Talk:Dipole

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Dipole moment points from negative to positive for an electric dipole. At least according to every other textbook and website I've looked at. You should be able to verify this by looking at the torque and potential energy equations.

which element on the periodic table are polar which are nonpolar?

  • Elements (atoms) are never polar, see the subsection on atomic dipoles --P.wormer 10:04, 8 February 2007 (UTC)

The directions of the field lines shown in the first image of a dipole field are NOT CORRECT. The direction is away from the north pole and towards the south pole. Someone please correct this.

The article says: "Any configuration of charges or currents has a 'dipole moment',..." What about a pure monopole, quadropole, whateveryoulikeapartfromdi-topole configuration? Haha :)

Contents

[edit] Dipole field lines

OOPs. someone just corrected the direction of the field lines. they are fine now.

Not so fast. It was I who 'corrected' the field lines, but now I am having second thoughts. I may have to restore them to what they were before, unless you can convince me otherwise.
The problem is that the drawing is of the Earth's magnetic field, not of a bar magnet. (It says so if you hover your mouse over it, but it would be more obvious if it had a permanently visible caption.) The Earth's magnetic pole near geographical North is a south-seeking, or negative, magnetic pole, whilst the pole marked "N" on a bar magnet is (by definition) a north-seeking, or positive, pole. (I hope this is not in doubt. It follows from the fact that opposite poles attract. See this NASA page for confirmation.) Therefore it is right that the field lines should be directed towards the pole marked "N" on the diagram, assuming that "N" represents geographical and not magnetic north. If the diagram were of a bar magnet, then the lines would indeed point the other way, away from the positive pole that we confusingly label "N".
By the way, according to http://edhiker.home.comcast.net/North_Pole_Positive_or_Negative.html, English and German bar magnets are labeled oppositely to French ones, so if you are French then what I said above will make no sense to you whatsoever! --Heron 19:44, 26 September 2005 (UTC)
I doubt this. According to French Wikipedia, North of a magnet points to geographic North. -- Wotan.

Yes, I agree that there is great confusion about north and south poles. If modeling the earth's magnetic field as a dipole field, then the magnetic pole in earth's geographic north is a south magnetic pole, with magnetic field lines pointing towards it. So -- if the diagram is intended to show the earth's magnetic field, then the labeling of the pole above, presumably in the geographic north, should be labeled S for south. I think my preference would be for you to just flip the image since geographic north is usually shown as up on a map. On the other hand, since the title of this article is "dipole", you might just stick to showing a standard magnetic dipole field with magnetic field lines pointing away from a magnetic north pole and towards a magnetic south pole. References: http://www.windows.ucar.edu/spaceweather/info_mag_fields.html and http://bell.mma.edu/~mdickins/TP2/Lectures/lectures21Cut.html

So what about the the flipping of the polarities? Anyone have any idea or theory of what that might or might not do to our technological way of life? Other than render our compasses North pointing have to change them to south? Will it disrupt our generating electricity? I mean when we get solar CME's from the sun flipping, and it reaches earth in from the sun in 8 and a half minutes if it is a southern it could wipe out all of the transformers....what would our own planets magnetical shift do to them?

I vastly prefer a picture like this one to help with the confusion. Brian 16:08, 1 December 2006 (UTC)

[edit] Other mathematical explanations

Isn't it quite common to say that a dipole is a source and a sink that has come infinitively close to each other, giving the potential function phi = ax/(x²+y²), "streamline function" psi = ay/(x²+y²)? I couldn't find any of those anywhere around here.

[edit] Lumping magnetic & electric dipoles together

I think that the opening sentences, which speak about electric and magnetic dipoles jointly, is confusing. It implies the existence of magnetic monopoles, whose existence must then be denied. An electric dipole is the limit of two charges, as the distance between them goes to zero and the product of the electric charge times the distance remains constant (electric dipole moment). A magnetic dipole is the limit of a loop of current, as the loop's area goes to zero and the product of the current times the loop area remains constant (magnetic dipole moment). Why not separate the two definitions? Why lump them into a single sentence?

[edit] Units

I think it would be a nice touch to add a statement of what unit system is being used when equations are being presented, this page is clearly in MKS while in my experience (I am a space physicist) most folks use cgs, so a mention of this being in MKS would be nice. In this case all the μ0's make it obvious but it is not always Brian 16:04, 1 December 2006 (UTC)

Clearly it is in SI Units. Even the cgs wiki deprecates cgs and MKS for SI. The only alternative that makes any sense to use in the context of electromagnetics is Planck Units, but that would require some serious rewriting.LeadSongDog 20:54, 18 May 2007 (UTC)

[edit] From negative to positive or vice-versa?

Physics textbooks (e.g. Jackson) tend to define the electric dipole moment as pointing from negative to positive charges (since it is \int \vec{x} \rho(\vec{x}) dV).

A recent anonymous edit changed this to say that it points from positive to negative, citing unnamed "chemistry textbooks". Can anyone verify this?

