As a permanent magnet, the material has more up electrons than down so there is potential for an overall magnetic field.

- As per the description in http://farside.ph.utexas.edu/teaching/302l/lectures/node77.html (figure 30), when influenced by an external field the electrons can line up in generally the same way to produce the overall pseudo-current (due to electrons acting the same way) around the perimeter

- This does not make up a complete current circuit, but at the same time there is still current in each atom due to the electrons lining up the same way. Because of this there would (if I'm understanding this right) still be an extremely small current.

- Because there is (albeit an extremely small) current, in theory it would induce a very small current in a coil going round the outside (due to the magnetic field from the magnet)

I think I got the positive and negatives the right way round this time!
So what the image is trying to show is a small magnetic field would be induced in the coil from the big magnetic field... This is what I have been wondering all this time and from what I've seen I thought this was the case (but on such a small scale it would barely be measurable)

...Have I understood that right?

 

[Charles Link]

Post #19 is what I considered to be the answer I was looking for- here's how I interpreted it:

- As a permanent magnet, the material has more up electrons than down so there is potential for an overall magnetic field.

- As per the description in http://farside.ph.utexas.edu/teaching/302l/lectures/node77.html (figure 30), when influenced by an external field the electrons can line up in generally the same way to produce the overall pseudo-current (due to electrons acting the same way) around the perimeter

- This does not make up a complete current circuit, but at the same time there is still current in each atom due to the electrons lining up the same way. Because of this there would (if I'm understanding this right) still be an extremely small current.

- Because there is (albeit an extremely small) current, in theory it would induce a very small current in a coil going round the outside (due to the magnetic field from the magnet)

View attachment 236106
I think I got the positive and negatives the right way round this time!
So what the image is trying to show is a small magnetic field would be induced in the coil from the big magnetic field... This is what I have been wondering all this time and from what I've seen I thought this was the case (but on such a small scale it would barely be measurable)

...Have I understood that right?

The magnetic surface currents are quite large. A solenoidal current of 1 ampere with 500 turns(=wire windings) per meter, results in a solenoidal current per unit length of $K=500$ amperes/meter. The magnetic surface currents (current per unit length $K_m$) in an electromagnet and/or a permanent magnet are much larger and can easily be 1000 x this number. And that is why an electromagnet which consists of a solenoid (wire windings carrying a current), plus an iron core will generate a magnetic field that can be 1000 x as strong as the magnetic field from the solenoid without the iron core. $\\$ The magnetic surface currents are of a similar strength in a permanent magnet. The stronger the magnetic field, the higher the magnetic surface current number for the same geometry. In principle, it is the magnetic surface currents that generate the magnetic field in an electromagnet and/or permanent magnet.

 

[sophiecentaur]

The magnetic surface currents are quite large.

It's fair enough to use a term like that as long as we realize that it's no more 'really like that' than all the other constructs we use in advanced treatments of Physics. There is nowhere we could put an ammeter to measure that current and that's why I have a problem with presenting this to the uninitiated because they will grab at the analogy and treat it as actuality. We have exactly the same for of problem when people grab at photons as if they will help with all the EM wave problems because the corpuscular theory dies very hard! Dear old Feynman has a lot to answer for, here I think - however brilliant he was.

 

[sophiecentaur]

Please note that "visualization" and "modeling" are meaningless if they are not accompanied by quantitative analysis. If what you visualize does not produce numbers that match experiment, your visual model is incorrect, no matter how beautiful, or how useful it is to you. Just because you are happy with it does not make it correct.

@atommo You have 'liked' the post with this quote in it but you do not seem to be taking its message on board. For heaven's sake get yourself familiar with the basic theory of permanent magnetism before you reach for 'fringe' treatments of it. How can you expect to grasp it without learning the basics?

 

[Charles Link]

It's fair enough to use a term like that as long as we realize that it's no more 'really like that' than all the other constructs we use in advanced treatments of Physics. There is nowhere we could put an ammeter to measure that current and that's why I have a problem with presenting this to the uninitiated because they will grab at the analogy and treat it as actuality. We have exactly the same for of problem when people grab at photons as if they will help with all the EM wave problems because the corpuscular theory dies very hard! Dear old Feynman has a lot to answer for, here I think - however brilliant he was.

