Talk:Magnetic moment/Archive 2

Article is absolute USELESS!
I don't know what is correct and what is not. But i do know, the article does not explain anything. You may be using the correct technical jargon but it is menaingless to everybody else but physicists specialising on magnetism. Those however would already know these. Thus this page is pointless but a battlefield of great egos: "I'm right and you are not". The whole lot is useless until you learn to express yourself. Now comes the argument "You have to know the technical jargon ... ". No, I DON'T! Especially because i come here to learn about it. If you don't support this you are useless. — Preceding unsigned comment added by 115.70.177.64 (talk) 12:35, 16 July 2013 (UTC)


 * I'm glad to see an revolting lambda :-) . The concept of magnetic moment should be explained with pictures, videos, descriptions of experiences. In those experiences, all would seem ununderstandable. But then would come the magnetic moment concept, that makes things clear. I studied physics at the university and I realized our teachers had a huge lack of pedagogy. We need more people like you who ask for clarity. Bête spatio-temporelle (talk) 12:19, 27 August 2013 (UTC)


 * The article is not exactly useless (!), but I went through hell to understand a quarter of it. They certainly don't make it easy for beginners. Its not the first article of the kind in Wikipedia and it won't be the last.2A02:587:4508:2C00:688F:C34C:D0FC:9449 (talk) 02:13, 25 July 2017 (UTC)

No monopole
Hi guys. I heard that there is no magnetic monopole. What do you think about it ? Bête spatio-temporelle (talk) 12:28, 27 August 2013 (UTC)
 * Hi, I'm afraid this is not the best page for questions like this per WP:NOTAFORUM.  However, if you have any specific suggestions or questions about the article content we would be happy to discuss.  Thanks, a13ean (talk) 14:22, 27 August 2013 (UTC)

Merge proposal: Magnetic pole strength
There doesn't seem to be much point to Magnetic pole strength. The substance of the article is discussed in Magnetic moment, Magnetic field, Magnetostatics, Demagnetizing field and Magnetic monopole, where in my opinion the context makes the discussion more useful. Also, as it is the article is rather dismissive of magnetic poles, but the magnetic pole model is widely used in permanent magnet applications (see Demagnetizing field). RockMagnetist (talk) 16:12, 7 April 2012 (UTC)


 * If anything it should be merged with magnetic monopoles (in my opinion), since this is their fundamental property. But then the definition of magnetic dipole moment can be given in terms of pole strength, which favours this article... F = q(E+v×B) ⇄ ∑ici 17:14, 7 April 2012 (UTC)


 * On second thought why not? I'll help, if there's anymore consensus and no objections. =) F = q(E+v×B) ⇄ ∑ici 17:18, 7 April 2012 (UTC)


 * Sorry about no edit summary in this edit (didn't fill it in then clicked save too early): I archived all terminated discussions up to 2012. F = q(E+v×B) ⇄ ∑ici 17:20, 7 April 2012 (UTC)


 * Quite the lean and mean discussion page now! I think Magnetic pole strength is mainly about the magnetic pole model of ordinary magnets. RockMagnetist (talk) 18:28, 7 April 2012 (UTC)


 * Maybe we should just do it? It’s as trivial as Normalizable wavefunction and wavefunction. We could literally copy and paste this article to the beginning of magnetic moment, trim it down (remove lengthy description of units), then spill anything else we decide on into the articles you link to at the top. What remains from pole strength will be the first section/s of this article (i.e definition of pole strength, mention of units, magnetic coulomb force), since this leads to the definition of the magnetic dipole and its definition in terms of the pole strength. F = q(E+v×B) ⇄ ∑ici 20:47, 7 April 2012 (UTC)


 * I don't think there is anything in Magnetic pole strength that isn't in some other article, so I would just blank it and not copy anything over. RockMagnetist (talk) 20:55, 7 April 2012 (UTC)

Then where to re-direct? Magnetic monopoles or here? F = q(E+v×B) ⇄ ∑ici 21:50, 7 April 2012 (UTC)


 * Not one of the links you provide above has the definition:
 * $$\mathbf{H}=\mathbf{F}/p$$
 * which may help people understand that p is associated with the sources of H and that $$\nabla\cdot\mathbf{H}\neq 0$$? Also what about the external links and references? Even if pole strength was blanked the refs and external links could still be recovered from the edit history, but it may become deleted if not re-directed. Its not essential though. F = q(E+v×B) ⇄ ∑ici 21:59, 7 April 2012 (UTC)


