Talk:Magnetic moment/Archive 1

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Magnetic field produced by a magnetic moment

Does anyone have a source for these equations? I'm pretty sure the form of the equation is geometry dependent. I recall deriving an equation for a magnetic field due to a bar magnet using the magnetic moment method and the answer was not that simple... I remember a three part triple integral w/ no closed form solution in the x or y directions unless the point of interest is specified —Preceding unsigned comment added by Jlvandruff (talk • contribs) 05:41, 4 September 2009 (UTC)

Cleanup required

I've marked this article for cleanup, due to its confusing and sometime incorrect explanations (see "Current, not poles" below). For a much better discussion of magnetic dipoles and magnetism, see magnetism.

ALSO The magnetic moment discussion here makes no mention to the magnetic moment in chemistry, only in physics. Can this hopefully be updated to include this information?

Also, torque is not measured in joules. http://en.wikipedia.org/wiki/Joule#Confusion_with_newton_metre 121.73.5.66 (talk) 06:09, 27 November 2010 (UTC)

• While you're cleaning up, please remove references to anonymous "textbooks" and replace them with real citations. Anonymous "textbooks" are mere euphemisms for "they", and are in no way preferable. Dlw20070716 (talk) 09:25, 27 March 2012 (UTC)

The sentence you are referring to is a statement about textbooks, so citing the textbooks themselves might qualify as WP:SYNTHESIS. Instead, I am citing the book by Brown. I admit the sentence could do with rewriting for clarity. RockMagnetist (talk) 15:15, 27 March 2012 (UTC)

Current, not poles

The explanation of a magnetic dipole moment as being created by two poles separated by some distance is incorrect. This is true for electric dipole moments, but magnetic dipole moments are created by current loops, or by the aggregate effect of many intrinsic dipole moments, such as by many electrons whose spins are aligned, as in a bar magnet.

Magnetic monopoles, such as those implied in the discussion here, have never been detected, although there remains some possibility that they do exist. Even if they do, however, it is extremely unlikely that they are responsible for magnetism in the everyday sense.

Definition of Spin

I hate to be a pedant, but the table in the article says that spin is dimensionless. Spin is an intrinsic angular momentum and as such most definitely does have units. Its units are Js, the same as Planck's constant. Unfortunately, Planck's constant gets dropped from all the literature and text books either for brevity or when working in some natural units system, where c = h = 1. Earlier on in the article it does mention that electronic spin is a half times Planck's constant, but the table says it is dimensionless, which is likely to cause confusion (aside from the factual inaccuracy). --Lateralis (talk) 20:34, 1 May 2008 (UTC)

Molecular Dipoles

The page mentions the dipoles of electrons and nuclei, but molecules themselves have a dipole. It is of great importance in the field of chemical solvents and their respective miscibilities. This should be added.

unclear phrase

the phrase

  Any rotating charged object has magnetic moment from the earth to the electron.

is quite cryptic. Can anyone elaborate or rephrase? ~~

Better rephrasing would be: "Any rotating charged object (from as large as the earth to as small as electron) has magnetic moment"

This is also untrue. A uncharged rotating object will have no magnetic dipole moment.

I'm not sure what people are saying here. The earth has zero net electric charge, yet it does have a magnetic dipole moment (due to currents in the core, most likely). The electron is charged, and it has a magnetic dipole moment, but is it rotating? Hard to say; it has "spin", but that's an internal quantum number, and it's a point particle. The neutron is uncharged, but it does have a magnetic dipole moment. HEL 18:47, 5 November 2006 (UTC)

Vectors vs scalars

Am I right that in the equation mew = I A the current I is a scaler? If it is, that should be noted.

Corrected LeadSongDog 21:23, 17 May 2007 (UTC)

...In which case, in keeping with the notation employed (the arrow over $\mu$ to denote a vector) the vector surface area, defined as "a" in the article, should also be given an arrow. It may also be useful to say that the "vector surface area" means a "surface area with a unit vector normal to the plane". Moreover, tying in with the comment further down the page, it is probably a good idea to change "a" to something else, such as "s", so there is no ambiguity with the magnetic vector potential. --Lateralis (talk) 20:34, 1 May 2008 (UTC)

Start simple, then work up

While a novice reader will easily understand the definition in terms of a permanent magnet, the "magnetic moment in a magnetic field" will throw them very early in the article. It is good to keep the explanations/usages of the term itemized and summarized at the top of the article, but phrases should be added to distinguish them as different definitions (not in meaning, but in explanatory mechanism.) The separate paragraphs don't quite achieve this on their own.