My generation (college days 1975-1980) was taught the magnetic pole model, with the magnetic surface currents mentioned only very quickly as an alternative theory which really wasn't quantified in detail. From what a University of Illinois physics professor who has taught E&M for 20+ years now has told me, they are now presenting the magnetic surface current approach to the undergraduate students, and only present the pole model as an advanced topic in the graduate classes. I think the magnetic surface current approach is indeed a good one. Yes, I agree, you can't measure the magnetic surface currents with an ammeter. That has its pluses in that the plastic laminations of a transformer don't block the magnetic surface currents. $\\$ Meanwhile, I have used the photon concept when considering the response of photodiodes, where essentially (with an efficiency number of .8 or thereabouts), you get one photoelectron per incident photon. It also comes in handy in deriving the Planck blackbody function, but I don't want to stray too far off the present topic which is the subject of permanent magnets and electromagnets, and trying to find a good way to explain how their magnetic fields arise.$\\$ The magnetic surface current approach is the best way I know of at present to explain the origin of the magnetic fields in magnets. It far surpasses the magnetic pole model for explaining the underlying physics. The pole model is mathematically accurate in the magnetic field vector $\vec{B}$ that it computes, but if it is used to try to understand the underlying physics, it can generate many misconceptions.

 

[ZapperZ]

The magnetic surface current approach is the best way I know of at present to explain the origin of the magnetic fields in magnets. It far surpasses the magnetic pole model for explaining the underlying physics. The pole model is mathematically accurate in the magnetic field vector $\vec{B}$ that it computes, but if it is used to try to understand the underlying physics, it can generate many misconceptions.

You and I have had disagreement on this before, and I have stated my argument in the relevant Insight article that you created.

And as I've stated there, the problem here is that you are giving the impression that this is a valid model to those who don't know any better. As you can already see, it is creating a huge amount of confusion here. Undergraduate physics students will learn about QM, and those who go into condensed matter/solid state physics, will get to see the quantum description of magnetism. But many people on here do not, and will not get to see the accurate picture! It is difficult to justify using this classical picture of magnetism as the end picture that we leave these folks with. That's like leaving the planetary model of an atom as the valid picture of an atom, regardless of how good of an approximation it is for H atom.

I do not have the same problem with the photo picture as sophiecentaur. I've worked with phothocathodes and photomultipliers, and the photon picture has no counterpart in many situations that we dealt with. The idea of multiphoton photoemission and quantum efficiency are very much well-established.

The OP needs to first learn about magnetic moments of individual atoms. Then migrate to how each individual moments then interacts with other neighboring moments. The arrangements of these individual moments can make a huge difference. For the same type of atoms, one arrangement can mean something being ferromagnetic, while another it can be something else! This basic fact cannot be explained by your circular surface current model.

Zz.

 

[Charles Link]

This surface current model isn't expected to explain even a good portion of the subject. It does provide accurate calculations though for magnetic field strengths $\vec{B}$ under the assumption that the magnetization vector $\vec{M}$ is approximately uniform. It doesn't begin to treat the quantum mechanical aspect, and I haven't even introduced the exchange effect in this thread. I'm also not trying to answer questions like why some materials make permanent magnets while others like soft iron have the magnetic field go back to near zero once the external magnetic field is removed. $\\$ I do think my response gave a reasonably satisfactory answer for the OP and the questions of his initial post. If he continues his studies of the magnetism subject, in another week or two he might come back fascinated by the force/torque on a current-carrying loop in a magnetic field and wonder whether it might be possible to make use of this to power machinery. (LOL) $\\$ Back to the magnetic surface current subject: If it wasn't of some importance, I don't think Griffiths would have spent a couple of pages doing a very detailed derivation in his E&M textbook. For me, the surface current model is a tremendous advancement beyond the magnetic pole model, which I saw both as an advanced undergraduate as well as in two graduate courses. In the graduate courses, we used J.D. Jackson's textbook, which was a very good book for its very thorough vector calculus, but his treatment of the magnetism subject is very incomplete. $\\$ IMO, the magnetism subject needs to be presented in the undergraduate curriculum, as it presently is. The case of the transformer problem with its Ampere's law=MMF (magnetomotive force) type solution is also very useful material for both physicists and EE's. $\\$ Although I'm sure it is an extremely interesting subject, but one that also takes an enormous investment of time and effort, an undergraduate does not need to know the details of the solid state and quantum mechanical descriptions of ferromagnetism=there are only a small handful of physics people out there who have that kind of level of understanding, and I will readily admit, I am not one of that small handful. $\\$ Edit: To add a little detail: The interactions of neighboring magnetic moments via the exchange effect makes any near-complete mathematical description of ferromagnetism extremely difficult. It would be nice to say that how a magnetic moment $\vec{\mu}$ in the material behaves depends only on the value of the magnetic field $\vec{B}$ at that location. Unfortunately, this is not the case, so that the problem is extremely complex from a mathematical sense. Weiss' mean field theory is an attempt to work through this difficulty, but that model has its limitations. A more complete treatment of the subject would take an enormous investment of time and effort. Ferromagnetism perhaps is a problem that quantum field theory may provide some insight, but the second-quantized operators of quantum field theory can be very cumbersome for those who are not extremely proficient with them. $\\$ Additional edit: Even though the mathematical mechanics of the magnetic pole method is rather clumsy, I am very glad that we were taught the method, and I am very glad I put the many, many hours into learning it. The magnetic pole method actually only really finally made sense to me after I did some calculations with the magnetic surface current method, and I was able to show how the two were connected. I do think it is a major plus that the magnetic surface current method is now included in the undergraduate E&M courses at many universities.