 * Actually I'll just blank and re-direct to here. It can be reverted. F = q(E+v×B) ⇄ ∑ici 22:04, 7 April 2012 (UTC)


 * Its done. Ignore the cross-outs. F = q(E+v×B) ⇄ ∑ici 11:40, 8 April 2012 (UTC)

has reverted this merge. The discussion only lasted a day, so I have re-opened it so more people can comment. RockMagnetist (talk) 18:24, 7 October 2013 (UTC)


 * I support that the article Magnetic field strength should be "merged" somewhere (even if this means simply deleting it), and merging to Magnet might be better. Not to Magnetic monopole (mentioned above), though.  A separate merge discussion might be Magnetic moment and Magnetic dipole. Too many related articles! — Quondum 23:32, 7 October 2013 (UTC)


 * Support merge I.e. restore to the previous state before FKLS's edit: magnetic pole strength merged into magnetic moment. Other articles like magnetism and magnet would induce confusion as the true origin of magnetism is not monopoles as far as we know (...). Magnetic monopole is not a wrong idea, since you have to introduce the notion of pole strength to a magnetic monopole to formulate them in theoretical physics. But this article (magnetic moment) is less technical and easier to read, and as RockMagnetist says above, pole strength can be used to model real world magnets. Pole strength seems to be most relevant and accessible here (magnetic moment) as it was before. M&and;Ŝc2ħεИτlk 20:16, 10 October 2013 (UTC)

Force equation between two dipoles is wrong
It should go like this:

$$ \vec{F}_{ab}= \frac {3 \mu_0} {4 \pi |r|^4} [ (\hat r \times \vec{m}_a) \times \vec{m}_b + (\hat r \times \vec{m}_b) \times \vec{m}_a - 2 \hat r(\vec{m}_a \cdot \vec{m}_b) + 5 \hat r ((\hat r \times \vec{m}_a) \cdot (\hat r \times \vec{m}_b)) ]

$$

where $$\hat{r}$$ is unit vector pointing from magnetic moment $$m_a$$ to $$m_b$$, and $$|r|$$ is the distance between those two magnetic dipole moments.

http://downloads.hindawi.com/archive/1998/079537.pdf

Ze-aksent (talk) 06:45, 23 November 2012 (UTC)


 * They're both correct. You can get from this expression to the one in the article using the triple product expansion. RockMagnetist (talk) 18:24, 23 November 2012 (UTC)

Go ahead and try it, see what you get. And how would you arrive at 1/r^4 relation from that equation in the article? By the way, reference given in the article links to document that is missing page 140 where that equation is supposed to be, so what kind of reference is that? Can you point some actual reference for that equation given in the article?

Ze-aksent (talk) 04:49, 24 November 2012 (UTC)


 * I did the calculation before I posted my comment. It's $$1/r^5$$ in the article because it's $$\mathbf{r} = r\hat{\mathbf{r}}$$ on top. I don't know about the reference - I didn't contribute this equation. Most textbooks don't bother giving an explicit equation for the force. I'm fine with having both versions in the article, as long as the formatting is consistent. RockMagnetist (talk) 05:22, 24 November 2012 (UTC)

$$\mathbf{F}(\mathbf{r}, \mathbf{m}_1, \mathbf{m}_2) = \dfrac{3 \mu_0}{4 \pi r^5}\left[(\mathbf{m}_1\cdot\mathbf{r})\mathbf{m}_2 + (\mathbf{m}_2\cdot\mathbf{r})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)\mathbf{r} - \dfrac{5(\mathbf{m}_1\cdot\mathbf{r})(\mathbf{m}_2\cdot\mathbf{r})}{r^2}\mathbf{r}\right] $$

Where do you see $$\mathbf{r} = r\hat{\mathbf{r}}$$ on top? There is not. There is no even any "unit vector" there but only "direction vector". That's not the same thing. And don't you see 1/r^2 in there? So you are telling me this equation in the article comes out as 1/r^4? And there is no any reference for it? Most text books don't have it because barely anyone knows about it, the equation did not exist until 1998. Guys who first derived it should be credited properly, their paper should be the reference for this equation, and that wrong equation given in the articles that has no proper reference should go into rubbish.

