Talk:Magnetic field/Archive 3

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Thanks for the quick response

I am still learning my ropes around the wiki and I appreciate your effort here. Science involves consensus and while a consensus of two isn't very good it beats a consensus of one by quite a bit. Personally, I think that a wiki like this should follow the model used for coding large software projects of having all articles peer reviewed before before committing. As long as you or someone else (and better yet a bunch of people) are willing to help, I will follow that model.

(By the way would it be better to edit your original response or to copy it as in here?)

• The second sentence of the article may be too early to bring up the fact that it takes a non-uniform B-field to exert a force on a magnetic dipole. How about something like "...magnetic field is a field that permeates space and which CAN exert a magnetic force on moving electric charges and magnetic dipoles"? or "...which can exert forces and torques on moving electric charges and magnetic dipoles"?

Point well taken. I don't think the concept is too complicated, but I agree it isn't necessary for the introduction and it would help with concept clutter in the beginning.

• B and H section: I like it. I don't think it's necessary to give the units of D. You should make it very, very clear that there's another B and H section later on in the article.

Personally, I don't think it is necessary to give units for anything, other than completeness. I thought I did make it clear that there was a more in depth B and H section later. (Is there something like main that I can use for that? I will look at it and make it clearer.

• Field lines: Your edits, which I generally like, seem to have had the (presumably unintentional) consequence of using the term "magnetic field lines" a couple of times before saying what they are. Is there a way to change the order or change the phrasing to fix that?

This is a problem, that I noticed and was not quite sure how to handle. I'll see what I can do about it. The problem was introduced when I rearranged the paragraphs. My reasoning was that I wanted to start with the familiar and build from there. Most people (or at least many) have seen magnetic field lines around a magnet. I still want to keep that philosophy.

• Pole labeling confusions: The terms "north pole" (and "south pole") are, as far as I know, much more common than either "N-pole, S-pole", or "North seeking pole, south seeking pole". Maybe you could say once near the beginning that it's helpful to think of the term "north pole" as shorthand for "north seeking pole", but once you establish that, maybe you should call it what most everyone calls it, north pole and south pole. Moreover, you may not have known this (I didn't until someone corrected a similar edit that I had made), but I think people who actually design magnets have precise definitions of "poles", such that a realistic magnet doesn't have a single north pole and single south pole, but rather a spatial distribution of each. So you shouldn't say things like "magnetic fields always enter a magnet at the S-pole and leave at the N-pole of a magnet"...rather they enter near each, usually. Likewise, you make it sound like the earth's magnetic-field south pole is just below the surface in Canada, when in fact it's probably deep in the earth's core. Finally, maybe it would make more sense to rename the section something like "field lines for a bar magnet", and then start by talking about how the field lines pass in and out of the magnet, and define north and south pole in terms of where they enter and exit, and then have a sub-subsection called "Earth's magnetic field and pole labeling confusions", where the north-vs-south issue could be mentioned. I think that might be a better organizational fit to the section, with material you've now added.

I will hunt and kill all N-Pole and S-pole references replacing them with north pole and south pole. Ditto for the near the north pole instead of at the north pole. (It seems rather picky to me, all you have to do is look at any field diagram to know that the poles are distributed rather than points. On the other hand the clutter of 'near the ' is small and may help avoid some ambiguity about what is meant by the north pole being at that location.) The rest of the suggestions seem reasonable to me as well.

I think we need an image here with Earth's magnetic field. That means I have to learn a new wiki skill.

I have been hesitant about renaming any sections just in case something is linked to any of them.

• "charged particles spiral along magnetic field lines"...I think "form a helix" is more unambiguous than "spiral". "Spiral" has other meanings.

Using the term helix seems too technical to me, but I agree it is more precise. I'll see what I can do.

• "Magnetic fields can do no work"...I know Griffiths says this (in boldface, in a big box, no less), but I haven't seen it in other textbooks and I think there are cases where it's arguably false, depending on definitions (e.g. an electron, with its spin magnetic moment, accelerates in a non-uniform magnetic field). I don't think this should be stated without some more research and references and examples and detailed discussion. You certainly can say that it doesn't do work on charged particles, though.