If there really are two common conventions (which would be annoying!), we should explain this, although I personally only recall the negative-to-positive convention.

—Steven G. Johnson 04:36, 28 December 2006 (UTC)

Personally I have known for more than 30 years that (organic) chemists let dipoles point from plus to minus and physicists from minus to plus, and, indeed, that it is confusing. There is, however, a large difference in the way chemists and physicist look at dipoles. For organic chemists it is an object with a single value: "the dipole of molecule A is x.y Debye". If they draw a dipole, they put a δ+ and δ- somewhere in the molecule. That is, chemists do not understand the concept of a point dipole. For physicists a dipole of a charge distribution (such as a molecule) is a vector with a well-defined formula for its three components (see previous contribution). A physicist would draw a dipole as an arrow. When a physicist would try to explain to a chemist that the chemist's value of a dipole is the length of a vector, he would meet utter disbelief. --P.wormer 12:11, 28 January 2007 (UTC)
  • Example to my previous remark: see the present article for "the" dipole moment of cyanamide, which is 4.27Debye. No direction, no frame of reference is given. For a chemist the value 4.27 is all (s)he wants to know about this object. --P.wormer 12:32, 28 January 2007 (UTC)

Examples of chemistry textbook: Solomons, T. W. G. Organic Chemistry, 5th Edition; Wiley:New York 1992, pp.39-40; Morrison, R. T.; Boyd, R. N. Organic Chemistry, 4th Edition; Alluyn and Bacon: Boston, 1983, pp. 22-23; Fox, M. A.; Whitesell, J. K. Organic Chemistry, 2nd Edition; Jones and Bartlett:Sudbury, MA, 1997, PP. 92-93; Bruice, P. Y. Organic Chemistry, 5th Edition; Pearson Education:Upper Saddle River, NJ, 2007, p. 12. In all these books a dipole vector points TO a negative charge. So the relevant sentence in the second paragraph is confusing. Evidently, chemists and physicist define the direction of a dipole vector in the opposite ways, which should be reflected in the article. --Fangol 09:23, 20 March 2007 (UTC)

  • Fangol, you sound if you disagree with me, but we are in complete agreement! In my 2nd paragraph above I did not state that chemists do not give direction to a dipole (this would contradict my first paragraph), but that direction for chemists is more a matter of polarity ("one side of the molecule is positively charged and hence the other side negatively". Very often the polarity is not even mentioned, because every chemist is supposed to know the polarity from electronegativity rules, see the cyanamide example that I quoted). The direction of a dipole has not the quite the same feel for a chemist as for a physicist, for whom a dipole is just any ol' vector. So, please go ahead and introduce a caveat into the main article citing the sources that you mentioned (and register yourself as a bona fide Wikipedian).--P.wormer 16:40, 20 March 2007 (UTC)

[edit] merge with electric dipole moment

This article has lots of overlap with electric dipole moment. May be it is a good idea to split the present article into two: magnetic and electric dipole and to move all things electric to electric dipole moment? Anybody has objections to this? --P.wormer 10:01, 8 February 2007 (UTC)

Also duplicates much of the existing magnetic dipole and magnetic dipole moment. Given the duality of the two, it's not surprising. This is perhaps the cleanest of the related wikis. I'd oppose the move (at least for now, until clean alternatives are ready). LeadSongDog 15:46, 18 May 2007 (UTC)

[edit] Quantum mechanical dipole operator

I think there is something wrong in the describtion of the quantom mechanical operator of the dipole in the article. If I am correct, there is just a general formulation of the classical expression of the dipole in part 4 of the main article. moritz vh, 12 March 2007

I don't quite understand what you mean, but remember that the QM operator has the same appearance as the classical expression. The only difference is that the position vectors are interpreted as multiplicative operators in QM. --P.wormer 19:48, 12 March 2007 (UTC)

[edit] Wrong Constants

According to Halliday, Resnick, and Walker's Fundamentals of Physics, 30-5, the magnetic dipole has a coefficient of μ_0/2π not, μ_0/4π. Either that, or the description of the angle λ is wrong. —The preceding unsigned comment was added by 72.82.207.39 (talk) 23:07, 28 March 2007 (UTC).


Also, article everywhere refers to \epsilon_0 and \mu_0 instead of \epsilon and \mu, yet text doesn't say this is only for the free-space case.

In the radiated power calculation, factor 12 seems suspicious. Can someone confirm? LeadSongDog 19:32, 18 May 2007 (UTC)

[edit] a corrected version of Image:Dipole field.jpg

I created a corrected version of this figure on Image:Dipole field.PNG (the field line arrows are reversed). You might want to use it, and cange the description accoefingly

eman 12:06, 9 July 2007 (UTC)

[edit] Euler potentials: wrong?

I don't see the reason for exponential term in euler potentials. What is the source for this formula?

Skwa 01:57, 3 October 2007 (UTC)