Ze-aksent (talk) 10:01, 24 November 2012 (UTC)

Is this what you mean, Ze-aksent?

$$\mathbf{F}(\mathbf{r}, \mathbf{m}_1, \mathbf{m}_2) = \dfrac{3 \mu_0}{4 \pi r^5}\left[(\mathbf{m}_1\cdot r\hat{\mathbf{r}})\mathbf{m}_2 + (\mathbf{m}_2\cdot r\hat{\mathbf{r}})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)r\hat{\mathbf{r}} - \dfrac{5(\mathbf{m}_1\cdot r\hat{\mathbf{r}})(\mathbf{m}_2\cdot r\hat{\mathbf{r}})}{r^2}(r\hat{\mathbf{r}})\right] $$

$$\mathbf{F}(\mathbf{r}, \mathbf{m}_1, \mathbf{m}_2) = \dfrac{3 \mu_0}{4 \pi r^4}\left[(\mathbf{m}_1\cdot \hat{\mathbf{r}})\mathbf{m}_2 + (\mathbf{m}_2\cdot \hat{\mathbf{r}})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)\hat{\mathbf{r}} - 5(\mathbf{m}_1\cdot \hat{\mathbf{r}})(\mathbf{m}_2\cdot \hat{\mathbf{r}})\hat{\mathbf{r}}\right] $$

Maschen (talk) 13:18, 24 November 2012 (UTC)


 * Irony aside, according to the paper and equation you give above, the equation is [1] and [4] below, and is equivalent to [2] and [3];



\begin{align}

\mathbf{F}(\mathbf{r},\mathbf{\hat{m}}_{1},\mathbf{\hat{m}}_{2}) & =\dfrac{3\mu_{0}m_1m_2}{4\pi r^{4}}\left[(\mathbf{\hat{m}}_{1}\cdot\hat{\mathbf{r}})\mathbf{\hat{m}}_{2}+(\mathbf{\hat{m}}_{2}\cdot\hat{\mathbf{r}})\mathbf{\hat{m}}_{1}+(\mathbf{\hat{m}}_{1}\cdot\mathbf{\hat{m}}_{2})\hat{\mathbf{r}}-5(\mathbf{\hat{m}}_{1}\cdot\hat{\mathbf{r}})(\mathbf{\hat{m}}_{2}\cdot\hat{\mathbf{r}})\hat{\mathbf{r}}\right] & [1] \\

& =\dfrac{3\mu_{0}}{4\pi r^{4}}\left[(\mathbf{m}_{1}\cdot\hat{\mathbf{r}})\mathbf{m}_{2}+(\mathbf{m}_{2}\cdot\hat{\mathbf{r}})\mathbf{m}_{1}+(\mathbf{m}_{1}\cdot\mathbf{m}_{2})\hat{\mathbf{r}}-5(\mathbf{m}_{1}\cdot\hat{\mathbf{r}})(\mathbf{m}_{2}\cdot\hat{\mathbf{r}})\hat{\mathbf{r}}\right] & [2] \\ & =\dfrac{3\mu_{0}m_{1}m_{2}}{4\pi r^{4}}[(\mathbf{\hat{r}}\times\mathbf{\hat{m}}_{1})\times\mathbf{\hat{m}}_{2}+(\mathbf{\hat{r}}\times\mathbf{\hat{m}}_{2})\times\mathbf{\hat{m}}_{1}-2\mathbf{\hat{r}}(\mathbf{\hat{m}}_{1}\cdot\mathbf{\hat{m}}_{2})+5\mathbf{\hat{r}}((\mathbf{\hat{r}}\times\mathbf{\hat{m}}_{1})\cdot(\mathbf{\hat{r}}\times\mathbf{\hat{m}}_{2}))] & [3] \\ & =\dfrac{3\mu_{0}}{4\pi r^{4}}[(\mathbf{\hat{r}}\times\mathbf{m}_{1})\times\mathbf{m}_{2}+(\mathbf{\hat{r}}\times\mathbf{m}_{2})\times\mathbf{m}_{1}-2\mathbf{\hat{r}}(\mathbf{m}_{1}\cdot\mathbf{m}_{2})+5\mathbf{\hat{r}}((\mathbf{\hat{r}}\times\mathbf{m}_{1})\cdot(\mathbf{\hat{r}}\times\mathbf{m}_{2}))] & [4] \\

\end{align} $$


 * where $$\mathbf{m}_1 = m_1\mathbf{\hat{m}}_{1}, \mathbf{m}_2 = m_2\mathbf{\hat{m}}_{2} $$ are the unit magnetic moment vectors. Maschen (talk) 14:19, 24 November 2012 (UTC)