Hmm... I have run across that in more than one place. You are right in that Griffith's hammers it home. It isn't worth the fight, though. I'll replace it with your statement.

• Is there a reason that you took out the Fleming's left-hand-rule and its diagram? A google search confirms that it's in pretty widespread use. Certainly your made-up replacement ("F.I.B.") is inadequate, since there's no diagram indicating how you're supposed to orient your fingers.

This is the reason I am so happy that you looked at this. I have never run across a left hand rule before, ever. Every book I have uses a right-hand rule. I have always thought that that was the established convention. I have some recollection of the (F.I.B.) rule although I have never used it. I was trying to make a quick edit and it was too quick. You are right about needing a replacement image.

Do some physics textbooks use left-hand rules?

• When describing the Gilbert model, you say "could express this force in terms of the magnetic moments of the poles". I think "...in terms of the attractions and repulsions of the poles" would be clearer. After all, "magnetic moments" are exactly what you want to be using. Also, as I mentioned before, you come across as awfully judgmental against the Gilbert model, which as I understand it is a quite widespread and rigorously correct and convenient model, when used correctly. Anyway, maybe you could try phrasing it in a less judgmental way, along the lines of "you can calculate the magnetic attraction via the attractions and repulsions of the poles in the magnets, in an analogous way to attractions and repulsions of electrical changes. One should keep in mind, though, that this is a calculational tool, and not meant to be a literal model of what's going on inside a magnet. In fact, all the forces physically arise because the microscopic dipoles in one magnet respond to the non-uniform B field of the other magnet." I dunno, something like that.

I wasn't trying to go against the Gilbert model, I was disappointed that there was not an article that I could link to, for instance. The Gilbert model works very well. (This isn't terribly surprising considering that the similarity between p and m from the outside. It wouldn't surprise me if the Gilbert model was rigorously correct either, because of the duality symmetry of Maxwell's equations.) Oops, I messed up by talking about the poles having 'magnetic moments'.

• You should fix the wikilinks to Faraday's law...I think you mean the article Faraday's law of induction.

Wow, you are thorough

• In "electrical currents", you should link to the article Jefimenko's equations.

Ditto. I am surprised that the wiki has Jefimenko's equations.

• In "magnetic field of a steady current", you might consider taking out the equation and just describing how the field lines tend to circle around the currents, as in the figure. You could say, See Biot-Savart law or Ampere's circuital law or Maxwell's equations for the exact quantitative equations.

It is in the plan. I may put this off. It is my feeling that I did too much in my first revision. I don't want to do any more cutting this round if I can help it. My general philosophy for this article is to kill off as many equations as I can, though.

• Changing electric field: You have a typo in the first sentence.

Just one. I am surprised.

• "Maybe a better way of saying this is that a magnetic monopole is a property of a particle(s)." I don't think this is the place to be wondering about how best to refer to monopole-ness...everyone just says that the particle would be called a magnetic monopole. Also, you forgot to put "main article: Magnetic monopole".

You never know what this wiki has and what it doesn't. Thanks. for the link.

Anyway, I'm just one guy, and you're welcome to disagree with and ignore any of those suggestions. Also, I don't know if anyone else will offer feedback, you may want to sit on it a few more days before posting, at your discretion. Also, I tend to prefer big all-at-once changes to a zillion small ones, but maybe that's just my opinion.

Thanks for everything. If I am ambitious, I will knock most of this stuff off tonight. I definitely won't commit anything without fixing the r.h.r. mess. I still have to learn how to upload images as well.

One thing you didn't mention was how bad my H field section was. That will be a separate effort.

TStein (talk) 19:19, 19 May 2008 (UTC)

"I think we need an image here with Earth's magnetic field. That means I have to learn a new wiki skill."

