Talk:Magnetic moment

Notation consistency

This article uses both $\mathbf {\mu }$ and $\mathbf {m}$ for the magnetic moment. In cgs it probably makes more sense to use $\mathbf {\mu }$, but Jackson uses $\mathbf {m}$ (in SI). My vote would be for $\mathbf {m}$ on the basis that the magnetic moment is rarely used as a scalar quantity, so there shouldn't be ambiguity (with mass). Turtle47 (talk) 12:24, 29 April 2014 (UTC)

Most of the article already uses $\mathbf {m}$, so I lean towards that. RockMagnetist (talk) 13:43, 29 April 2014 (UTC)

ackk! I have been working on this article starting more or less from the top which uses mu. I will try to follow RockMagnetist's suggestion to use m which is the symbol used by the physics textbooks that I use. Unfortunately both symbols have conflicts with other quantities that use their symbols. In the case of m there are two conflicts (with mass and with the quantum number for lz). TStein (talk) 06:12, 17 April 2018 (UTC)

I converted the article to just use m for the magnetic moment. I left the bohr magneton as \mu_B for now. Along the way I had to change the symbol used for mass from m --> \mu in order to avoid using m for two different things. TStein (talk) 15:31, 18 April 2018 (UTC)

Image somewhat in error?

Neutron spin dipole field

I noted that the image Magnetic_ring_dipole_field_lines.svg (red field lines) seems to be slightly in error. The field lines seem to be circles, whereas that is not precisely the case for a dipole field. (Compare to the figure below it). Bdushaw (talk) 03:05, 23 January 2015 (UTC)

It is clearly an approximation. All the lines of a perfect dipole should go through the centre as well, not remain separated as in the diagram. There is a further problem: whereas the lines can give the exact direction, the line density (with continuous lines) cannot simultaneously be proportional to the field strength when a three-dimensional field is depicted in two dimensions. Allowing for this, it would be nice to have a diagram that accurately reflects the direction of the field lines in a plane section through the dipole. Perhaps an editor with the necessary software can graph lines using an accurate formula? —Quondum 06:25, 23 January 2015 (UTC)

While we wait for that to happen, I suggest we remove this figure. The one below it is very similar, but better, so at best Magnetic_ring_dipole_field_lines.svg is redundant. RockMagnetist(talk) 16:12, 23 January 2015 (UTC)

I have no problem removing that image. I notice another that is close to what we are discussing (see thumbnail), but for the black arrow and thicker lines than necessary. Bdushaw, it seems your skills might be up to the challenge . —Quondum 01:38, 25 January 2015 (UTC)

Done, as penance for my electron magnetic dipole moment snafu...thinner lines even, by request. :) Bdushaw (talk) 04:14, 25 January 2015 (UTC)

(BTW, that figure was not so easy...computed in matlab using the equation to the left of the figure, with a home-grown solution for adding the arrows.) Bdushaw (talk) 04:15, 25 January 2015 (UTC)

BTW2, for depictions of other magnetic fields, and software to make them, see VectorFieldPlot. There is a figure for a magnetic dipole at the bottom that might be used here instead. Bdushaw (talk) 04:30, 25 January 2015 (UTC)

Thanks, I was not aware of VectorFieldPlot; it might be good to use this in general. Is it my imagination, or is there a difference in the density of field lines in different places, though the shape of the lines is the same? Somehow, yours seems "more" accurate in this respect – though as I've already remarked, field line density is not proportional to field strength due to the third dimension. It would be nice if there was a power-law relationship in special cases such as this (though even this might be a lot to ask; representing a section of a field accurately is clearly nontrivial). Let's see what other comment we get on the diagram – I think that is much better, and would only think that black might do better for consistency with the other diagrams. —Quondum 05:36, 25 January 2015 (UTC)

Hmm - yes it occurs to me that getting the field lines right such that the density of lines accurately represents field strength may not be so easy. One computes these (as VectorFieldPlot does it seems) by specifying a set of starting points, and integrating the path out. So the trick would be to specify a uniform set of points where the field is uniform. For a perfect dipole there is no such place! One could use a loop current likely, as in the figure below, and then plot it on a large scale such that the loop current dimension goes to near zero. I made my figure by specifying a set of equally spaced starting points just above the center of the dipole, which may not be too bad. One can also plot a color contour plot of |H(r)| to get the intensity directly - this looks somewhat like my plot, with an oval shaped region at the center - lines of constant field intensity are oval shaped. The VectorFieldPlot figure does look a bit peculiar near its center. I can certainly do black lines; red looks more exciting... One might note that a magnetic dipole strength falls off quite quickly with distance 1/r^3 and that the field is cylindrically symmetric. Bdushaw (talk) 06:52, 25 January 2015 (UTC)

"... to specify a uniform set of points where the field is [of] uniform [intensity]." It is straightforward to show that this cannot work (in the sense of having a given line density representing a given intensity) in the general case, but if you can show it holds in this case, this might give the most intuitively valid result.