 

[sophiecentaur]

Meanwhile, I have used the photon concept when considering the response of photodiodes

Naturally; there is no better way of considering quantum phenomena and Photon Efficiency is a very meaningful performance descriptor. But would you discuss light from a distant star in terms of a string of little bullets arriving? That is an analogy too far but its one that's used with gay abandon by people who do not know much Physics.
But, back to these 'currents . . . .

 

[Charles Link]

Some very simple calculations can be done with a cylindrical bar magnet and using either the magnetic pole or magnetic surface current model: $\\$ e.g. If you assume the magnetization $\vec{M}=1.0$ (M.K.S. units with $\vec{B}=\mu_o \vec{H}+\vec{M}$) and is uniform along the z-axis, $\vec{B}$ can be computed everywhere by identifying the magnetic poles, and using $\vec{B}=\mu_o \vec{H}+\vec{M}$ and the inverse square law. $\\$ Alternatively, using the magnetic surface current model, $\vec{B}$ can be computed everywhere using Biot-Savart, once the magnetic surface currents are identified. $\\$ The tremendous level at which we we taught the magnetic pole model in my advanced undergraduate E&M class in 1975 using $\rho_m=-\nabla \cdot \vec{M}$, and $\vec{H}=\int \frac{\rho_m(x')(x-x')}{4 \pi \mu_o |x-x'|^3} \, d^3x'$ + $\vec{H}$ from any currents in conductors using Biot-Savart, and after all that we still couldn't do simple calculations with a cylindrical bar magnet was really carrying the physics and mathematics to the extreme. The very mathematical description is a good one, but the simpler explanations are also necessary. $\\$ Both the magnetic pole method and the magnetic surface current model are highly mathematical. They may be a watered-down version of a many body quantum field theory approach, but they still provide very useful models which offer a reasonably good explanation for many of the phenomena that occur.

 

[ZapperZ]

... but they still provide very useful models which offer a reasonably good explanation for many of the phenomena that occur.

Where exactly do they become useful?

Zz.

 

[Charles Link]

Where exactly do they become useful?

Zz.

IMO, it is a very useful thing to be able to estimate the magnetic field from a cylindrical bar magnet that has $L=4$" and diameter of 1/2". For many commercially available magnets $\vec{M} \approx 1.0$ M.K.S. units within a factor of 2. For a laboratory experiment that a couple of students did using some of these "simpler" theories, and I think it was highly educational for them, see: https://www.physicsforums.com/threa...-function-of-temperature.950326/#post-6020315 $\\$ Quite a lot can be done in studying ferromagnetism without employing advanced quantum field theory.

 

[ZapperZ]

IMO, it is a very useful thing to be able to estimate the magnetic field from a cylindrical bar magnet that has $L=4$" and diameter of 1/2". For many commercially available magnets $\vec{M} \approx 1.0$ M.K.S. units within a factor of 2. For a laboratory experiment that a couple of students did using some of these "simpler" theories, and I think it was highly educational for them, see: https://www.physicsforums.com/threa...-function-of-temperature.950326/#post-6020315 $\\$ Quite a lot can be done in studying ferromagnetism without employing advanced quantum field theory.

How useful and accurate is this? I have several cylindrical bar magnet in my class lab, and they ALL have different strengths even though they all have identical size.

Zz.