That's better. Thank you for that, it makes sense now. I don't see irony though, I think that was terrible way to write it. It surely confused me. Not that I'm much familiar with math and different notations, but still the one you wrote I expect would be far more readable and self-explanatory to other people as well:

$$\mathbf{F}(\mathbf{r}, \mathbf{m}_1, \mathbf{m}_2) = \dfrac{3 \mu_0}{4 \pi r^4}\left[(\mathbf{m}_1\cdot \hat{\mathbf{r}})\mathbf{m}_2 + (\mathbf{m}_2\cdot \hat{\mathbf{r}})\mathbf{m}_1 + (\mathbf{m}_1\cdot\mathbf{m}_2)\hat{\mathbf{r}} - 5(\mathbf{m}_1\cdot \hat{\mathbf{r}})(\mathbf{m}_2\cdot \hat{\mathbf{r}})\hat{\mathbf{r}}\right] $$

And this one still even more elegant, and specific having magnetic moments denoted as vectors:

$$ \vec{F}_{ab}= \frac {3 \mu_0} {4 \pi |r|^4} [ (\hat r \times \vec{m}_a) \times \vec{m}_b + (\hat r \times \vec{m}_b) \times \vec{m}_a - 2 \hat r(\vec{m}_a \cdot \vec{m}_b) + 5 \hat r ((\hat r \times \vec{m}_a) \cdot (\hat r \times \vec{m}_b)) ]

$$

So now having that sorted out the other point still stands, which is that reference given for it does not contain the page where that equation came from. And also, if those people in that paper I gave a link to were the first ones to derive it, then I think their paper deserves to be the reference for this equation.

Ze-aksent (talk) 14:42, 24 November 2012 (UTC)


 * When I said "ironic" I never meant to be offensive, it's just that you claimed that they're inequivalent when they are equivalent in the very paper you gave! And yes that paper can be used for reference - by all means add it. The cross-product version isn’t really "more elegant" though – because it's far easier to calculate the dot product of vectors than the cross, and the length of the equation is still roughly the same, though I'm not too fussed about that point... Maschen (talk) 14:51, 24 November 2012 (UTC)

I think it's fair you laugh at me. I simply could not make sense of it, and I think if I didn't take that attitude you probably wouldn't be motivated to explain all that as you did. Basically I used arrogance to get a free lesson. Hope you don't mind, I really appreciate it. -- I'd like to see the equation in the article be change to either one you wrote or the one from the paper I gave a link to. And if you would put the equations you wrote in the article then I think magnetic moments should be denoted as vectors. By the way, I don't know how to add references, tried once and screwed some things, not really that much motivated about it to learn it. Anyway, thanks again! Cheers.

Ze-aksent (talk) 15:13, 24 November 2012 (UTC)


 * No worries, and I'm not laughing at you, it's fine to ask (but please also note that Wikipedia talk pages are not forums for extensive discussion of topics, which is for the WP:reference desk, unless the discussion is aimed at improving the article - as you have done here).


 * The link to the current source is on Google books and they disallow preview of all pages, for copyright reasons etc. So p.140 is not necessarily "missing", I don't have the book but the person who added the citation surely has it and knew what he/she was looking at. Rather than removing the reference, we could add this paper as a supplementary ref (I'll do it now).


 * Adding references is very easy; ...(insert ref here)... . Its good to use the template citebook, citejournal or similar (they are link from these templates), and just fill the parameters in with the information provided. See Citing sources for fuller descriptions, if not already.


 * About "magnetic moments should be denoted as vectors"... The boldface notation for vectors is very widespread, so the formulae above already denote magnetic moments as vectors, and the paper gives the same equation as the WP articles.


 * Are you saying the cross-product form is preferable, and/or arrow notation for vectors, and in terms of unit vectors and not the separation vector? The advantage of not using unit vectors is that less notation is needed to be introduced.