Here, from the article "Dipole". I also don't know much about how to properly code and format pictures, but I do know how to copy-and-paste them from other articles. :-) --Steve (talk) 23:20, 19 May 2008 (UTC)

The Earth's magnetic field, which is approximately a magnetic dipole. However, the "N" and "S" (north and south) poles are labeled here geographically, which is the opposite of the convention for labeling the poles of a magnetic dipole moment.

An newer improved version of this article is up on my page

Here's the link to the page.

Ok, I am almost to the point where I am ready to commit my changes. I dealt with most of the issues that Steve raised. I left a minor issue or two for others to fix. I made some larger changes to the visualizing the magnetic field section. I also got a good start on the H field section. I am not as comfortable with the H field as I would like to be, so help would be appreciated there.

Other problems: I included 2 images as placeholder until better images can be found.

I did not like any of the right hand rule images because most of them had little to do with F = v×B. They mostly had to do with generating current when pulling on a wire. The signs are reversed for these 2 cases.

The second image was an attempt to sketch Earth's magnetic field including a 'magnet' in the interior. I thought it might help people to see approximately where Earth's magnetic field is generated and what is the polarity of that magnet. I found a lot of similar stuff on the web but all of them had the wrong pole of a magnet (N) under Earth's N magnetic field. Unfortunately my attempt had the field lines to rounded, which I could fix if I had the time. Worse, the arrow heads that show up on my svg viewer fx 2.0 are not showing up on wikipedia's viewer.

TStein (talk) 05:57, 23 May 2008 (UTC)

You've got B and H, how about links to magnetic permeability and magnetic constant? To Moving magnet and conductor problem? To Weber (unit)? Brews ohare (talk) 19:02, 23 May 2008 (UTC)

Intro

Incarnation seems to me a poor word choice. It means in the flesh, kind of like when a god becomes human. I'll think of a better word. Daniel.Cardenas (talk) 14:53, 23 May 2008 (UTC)

Hehe, I agree, an "incarnation of the magnetic field" makes me think of some scary-looking monster that a bored physics student doodled in his/her notebook. I actually may be to blame for putting that word in. Maybe "instances"? "Examples"? --Steve (talk) 22:28, 28 May 2008 (UTC)

"Instances" sounds good.  :-)   Daniel.Cardenas (talk) 00:00, 29 May 2008 (UTC)

Earth's magnetic field fadin'

I saw this fifty-minute NOVA documentary that addressed the problems our magnetic field has -- it's getting less and less powerful by the day. Some say it's just a normal 10,000-year cycle from which the source of magnetism will change direction. Basically, said in everyday language - "south" becomes "north" and vice versa. As the transicion will occur, it is expected a 10% increase of people with skin cancer. Now, does anybody think we should add this piece of information?

--96.232.60.47 (talk) 21:09, 28 May 2008 (UTC)

See geomagnetic reversal, and also Earth's magnetic field. Maybe this article could use some better links to those pages? After all, if you came to this page for information on the earth's magnetic field, probably other people do too, and those people should be able to find their way to the proper article, namely Earth's magnetic field. Perhaps the article could use an additional disambiguating note at the top? Or a section on Earth's magnetic field? --Steve (talk) 22:28, 28 May 2008 (UTC)

I pulled the trigger on the Editing changes I discussed earlier

There is still a lot of work that has to be done in cleaning this article up. There were some problems introduced, such as with the 3 figures I added. (Magnetic field with compasses, right hand rule and Earth's magnetic field.) Overall, though I think it is a large enough improvement over the previous case to justify the edit.

The main reason I am doing this, though, is to avoid the difficulties in keeping my branch in synch with the article. Plus I am hoping to get more eyes on this page.

Still needs:

• Cleanup of last sections (From History of B and H forward)
• Major fix of images (I am not the person to do this, I am afraid). The images should be cleaner and all in the same style, IMHO.
• Review of my H field section. (I don't use the H-field that much.
• Wikify some of contents (In particular the ordered lists) I tried to do that but failed.
• Outside grammatical review. (I have fixed a lot of the problems myself, but it is difficult to find my own mistakes.)