Moreover, this lumping of a simple bar magnet in with a more complex relationship between magnet and another body/field continues in the "Explanation" section. It might be best to explain the stand-alone bar magnet longhand, e.g. give a few examples of a short powerful magnet and a big weak one. This may sound braindead, but it will help build confidence in the reader and encourage them to try to understand the next section, which would hopefully also be made more explicit. (71.233.165.69 01:49, 10 June 2006 (UTC))

A is a bad vector name

A is commonly used as the magnetic vector potential, and "a" as the surface normal vector.

Magnetic quadrupole moment and higher?

This page seems to equate "magnetic moment" with "magnetic dipole moment". This is wrong since there are magnetic quadrupole, octupole, ... moments as well. Either the page should be renamed "Magnetic dipole moment", or it should be changed to state "the most commonly encountered type of magnetic moment is the magnetic dipole moment" and then also mention the quadrupole, etc. HEL 18:50, 5 November 2006 (UTC)

Cant quadrupole and octupole moments be broken down into dipole moments? just-emery (talk) 19:02, 3 June 2009 (UTC)

Changes made

I'm a Physics-Major and currently enrolled in a course on Electricity and Magnetism, I am in the process of editing some of this page. I have changed the definition of the magnetic dipole at the top of the page to its correct definition.

phrase

This: (Curl your fingers in the direction of the current, your thumb will point in the direction a). sounds funky. Am I wrong in saying that you cannot curl your fingers in the direction of current (in a wire)? How do you curl fingers around the direction of a straight-moving thing? You curl your fingers around a conductor/wire in the direction of the magnetic lines of flux and then your thumb should be pointing in the direction of the current... or at least, that's what I was taught (and the old-timers with their "conventional current" will use a "left-hand rule"). However, I could have this all wrong/misunderstanding the author's meaning. Gaviidae 15:46, 2 March 2007 (UTC)

Partly right, in the example then the current is in a wire loop so it is a circular current and it is possible to curl your fingers along the current but the original author mixed up the right-hand rule and the right hand grip rule, I have fixed that.

Better to delete the funky elaboration, which is redundant at best. There's no need to explain the right hand grip rule here. Will snip it. LeadSongDog 20:19, 17 May 2007 (UTC)

proposed merge

No. Each article has a link to the other. I think a merge would not be in order.--Loodog 17:33, 25 April 2007 (UTC)

No. I dont think merging the two articles makes sense. —Preceding unsigned comment added by 130.89.95.129 (talk) 13:09, 8 October 2007 (UTC)

I agree, these pages shouldn't be merged. Magnetic moment is an abstract notion, and requires a definition, as well as explanations how to use it in calculations. The electron's moment is a natural phenomenon, and as such requires mention of its discovery, history, etc. —Preceding unsigned comment added by Xenonice (talk • contribs) 23:18, 7 November 2007 (UTC)

Template:Electromagnetism vs Template:Electromagnetism2

I have thought for a while that the electromagnetism template is too long. I feel it gives a better overview of the subject if all of the main topics can be seen together. I created a new template and gave an explanation on the old (i.e. current) template talk page, however I don't think many people are watching that page.

I have modified this article to demonstrate the new template and I would appreciate people's thoughts on it: constructive criticism, arguments for or against the change, suggestions for different layouts, etc.