Your mention of symmetry (in this instance rotational, not cylindrical) give us another quantity that the line density can represent while the direction is accurate for the planar section when such a symmetry holds: r times the field strength, where r is the distance from the axis of symmetry. One could represent this "bleeding off" of intensity into the third dimension via something like the lines getting thinner away from the axis, or by a fading background colour intensity. In the former case, total line width per unit width would represent field intensity. —Quondum 16:00, 25 January 2015 (UTC)

I've thought about it, and will punt on the matter, if that's ok. You are likely right that the figure is not 100% correct in line density to field strength, but on the other hand it is not THAT in error I don't think. The region where the lines merge to form a continuous red gives an oval shape equivalent to what |H| shows. Insofar as wikipedia goes, I claim the figure is "good enough". Anyone attempting to use the figure for quantitative research has only himself to blame... I could upload the figure of field strength, if you like. I thought about including it in this article, but it seems not essential. Bdushaw (talk) 23:02, 26 January 2015 (UTC)

I think I'd have to agree with you. Aesthetically pleasing + direction of lines accurate (as in the diagram) is the best we should go for, and I think any field strength detail belongs in Magnetic dipole, not here. We've gone from something that feels odd to something that feels right. —Quondum 23:30, 26 January 2015 (UTC)

Measurement

The article does not specify how this quantity is measured.--79.119.209.81 (talk) 15:59, 30 March 2015 (UTC)

Good point. We should have a summary of Magnetometer#Laboratory magnetometers in this article. RockMagnetist(talk) 18:18, 30 March 2015 (UTC)

Units Problem

In the units section of the main article, it states that units for magnetic dipole are N.m/T = A.m^2 = J/T.

I shrieked when I saw a torque of N.m converted to J. I'm a mechanical engineer, and for me, that is a huge no-no. You never convert torque to energy. Torque, of course, is meters cross-product newtons, whereas work is meters dotted with newtons. Our unit system does not carry along this vector operation aspect, so we have to know that we cannot interchange them. We represent torque as a vector, whereas work is a scalar. To my mind, it is fundamentally incorrect to convert newton-meters of torque to joules. In the end, units can be manipulated and the answer will come out the same, so if you guys tell me that in the magnetics world, you cover your eyes and commit this atrocity all the time for some reason, then fine. I'll simply not like it. But if you agree, then I'd like to delete the bit where it shows J/T. Kimaaron (talk) 15:19, 19 February 2016 (UTC)

Kimaaron, your argument makes sense, but IEEE Magnetics and NIST accept J/T (I have added a citation). Surprisingly, I can't find any sources that suppose N.m/T, so I'm going to tag that. RockMagnetist(talk) 16:58, 19 February 2016 (UTC)

Kimaaron,the distinction between Nm and J is important to me as well. In this case though both makes sense for different reasons: the former comes from the equation N =mxB and the latter because U = -mdotB. That being said from a limited search I usually see J/T. TStein (talk) 06:13, 17 April 2018 (UTC)

@Kimaaron: RockMagnetist, TStein: Have added a short explanation in the "Units" section with source citation that may "resolve" this issue, or give a start to a satisfactory formulation. Sdc870 (talk) 14:57, 23 March 2019 (UTC)

Magnetic moment of an atom

Here "m" is taken as "m_j", whereas in the Wikipedia page "Magnetic quantum number" m is taken as "m_l", with different meanings. Please reach an agreement between the two pages, or use the unambiguous notation m_j, m_l respectively.Grausvictor (talk) 16:51, 24 May 2016 (UTC)

"Magnetic Quantum number" relates only to orbital angular momentum L, so using a j subscript is incorrect. Magnetic moment relates to total angular momentum J, which is equal to L+S. The second suggestion is correct, and has been applied to the "Magnetic Quantum Number" page. — Preceding unsigned comment added by 128.252.1.101 (talk) 19:58, 17 August 2018 (UTC)

The complaint here appears to be an issue of notation rather than of substance. Not using the same notation as another Wikipedia page does not make the information wrong. I am therefore going to remove the {{dubious}} tag that has been sat there for over four years without action. If the claims in the article are actually wrong, that is a another matter, but can't you WP:JUSTFIXIT rather than leave it sitting there for years? SpinningSpark 11:41, 22 July 2020 (UTC)

factor 1/2

funny talk: "If one had started from a differential definition" ... how about deleting these confusing lines with contradicting equations? Ra-raisch (talk) 16:08, 15 April 2018 (UTC)

I did not understand that section either. I redid that section, but much more work needs to be done. Please check to make sure that the section makes sense. Thanks. TStein (talk) 06:15, 17 April 2018 (UTC)

I need thoughts on organizing parts of the article into 3 size regimes

It occurs to me that the dipole moment is used for 3 very different cases corresponding roughly to 3 different size regimes:

1. atomic size magnetic moments depend heavily on the relationship with the quantized angular momentum
2. engineering devices such as electromagnets, permanent magnets, quadrupole magnets, etc use the concept of magnetic moments differently. Here the currents are well controlled and known. Magnetic moments may use Amp turns instead of Amps, etc.
3. planetary sized (and larger) where the dipole moment is inferred from the measured magnetic fields and is used to get a better understanding of what layer produces the magnetic field and how.