I am willing to push this article forward as far as I am able. Right now I need more eyes on the problem, though. Personally, I think this can and should be pushed to being a FA.

TStein (talk) 16:56, 2 June 2008 (UTC)

Magnetic field vs electromagnetic field

Pol098 has just put in a paragraph differentiating between the "magnetic field" and the "electromagnetic field", in particular saying that the latter is only associated with electromagnetic radiation. I don't think that's right. I think the "electromagnetic field" is just the electric field and the magnetic field put together into two words. A magnet does have an electromagnetic field: its electromagnetic field consists of zero electric field and whatever magnetic field.

I don't have a textbook on hand to confirm this, but one piece of evidence is that the so-called "electromagnetic field tensor" (which, one would think, should describe the electromagnetic field) includes any magnetic and electric fields, not just those that make up EM waves. In particular, the electromagnetic field tensor (and hence, I think, the electromagnetic field itself) is certainly not zero around a stationary magnet.

Or maybe I'm way off. What do other people think? Can anyone find reliable sources on this? --Steve (talk) 20:28, 29 August 2008 (UTC)

Removed his nonsense (for now): "electromagnetic fields are associated with electromagnetic radiation... an electromagnetic field decreases more slowly, with the square of the distance". Really? News to me. Last I checked, EM (far field) radiation goes as 1/r. Alfred Centauri (talk) 02:43, 30 August 2008 (UTC)

Failure at a basic level

I am trying to do a back-of-the-envelope calculation and navigated to this page for a definition of B and H fields. The page fails to cover these very fundamental in anything approaching clarity. The B field defintion is turgid and buried halfway down the page. The H field definition is more alluded to than buried. ~Paul V. Keller 16:25, 1 January 2009 (UTC)

Thanks for your input. I would love to fix these problems, but it is not as obvious to me exactly what they are. As far as I am aware there are no one well agreed upon definitions for the magnetic B field. The Lorentz force law F = q v x B is probably the closest thing to a definition. That definition is not very accessible to the average reader, though. (Not that there is not other things in here that need paring down as well.) What part of the article did you find as the most useful definition for you? Perhaps I can find a way to elevate it?

The definition of the H field is given in the first paragraph as B = u_o H + M (SI). There is a lot of technical things that can be added, but at a price of complexity.

Mostly I think what is needed is someone to go through with a hatchet. TStein (talk) 22:55, 16 January 2009 (UTC)

Finding consensus on Magnetic Field as relativistic portion of E-field

I have objections with the following statement as being misleading:

Magnetic field is a relativistic part of electric field. When electric field is seen by an observer moving with some velocity v (or vice versa) then Lorentz transformations of space and time from one reference frame to another result in the origin of cross product of electric field E and velocity of motion v. This cross product vxE is what we call magnetic field.

It is true that starting from a pure Electrostatic term one can arrive at the above statement. It is also true that all of Maxwell's equations can be derived from Coulomb's Law (with some difficulty). (One has to be careful, amongst many other things, not to also 'prove' that gravity follow Maxwell's equations as well.) The first problem I have is that, as far as I can tell, there is no reason to assume there is one reference system that is purely electrostatic for real systems of charges. A more important pedagogical reason is that there is no reason to favor the Electric field in this way. In relativity, boosting a pure electric field will result in an electric plus a magnetic field, true. It is also true, though, that boosting a pure magnetic field will result in a magnetic field plus an electric field!. There is no a priori reason to think that the electric field is any more fundamental then the magnetic field. Advanced theories all treat E and B on equal footing. (Together they form an anti-symetric 4-tensor).

All of this is covered above from the last time that this type of statement was placed in this article. The end result was that the sentence was altered to be more technically true from the above perspective. This process already been done at least twice and vestiges from this slow motion edit war can be found sprinkled through the article.