 * Recently there was a debate on vector notation here at Wikiproject physics, so either bold or arrow notations will be accepted, but let's not change styles throughout articles which already use only one notation to another - only change notations all the way through if there is a mixture of different styles.
 * Best! Maschen (talk) 16:36, 24 November 2012 (UTC)


 * Thanks for handling this, Maschen. While we're discussing this section, the discussion of "local" and "global" coordinates seems orphaned and sheds no light on the subject. I replaced it by a much shorter discussion. RockMagnetist (talk) 17:43, 24 November 2012 (UTC)

That's fine RockMagnetist. Although that section is deleted, just in case it's reinstated, or used elsewhere, I thought to let the editors of this article know that I colourized and redrew to this;



Nice trim though! Thanks, Maschen (talk) 17:50, 24 November 2012 (UTC)
 * Sorry I didn't do it before you went to all that trouble! RockMagnetist (talk) 18:02, 24 November 2012 (UTC)


 * Don't worry, it wasn't any trouble! The important thing is the discussion was redundant and has now been removed. Maschen (talk) 18:21, 24 November 2012 (UTC)


 * I like the two unit vector expressions discussed above - very tidy. I have added both, replacing the previous expression. RockMagnetist (talk) 18:24, 24 November 2012 (UTC)

Great. Thank you RockMagnetist, that makes me happy, and I think it will be more readable for other people as well. If for nothing else then it makes obvious it is 1/r^4 relation and not something else. -- As for vector notation, I don't think bold is very good way, especially if the whole equation ends up in bold. And while it certainly is not an issue, I would also put all the multiplicands before brackets rather than behind, like this:

$$ \vec{F}_{12}= \frac {3 \mu_0} {4 \pi |r|^4} \bigg [ \vec{m}_1 (\hat r \cdot \vec{m}_2) + \vec{m}_2(\hat r \cdot \vec{m}_1) + \hat r(\vec{m}_1 \cdot \vec{m}_2) - 5 \hat r (\hat r \cdot \vec{m}_1)(\hat r \cdot \vec{m}_2) \bigg] $$

Ze-aksent (talk) 22:38, 24 November 2012 (UTC)
 * Ze-aksent, thank you for bringing up the issue in the first place. I think that section is much improved as a result. I moved the multiplicands as you suggested, but I'm keeping the bold. As Maschen said, there have been attempts to change the representation of vectors before, and they weren't popular. Every representation has its good and bad points, but boldface for vectors is widely used in the literature, so there is no reason to change it. RockMagnetist (talk) 23:24, 24 November 2012 (UTC)

I'm sure glad it turned out that way and that I could be of some help while not really understanding much of it. Funny. Now, there is that other article: http://en.wikipedia.org/wiki/Magnetic_dipole ...it still has the old equation. It makes me wonder if such redundancy of the same material across different articles is a good thing, since if you don't synchronize some change and modify all the articles that deal with the subject some articles might end up to contradicting others. In any case I'll leave all that to you and other experienced Wikipedia editors. I got the lesson I was after and I'm happy if it also makes that part of this article more clear for everyone. Good-bye!

Ze-aksent (talk) 01:07, 25 November 2012 (UTC)

Suggested section move
One suggestion: move the section Effects of an external magnetic field on a magnetic moment into the Magnetic dipoles section, so that the units and Gyromagnetic effect (more useful/informative to reader) come before the formal maths (useful but not for all readers)? Maschen (talk) 18:21, 24 November 2012 (UTC)


 * "Effects" shouldn't go into the dipole section because it is more general, but switching it with Gyromagnetic effect would be fine. RockMagnetist (talk) 01:48, 25 November 2012 (UTC)


 * Ok - done. Maschen (talk) 09:40, 25 November 2012 (UTC)

Magnetic Pole Strength
Since the page on magnetic poles was merged here, the hyperlink on "pole strength" redirects back to the same page. Maybe a few lines need to be added to clarify whether pole strength is defined in terms of the dipole moment or the dipole moment in terms of the pole strength. — Preceding unsigned comment added by KitchiRUs (talk • contribs) 13:50, 4 March 2013 (UTC)


 * Section Magnetic pole definition already has the explaination, but you're right there is a link to that page, so I just removed it and added a notice . Better? M&and;Ŝc2ħεИτlk 17:25, 4 March 2013 (UTC)


 * Much better! (KitchiRUs (talk) 10:24, 6 March 2013 (UTC))

Your point is correct, the definition as stated is outdated by about 100 years.

The magnetic moment is now defined in terms of how magnets are affected by magnetic fields, rather than the other way around.

This is in contrast to the electric dipole term of the multipole expansion, which is defined based on a distance approximation to the field.

In practice, of course, magnetic moments can be measured either way. — Preceding unsigned comment added by Jwkeohane (talk • contribs) 22:43, 1 March 2014 (UTC)

angular momentum for the current loop case
can anyone talk about the angular momentum for the current loop case? Jackzhp (talk) 04:02, 30 September 2013 (UTC)