Talk:Dipole/Archive 1

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Untitled

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 :)

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)

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.

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?

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 \mu_0'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)

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)

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)

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)

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)

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)

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)

Dipole gif: opposite signs for left and right

Dipole.gif

Is the gif correct, that the fields to the left and the right of the y-axis have opposite sign (blue and green)? And if it is correct: Where does this effect come from, I do not see it in the formulas. --Geoemyda (talk) 14:08, 4 November 2008 (UTC)

On this webpage of a german university the dipole radiation is symetric: http://www.temf.de/Dipol-und-Hornantennen.59.0.html --Geoemyda (talk) 14:17, 4 November 2008 (UTC)

They may have plotted absolute value for simplicity. If you look at the formula, it should be antisymmetric in x.--Loodog (talk) 17:45, 4 November 2008 (UTC)
"They may have plotted absolute value for simplicity" - but it becomes negative, what absolute values usually do not.
What did you plot there? It is difficult to plot a vector using colours. Did you only plot one componenent E_y or the absolute value \sqrt{E_x^2+E_y^2} artificially adding a sign by looking at the direction of the vector E?
What would I see if you would look at the dipole from the top - where do the negative fields and the positive fields of the opposite signs fit together? --Geoemyda (talk) 08:43, 5 November 2008 (UTC)
Also comparing to this image the electric field is symmetric around the y-axis. I am really not sure wheather the sign change is correct or not, and would like to understand it.--Geoemyda (talk) 13:10, 5 November 2008 (UTC)
DipoleRadiation.gif

I have uploaded a dipole radiation to wiki commons. (A description of the file can be found there). I still believe the existing file is wrong, but I can not say this for sure, nor do I see where the sign change in Loodog mathematica file does come from, so I have not changed the image in the article. --Geoemyda (talk) 10:06, 11 November 2008 (UTC)

I am certain that at least the caption of dipole.gif is wrong. a) The Frequency, as displayed, is 1.0Hz not 0.16Hz. Easy to see for yourself: count the arrival of blue bands at any point on the picture-edge; arrivals are spaced 1 second apart. So, periodicity =1.0 second, and frequency = 1/period = 1.0 Hz. Difficult and pointless for a viewer to imagine that the plot represents phase in radians, even if the generating author wrote his simulation with that intent. The repetition frequency of the figure itself is 1 Hz. b) The dipole shown must be Horizontal, not Vertical, because Phase shown is opposite between left-and-right, but not up-and-down. Remember a dipole is about like tiny (+) and (-) charges aligned on the axis, close together compared to the size of one's picture of the field they project. So a vertical dipole oscillator would definitely have: - matching fields left-&-right (in fact, all around dipole's "equator"), - optional opposites up-&-down. c) After so much discussion, it's important to specify more precisely what aspect of the field was being displayed. There too many possible mathematical derivatives of the field that this plot might show, but Wikipedia deserves enough specificity for the rightness of this figure to be seen and acknowledged by somebody here. This plot can't be the magnitude of the electric field because, in a dipole, the electric fields don't have places of zero strength as shown here on the vertical axis. Perhaps this display plots the x-, y- or z- component of the 3-D electric field. It looks about right to represent: the electric potential of a Horizontal electric-dipole oscillation, the magnetic field component perpendicular to the page (of a Vertical E-dipole!), or the pressure field of a Horizontal sonic dipole. If so, "potential" should be distinguished from "field" or some such. d) perhaps somebody should generate a plot of a sonic dipole field, which has fewer field components to confuse the interpretation . It would represent an abstract dipole field more concisely.-- jimswen (talk) 18:52, 29 January 2009 (UTC)