This article is already polluted enough. As an example of this see the section directly above this one on this discussion page. (Unfortunately, I myself have contributed a good portion of that pollution as I have strugled to learn how to write encyclopedia articles well.) I was hoping that we can reach a consensus so that we can stop the madness. TStein (talk) 15:44, 23 January 2009 (UTC)

I 100% agree with you, see my post on User Talk:Enormousdude. I support undoing that particular edit. --Steve (talk) 17:34, 23 January 2009 (UTC)

I removed it but I modified the last paragraph of the introduction to say that one could view the magnetic field as relativistic part of an electric field in another reference frame. Hopefully, this compromise will stick and that I kept faithful to the facts. TStein (talk) 22:31, 2 February 2009 (UTC)

B and H

There seems to be too much in this article about the difference between B and H. The two are related by the simple equation B =μH where μ is the magnetic permeability. B is the magnetic flux density which depends on the permeability of the medium. In Maxwell's books the permeability would be a measure of the density of magnetic field lines for a given magnetic field strength H.

I agree a little about there being too much about the difference. The distinction is often not that important. On the other hand, not understanding the difference between the two can bite you in the arse when you are least expecting it. One particular example of this is the confusion that results because of demagnetization of magnetic samples in a magnetic field. TStein (talk) 22:19, 30 March 2009 (UTC)

B is essentially a weighted version of H which seems to make it a more useful quantity. Maxwell tended to use the product μH. I can't think that there's much more that needs to be said about the inter-relationship between these two quantities. I might want to take everything out regarding B and H, and put this short paragraph in instead. David Tombe (talk) 20:56, 28 March 2009 (UTC)

First of all, to put it bluntly, Maxwell's interpretation of the relationship between B and H is completely wrong and irrelevant to the discussion. B is NOT a weighted version of H. There is a good reason why Electrodynamics textbooks like Griffith's spend sections of the textbooks explaining the difference. We can debate whether or not the difference between B and H deserves even the small section it is given near the end of this article. (Personally, I don't see the harm with the little attention it is given in the article. It would be better to emphasize B = mu H a little more IMHO.) But replacing it by the idea that B is a weighted version of H is unacceptable to me as a physicists. I can understand that engineers may feel differently but that is not what is actually happening. TStein (talk) 22:19, 30 March 2009 (UTC)

That works fine provided that μ is constant (and meaningful). In many situations it isn't, hence the distinction. Stannered (talk) 22:23, 28 March 2009 (UTC)

Yes, for example in a bar magnet H is precisely zero but B is huge. --Steve (talk) 02:09, 29 March 2009 (UTC)

Steve, In a bar magnet and all ferromagnetic materials, μ is huge. Therefore B will be huge even if H is small. I cannot believe that the magnetic field strength H is precisely zero inside a bar magnet. That wouldn't make any sense. There has to be a magnetic field inside a bar magnet. Who told you that there is no magnetic field H inside a bar magnet? David Tombe (talk) 17:16, 29 March 2009 (UTC)

I'm sorry you've never had a good education about ferromagnets. I recommend that you get a modern textbook about magnetism (or even electromagnetism), and read it. In particular, make sure to carefully read the description of ferromagnetic hysteresis loops (H is usually the x-axis). Sorry I don't have time to teach you this myself. Good luck! --Steve (talk) 17:46, 29 March 2009 (UTC)

Steve, There is still a magnetic H field present when hysteresis takes place in a bar magnet. Hysteresis is a topic similar in principle to when Hooke's law breaks down due to elastic deformation. You are quite entitled to write a section on hysteresis. My point was simply that B =μH is the general rule, as like Hooke's law, and that there is an awful lot of unnecessary discussion in the main article about the distinction between B and H. I explained that distinction. Why do you need to bring hysteresis into it? It's like having endless discussion about the difference in meaning between 'Force' and 'extension' in Hooke's law. Someone points out Hooke's law as F = -kx and then you come along and introduce cases where Hooke's law breaks down due to the elastic limit having been reached. David Tombe (talk) 23:15, 29 March 2009 (UTC)