To see an example of a similar template style, check out Template:Thermodynamic_equations. This example expands the sublist associated with the main topic article currently being viewed, then has a separate template for each main topic once you are viewing articles within that topic. My personal preference (at least for electromagnetism) would be to remain with just one template and expand the main topic sublist for all articles associated with that topic.--DJIndica 16:48, 6 November 2007 (UTC)

units

Doesn't magnetic moment have a unit of measurement? If so, what is it? —Preceding unsigned comment added by 76.126.101.24 (talk) 15:56, 16 July 2008 (UTC)

Intuitive interaction of magnetic fields

There is no actual position of a north or south pole, as is usually stated in diagrams and articles, and one can get confused when researching this phenomina. Using the convention that magnetic field vectors run from north to south, one can assertain how two magnets will interact, and how they will allign themselves, by knowing that attraction occurs when the directions of their two field vectors are in opposing directions (N can be anywhere, as long as it precedes S on a field vector line). It is the change in direction (N to S or S to N) that governs the direction of the interactive forces, and the magnitude of this change, that governs the amount of force. That is, it is the relative positions, not the actual positions, of N and S, that count. Dipoles cannot be sepperated into monopoles, all calculations must treat them as one; that is, a change, or difference, or relative value. 124.185.155.44 (talk) 11:20, 15 December 2008 (UTC)

direction of electron spin

teh article states that the charge rotates in the opposite direction from the electron itself (or its mass). how can the direction that the electron (or its mass) rotates be known? Em3ryguy (talk) 19:00, 16 March 2009 (UTC)

Larmor frequency

the section on the effect of externally applied magnetic fields should mention that simple dipoles will resonate. this is non-intuitive and is very important so it ought to be mentioned. maybe a link to Larmor frequency too. just-emery (talk) 18:58, 3 June 2009 (UTC)

Confusing Introduction

As a newbie to the topic, the opening definition makes little sense:

In physics, astronomy, chemistry, and electrical engineering, the term magnetic moment of a system (such as a loop of electric current, a bar magnet, an electron, a molecule, or a planet) usually refers to its magnetic dipole moment, and quantifies the contribution of the system's internal magnetism to the external dipolar magnetic field produced by the system (that is, the component of the external magnetic field that drops off with distance as the inverse cube).

Is there a way to simplify this? Could you say that magnetic moment is a measurement of the strength of a magnet and use a more technical definition later? --72.253.129.59 (talk) 18:44, 12 June 2009 (UTC)

Your welcome to give it a try. The worst that can happen is that someone will revert your change.  :-)   Daniel.Cardenas (talk) 05:57, 13 June 2009 (UTC)

Well, I've given it a try. I'm a little more worried about accuracy. Here's my revision:

The magnetic moment of a system is a measurement of strength of magnetism in physics, astronomy, chemistry, and electrical engineering. More technically, the term magnetic moment of a system (such as a loop of electric current, a bar magnet, an electron, a molecule, or a planet) usually refers to its magnetic dipole moment ...

--72.253.129.59 (talk) 20:30, 13 June 2009 (UTC)

Why does Magnetic dipole redirect to this article?

If "The magnetic moment of a system is a measure of the strength and the direction of its magnetism" why does Magnetic dipole redirect here?? As I understand it, a magnetic dipole is a thing that may get measured and have strength and direction but is not, itself, a measure, strength or direction. From the Dipole article we have: "A magnetic dipole is a closed circulation of electric current. A simple example of this is a single loop of wire with some constant current flowing through it." While that does not seem like an accurate definition (leaving out permanent magnets), at least one exists there whereas I cannot find a definition of magnetic dipole in this article. Why not redirect Magnetic dipole to the Dipole article and clean it up? Jojalozzo (talk) 03:14, 9 July 2009 (UTC)

Probably because the most important thing about a magnetic dipole is its magnetic moment. Others may disagree. 121.73.5.66 (talk) 06:07, 27 November 2010 (UTC)

I came here because I was confused about this too. Specifically, I tried to look up Magnetic Dipole and it redirected to the Magnetic Moment article. There is a sub-category "Magnetic Dipole" on this page but for someone who doesn't know what a Dipole is (me), that really should be linked somewhere in that section. I didn't think of looking up dipole until I came here to the Talk page and saw this discussion. Also, the section on the page that talks about Magnetic Dipoles includes a link to the wikipedia entry on Magnetic Dipoles... which links back to this article. So, self referencing links are not very helpful. I'm going to go ahead and edit the Magnetic Dipole section of this page to link to the Dipole page. Makes sense to me. I don't edit often so feel free to change it back or something if you really disagree, but I can't imagine having the page reference itself is more helpful.Facegarden (talk) 21:20, 11 July 2011 (UTC)

Gyromagetic factor

The value that appears in the article is the experimental or the theoretical value? Both should appear in the article, as well as on the gyormagnetic ratio article.--190.190.87.136 (talk) 17:12, 30 July 2009 (UTC)

Will the REAL magnetic dipole moment please stand up?