PLEASE FIX. This image and its caption contain numerous errors. My grad students unfortunately come to Wikipedia first and then I have to clear up their misconceptions. And the misconceptions in the comments above only add to the confusion. Let me try to clear some of this up. (1) Ideal electric dipoles do not radiate along their axis, so the dipole in the image must be vertical. (2) Due to its symmetry, a vertical dipole (aligned on the z axis) emits radiation that is azimuthally symmetric. The right half of the picture should look identical to the left half no matter what property is being plotted. There is clearly an error in this image in this regard and it should be fixed to be symmetric (3) It is very difficult to visualize a vector field. The most effective way is to take the magnitude (overall strength) of the total electric field and plot it as a contour map or color-coded image, which I assume was done here. The lines in this image are NOT electric field lines but contour lines in the magnitude plot. This should be obvious from the fact that the fields are always transverse in the far-field, i.e. the electric field vectors always lie on a sphere centered on the dipole. This should be mentioned on the caption. —Preceding unsigned comment added by 71.174.28.9 (talk) 01:19, 26 March 2009 (UTC)

Okay, the code is here. It absolutely is 1hz. I was using omega *t as a variable instead of t, basically setting omega to 1, but the 2 Pi is already there, so I've essentially set f = 1hz.--Louiedog (talk) 03:57, 5 September 2009 (UTC)
I changed the caption in the article (to a less incorrect version) before inspecting the code: The far-field approximation has been used yielding incorrect results for the inner half of the plot. Please consider to delete the file. – Rainald62 (talk) 14:39, 9 October 2010 (UTC)

order of sections

I'd like to suggest moving Molecular, Quantum-Mechanical, and Atomic dipole sections down to the bottom. They're important, but more involved, than the original concept of a dipole. Logical evolution of the dipole idea would go something like 1) physical, ideal, approximate dipoles (as we have), 2) formulas for typical classical dipole fields (Magnetic, Electric, and maybe sonic), 3) important examples of classical dipoles such as molecular dipoles, 4) mathematical form of quantum dipoles (section as written), 5) atomic dipoles, an important dipole example which tends to depend on quantum aspects. In the present order, I'm trying to look up formulas for a classic dipole field, but have large sections of beyond-my-needs stuff coming first. I think more people would be served by remedying that. -- jimswen (talk) 20:20, 29 January 2009 (UTC)

Atomic dipole

Can someone please clarify what is an S-state atom? Is this the ground state of Hydrogen atom? Thanks! —老陳 (留言) 2009年5月19日 (二) 06:28 (UTC)

induced dipoles

Written that :

Induced dipoles: These can occur when one molecule with a permanent dipole repels another molecule's electrons.

It doesn't have to have a permanent dipole to gain an induced dipole, does it?? —Preceding unsigned comment added by Lifeisaserenade (talkcontribs) 13:18, 11 March 2010 (UTC)

Field from an electric dipole

The formula is not correct! It is not correct dimensionally, anyway. Correction is needed. —Preceding unsigned comment added by 84.3.105.214 (talk) 11:21, 7 January 2011 (UTC)

field of an magnetic dipole

The first eqaution in this section,

B= mu0 m / (4 pi r^3) x sqrt(1-cos^2)

ist nonsense, as the magnitude of the field of a dipole can of course not be zero anywhere. The orthogonal portions Bx,By have crosspoints, as does the interaction between two dipoles (at 54°). --129.13.72.198 (talk) 13:51, 4 February 2011 (UTC)

Etymology of the word

I suspect the current information on the origin of the word is wrong. The assumption seems to be the same flawed one often assumed for 'dielectric' and 'dialog'. I suspect it's actual etymology is Dipole as in 'along an axis' (dia- meaning across or through), not 'two axis' as is stated now. The current explanation would indicate something having two separate axis rather than two endpoints.Obbas (talk) 14:02, 20 February 2011 (UTC)

What about monopoles, quadrupoles, multipoles? The use of “pole” in electricity may originate from the rods of a galvanic cell. – Rainald62 (talk) 01:19, 4 November 2011 (UTC)
The etymology is correct. I have provided a citation for it. RockMagnetist (talk) 01:41, 4 November 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) 03:11, 3 August 2011 (UTC)

The discussion has been archived to here. RockMagnetist (talk) 15:52, 10 October 2011 (UTC)