I may be misunderstanding you, but it sounds to me like you're arguing that "force" and "extension" (aka stress and strain) are essentially the same thing, and it's not worth spending much time explaining the distinction between them. I hope you're not a civil engineer in real life.... :-) --Steve (talk) 01:11, 30 March 2009 (UTC)

Steve, with hubris like that (especially coming from a graduate student) coupled with the ignorance, your EM prof should flunk your ass. H is not zero inside a bar magnet and you're foolish to "teach" us that it is. 71.254.8.148 (talk) 02:59, 30 March 2009 (UTC)

You're right, I'm sorry, I should have been more specific. When I said "bar magnet", I should have said "a long, cylindrically-symmetric, uniformly-axially-magnetized permanent magnet". Inside such a magnet, the H-field is zero. (B=μ₀M and H=B/μ₀-M=0). That's the situation I had in mind when I said "bar magnet", but of course that wasn't clear.

I understand very well that H is not always zero inside permanent magnets with other shapes. For example, inside a spherical magnet with uniform magnetization, B and H are both nonzero and point in opposite directions (B=-2μ₀H=(2/3)μ₀M). Again, sorry. --Steve (talk) 05:09, 30 March 2009 (UTC)

Steve, I wasn't saying that force is the same as extension. I was making the analogy between 'Force' and H on the one hand, and between 'extension' and B on the other hand. It is by no means a perfect analogy. I was merely using a cause/effect analogy to make the point that in an encyclopaedia, one begins with the most basic general rule.

Hysteresis is a specialized topic, not fully understood, and which involves a certain amount of shielding and delayed reaction as between cause and effect.

In principle, B =μH as a general rule. There are far to many instances on the main article of people trying to learn this general rule on the job. It could be grossly simplified. Then if you want to write about Hysteresis, feel free to do so. David Tombe (talk) 21:28, 30 March 2009 (UTC)

Tstein, That's about the height of what I was saying. B =μH . That's pretty well all that needs to be mentioned in the article. The physical significance is no longer in modern textbooks. You disagree with Maxwell's physical explanation for it, but that's entirely your opinion. You expressed your opinion very boldly indeed. It's hard to believe that this guy Maxwell always seems to be so wrong when it comes to the issue of how he established the equations which are still hailed today as being one of the greatest triumphs of modern physics. David Tombe (talk) 11:18, 31 March 2009 (UTC)

Maxwell was a genius and his equations were one of the great revolutions in physics. That does not mean he was right about everything. (Newton set back optics by a hundred years by his failure to understand light as a wave even as he was the first to measure the wavelength of visible light.) Maxwell's ideas of the physical nature of B and H is in many cases flat out wrong. (One has to be careful here to mention that Maxwell's idea on that subject changed over his career.) That should not be surprising once you realize that he developed the equations with no idea of what an atom was or of quantum mechanics. That is not anyone's opinion. The interpretation given in the article is correct. It is for the most part straight out of any physics textbook on electricity and magnetism. I am sorry if that seems rude and I wish I had the skills to be more diplomatic about it.

More importantly, you still haven't answered (at least to my satisfaction) Steve's objection to your Hook's Law analogy. (If I may paraphrase the above argument. The analogy was nobody goes into as much detail to distinguish the quantities (F and x) of Hook's Law F = kx as what this article does to distinguish B and H (B = mu H). Further B = mu H is only invalid for a special case of hysteresis and therefore should not be dealt with in a basic article like this one.) Steves response can be 'simplified' into the following (with my own spin as well):

1. F and x do not need to be distinguished since they are obviously different quantities that everyone understands. The exact opposite is true for B and H. Few people understand why we need both a B and an H and what the difference is between them. If there is no difference then why do we need a relationship between them?
2. B = mu H is invalid for not just hysteresis but for the quite common case of any permanent magnet.
3. Even when B = mu H is valid, there are many cases where people's belief about the value and nature of H is completely wrong. One prominent example of this is a magnetic material placed inside of a uniform magnetic H field = Ho. If you assume that H remains the same because the current 'creating' it remains the same and use B = muHo you will arrive at extremely wrong answers. Your only recourse is to rely on a confusing fiction called 'demagnetization'. (Why would a magnet demagnetize?) The correct physical picture only comes into focus when it is realized that the magnetization also contribute to H.