Each of these regimes overlap but each also treat the magnetic dipole differently. I don't want to reorganize this article willy-nilly, especially when I am uncertain both what the proper way to reorganize it and how much time I will be able to dedicate to this. Any thoughts on this will be greatly appreciated. TStein (talk) 06:33, 17 April 2018 (UTC)

User:MaoGo one of your section edits is probably breaking links

I have no problem with your shortening of the section titles. The middle edit (Two models of ...) may be an issue, though. There are a large number of links to that particular section. I wouldn't mind fixing all of these links (I have fixed many of those links from an earlier change before) myself, except that I have the most generic settings for my editor. I am fairly certain there are tools to fix this but I have no clues where to look to find them or which tool to use. The what links here is useless as far as I can tell. TStein (talk) 15:58, 4 May 2018 (UTC)

Oh, sorry, my bad. Didn't think it was important, you may revert that particular title. I'm having the same issue with another article. Maybe we need expert help with that. --MaoGo (talk) 16:05, 4 May 2018 (UTC)

he, he. I was hoping that you had better tools for fixing links. I like your title for the subsection. I will chase down fixing the broken links later. TStein (talk) 19:14, 4 May 2018 (UTC)

I tried asking in the help desk but it doesn't seem to exist a standard way of fixing this issue. Check out the discussion Wikipedia:Help_desk#Is_there_a_Whatlinkshere_for_sections --MaoGo (talk) 16:00, 5 May 2018 (UTC)

I don't see any sign that there were links to this section title, except for one inside Momentum. If there were an issue, I think {{anchor}} would solve it. RockMagnetist(talk) 16:27, 5 May 2018 (UTC)

Magnetic pole strength

I've just discovered that magnetic pole strength redirects here to a heading that no longer exists. I've corrected that, but I have to say that I think the redirect into this article was a terrible idea. Look at it from the point of view of someone who has typed the term into the search box. They want to know about pole strength, not magnetic moment. So yes, now they are redirected to the right place they can learn that m=pl, but now they still have to go back to the top of the article and read the whole thing because now you have forced them to understand magnetic moment before they can understand pole strength. SpinningSpark 07:14, 19 May 2018 (UTC)

You have a point, but I don't think the answer is to restore the article, which only had one link leading to it. Instead, we could import more of the material from the article into that section - most importantly, the definition of an oersted in terms of magnetic poles. RockMagnetist(talk) 21:18, 20 May 2018 (UTC)

I think there is an error in how you describe m as the volume integral of remanence Br. In fact it is the volume integral of the internal magnetization M, which is not the same except in special/unusual cases. See this link https://farside.ph.utexas.edu/teaching/jk1/Electromagnetism/node61.html 98.159.210.62 (talk) 15:36, 4 April 2022 (UTC)

Behavior of magnetic dipole in an electric, NOT magnetic, field.

I need to know how a magnetic dipole behaves in an electric, not a magnetic, field. I suppose it precesses in some way, but not in identically the way that a magnetic dipole behaves in a magnetic field. Can anybody direct me to information on this? Slyfox4908 (talk) 22:01, 29 May 2019 (UTC)

@Slyfox4908: This WP:NOTFORUM. Anyway, I think you are looking for Spin–orbit interaction (in the case of an atom the electron spin is a magnetic dipole, and the electric field comes from the nuclei).--MaoGo (talk) 22:07, 29 May 2019 (UTC)

Traveling magnetic dipole is affected by electric field through Lorentz force. To see it, change for a moment coordinates such that magnetic dipole is resting - now the source of electric field is traveling in magnetic field, and the resultant Lorentz force also acts on the magnetic dipole through third Newton's law. Here is such simple Lagrangian with magnetic dipole $\mu$ from https://en.wikipedia.org/wiki/Free-fall_atomic_model: $\mathbf {L} ={\frac {1}{2}}m\mathbf {v} ^{2}+{\frac {Ze^{2}}{r}}+{\frac {Ze}{c}}\left[\mathbf {v} \cdot \left({\frac {\mu \times \mathbf {r} }{r^{3}}}\right)\right]$ — Preceding unsigned comment added by 89.64.54.158 (talk) 04:59, 14 October 2019 (UTC)

Fractal what?

Theres a phrase in the introduction ;- "..and all other fractal expressions of electromagnetic energy". What in the dickens can that possibly mean? Theres no other reference to fractals in there. Looking on the net, this isn't a wingnut special, its a real thing, but the article doesn't expand what exactly is meant by that turn of phrase. As a non physicist, that phrase has thrown me on a loop, could someone please add something to explain what that means? Duckmonster (talk) 08:02, 16 February 2021 (UTC)

It was some random IP that added that without proper justification. I reverted it to the previous stable version. Thanks for pointing it out. Also, remember that new sections should go at the bottom of the page.--ReyHahn (talk) 08:41, 16 February 2021 (UTC)

what's that?

"Intrinsic magnetic moments and spins of some elementary particles"

Is it a fake? Where You got such values from? 79.204.137.204 (talk) 11:43, 22 March 2024 (UTC)

The table is referenced in the caption as CODATA Johnjbarton (talk) 16:01, 22 March 2024 (UTC)