The author says that the SI unit of magnetic dipole moment has two equivalent representations:

1 m²·A = 1 J/T.

Unfortunately, this is true: The SI unit of so-called magnetic dipole moment is, in fact, so defined. I say "unfortunately", because real magnets don't pay any attention to decisions reached in committees of the International Electrochemical Commission. In "Experimental Demonstation that the Couple on a Bar Magnet depends on H, not B" [Nature 234,31(1971)], authors R. W. Whitworth and H. V. Stoppes-Roe have shown that real magnets pay attention to a different equation that was first introduced by Giorgi [Atti Ass. Elettrtec. Ital. 5,402(1901)], but now named after one Arthur E. Kennelly. According to Giorgi, Kennelly and real magnets, the magnetic dipole moment has two equivalent representations, in SI units:

1 m ^-1·A = 1 Wb·m.

The authors write:

"If we take the moment of the magnet to be independent of the surrounding medium, this result is consistent with the Kennelly equation (1) [magnetic dipole moment = J/H] and not with the Sommerfield equation (3) [magnetic dipole moment = J/B(=J/T)]. Allowance for any change in the moment of the magnet due to insufficient hardness could not reverse this conclusion because the effect does not have the required sign. We therefore conclude that the couple on a long hard magnet which has a constant magnetic moment j is given by the Kennelly equation."

In closing they comment:

"The conclusions of this paper can be reached from the basic concepts of electromagnetism and, indeed, many authors have done this. Nevertheless, the importance of the B-field in problems of current electricity has led many others to incorrect conclusions. The subject is of immediate importance because of the danger that decisions about standardization of systems may be taken in the belief that only matters of convention need be considered. We have set out to demonstrate that matters of physical fact are involved."

When it gets right down to it, the Sommerfeld theory and the Kennelly theory are both approximations due to the fact tha real magnets and real coils both displace part of whatever permeable medium they may be immmersed in and the materials, out of which the magnets and coils are made, have their own permeabilties, too. In order for magnetic fields due to real magnets and real coils to be directly comparable, it would be necessary to use magnets and coils, with the same shape and dimensions, made out of materials with the same permeability and immersed same medium. Such an experiment has yet to be performed.Koilon (talk) 07:27, 8 February 2010 (UTC)

Does electric field affect movement of magnetic moment?

I couldn't find it in the article, but it seems that there is kind of dual Lorentz force for magnetic moment travelling in electric field (like Aharonov-Casher effect is dual to Aharonov-Bohm).

For example imagine classical electron traveling in proton's electron field - let's change reference frame such that for infinitesimal time electron stops and proton is moving in also magnetic field created by quite large electron's magnetic moment - because of 3rd Newton's law, resulting Lorentz force should also work on electron ...

(3) equation here is Lagrangian for such electron's movement: $\mathbf {L} ={\frac {1}{2}}m\mathbf {v} ^{2}+{\frac {Ze^{2}}{r}}+{\frac {Ze}{c}}\left[\mathbf {v} \cdot \left({\frac {\mu \times \mathbf {r} }{r^{3}}}\right)\right]$

the last term here corresponds to this electric field-magnetic moment interaction.

This force seems to be quite important (if true?) - maybe it should be somehow included in the article? —Preceding unsigned comment added by 195.150.224.239 (talk) 11:22, 21 February 2011 (UTC)

This is discussed in spin-orbit interaction, which takes the approach of Lorentz-transforming to the instantaneous rest frame of the magnetic moment and finding that there's a magnetic field there. I also learned it that way. Anyway I agree this is a topic worth discussing in the article. --Steve (talk) 18:54, 21 February 2011 (UTC)

Treatments of the magnetic dipole field

I have started a general discussion of the many treatments of the magnetic dipole field at Wikipedia talk:WikiProject Physics#Treatments of the magnetic dipole field. RockMagnetist (talk) 03:08, 3 August 2011 (UTC)

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