That being said, I am open to greatly simplifying the article (despite the fact it will erase a lot of my hard work.) The article as it stands is probably too complicated for an introductory article. (To be fair though, the aforementioned Hook's law is much worse in many ways than this article.) What I am hoping from you though is to answer the above objections as well as:

1. Why does this bother you so much when it is buried near the end of the article where such complications belong? (Articles have to serve many audiences not just the lowest common denominator.)

In the meantime, I will work toward the goal of simplifying the relationship and emphasizing B = mu H a little more. TStein (talk) 14:12, 31 March 2009 (UTC)

First attempt to address spirit of complaint

In order to make at least a first stab to address the problems of the relationship between B and H, I made a number of small changes that I hope are in the right direction. (Although, I think the net effect is to increase the size of the article :( ). First of all I added the linear relation B = mu H to a number of different locations that should have already have it. I tried to emphasize the importance of that equation even while stating directly what the problems were. Second I tried to clarify the difference between B and H and hopefully made it a little more evident why this is important.

The second step is far from complete. I also realize that there is a difference in how physicists (like myself) approach this then how engineers approach it. The B vs H section is written from the physicists perspective. I have tried to respect the engineer's approach by making this section as short and as clear as I can (at this point) while keeping the fundamentals that are necessary from the physicist's point of view. There is still work to be done there, though I think that it is much better from my point of view. TStein (talk) 16:17, 31 March 2009 (UTC)

Tstein, in the main article, in the section entitled "The History of B and H", you have spoken highly of Poisson's approach. Poisson's approach was basically the same approach that Maxwell later used. You have spoken of the merits of this approach, and then summarily dismissed it without putting anything in its place, or explaining why they were wrong in your opinion. It is my opinion that Poisson was correct to make the analogy that H is to E, where B is to D. In fact the relationship E = (1/ε)D is in turn even more similar in principle to Hooke's law, F = -kx.

As a side note an even better analogy to Hooke's Law is E = 1/xi P. One can 'derive' this relation directly from Hooke's Law. D may be thought of as an effective polarization that includes an effective polarization of the vacuum (Maxwell would say aether) = epsilon_o E in addition to the polarization P of the material. TStein (talk) 04:22, 1 April 2009 (UTC)

Basically the textbooks have abandoned this 19th century explanation and put nothing in its place. There is no existing official explanation for the distinction between B and H, or for the meaning of the magnetic permeability μ. Maxwell explained μ in terms of the density of the magnetic lines of force. But if we are not allowed to use Maxwell's explanations for these phenomena, then we have got nothing else, and so the matter will have to be left unexplained.

I feel your pain here. It drove me nuts as an undergraduate that the physical nature of neither H nor D was explained. There is an official explanation. H is merely a useful quantity with no real physical meaning that is defined from B = u_o (H + M). H helps to simplify equations, but has no meaning beyond that. Similarly magnetic permeability has no real meaning. The quantity that has a physical meaning is the magnetic susceptibility which represents a linear relationship between B and M. It is not very satisfying I know. This interpretation is well supported both by the absence of an aether and by the extreme success of theoretical models based on B as the fundamental field. No explanation is given in textbooks not because there is none but because that explanation involves a mixture of quantum mechanics and special relativity that is difficult to explain qualitatively. TStein (talk) 04:22, 1 April 2009 (UTC)

There is absolutely no explanation in the article for the difference between B and H, and neither can there be any explanation based on what it says in modern textbooks, because the explanation has been purged from these textbooks. That is the source of all the confusion. People are asking why, but the answer lies in the works of 19th century pioneers that are now forbidden. David Tombe (talk) 18:33, 31 March 2009 (UTC)

The works are not forbidden, just outdated. Personally, I would purge H and D altogether as unnecessary anachronisms from a time where people believed in aether. The equations will get messier, but they would be more understandable. TStein (talk) 04:22, 1 April 2009 (UTC)

Does B = μ₀(H + M) not explain this just fine? Stannered (talk) 21:45, 31 March 2009 (UTC)

Tstein, you're learning. The next stage is to see if you can explain displacement current in a way that fits with the EM wave equation, without using the aether.

Meanwhile, I think we are stuck here with no choice but to list B = μH as an accepted fact of which the explanation which existed in the 19th century is now denied. There is no existing explanation, and that is what has caused all the confusion on the main article. Relativity cannot explain μ. The equation which Stannered mentioned above is merely an extension of the basic equation. David Tombe (talk) 13:16, 1 April 2009 (UTC)

David, the title of your last edit got me thinking of a way that I might be able to explain things better. The title said in effect that M is an extension of H. In reality, that is exactly reversed. The two fundamental fields of magnetism are B and M. B is defined by the Lorentz Force Law or whatever suitable definition you want to use. (Whether in or near a magnetic material or not it is the B field that pushes stuff around.) M is defined as the density of magnetic dipoles in that material. (In other words, M tells us the state of the magnetic material.)

Much like Hooke's law, B = mu_o*chi*M represents a natural law between two distinct and physically meaningful quantities. In Hooke's law one quantity represents the 'push' on the material, the other quantity represents what happens to that material. Same with B and M. (Like Hooke's law as well where strain can exist without stress, the relationship NEED NOT be linear and often is not.) The same is not true for B = mu H. Very few people understand the difference between B and H. Even fewer people have any physical understanding of what H is. Further H is not just a property of the material but it is a (an unholy, IMO) mixture of the field that is pushing stuff around with the responce of the material to that field.

Maxwell's equations in matter can easily be written in terms of B and M only without any loss of information. Indeed, H is not needed at all in any equation! The same is NOT true for writing the equation in terms of B and H. There are simple cases where M is not needed, but in general this is not the case. TStein (talk) 05:20, 13 April 2009 (UTC)

Tstein, I have no quarrel with the existing equations. But from what I can see, the original meaning of the relationship B = μH was lost when Maxwell's vortex sea was abandoned. Nevertheless, I agree that B is by far the more useful quantity, and indeed in most of Maxwell's writings, he uses the product μH. From what I can see, M is to atomic and molecular matter, what H is to the abandoned vortex sea. And becuase the vortex sea has been abandoned, nobody knows how to explain the meaning of H. All they know is that it fits into the maths.

For Maxwell the quantity corresponding to H was more accurately the circumferential speed of one of his molecular vortices. This circumferential speed is what contributed to the pressure between adjacent magnetic lines of force, and hence caused magnetic repulsion. The vortices were aligned solenoidally along their rotation axis, and as you know, the magnetic field lines between like poles spread outwards in the space between the poles, and come together laterally. Maxwell ascribed the magnetic repulsive force to centrifugal force in the equatorial plane of the vortices. The circumferential speed of the vortices was a measure of their vorticity, and so H can be seen as a measure of the vorticity of a single magnetic line of force. With the μ term added, we then obtained a kind of magnetic flux density B, because μ was considered to be related to the density of the vortex sea. Hence B is magnetic flux density and it is equal to μH, where H is a measure of vorticity or magnetic field strength. The modern definition of H is purely mathematical, and so it will be very hard to ascribe a physical significance to it in the absence of Maxwell's vortex sea. David Tombe (talk) 19:08, 13 April 2009 (UTC)

Reorganizing Article

Since this article has finally gotten some of the attention it deserves with more then one active editor I thought it prudent to discuss possible large scale changes to avoid butting heads.

Issue 1: large scale reorganization. A while back I reorganized this article to enumerate the field sources together and then the elementary effects together. This brings me to Brews Ohare's edit to magnetic dipoles. At first I was mildly annoyed that Brews Ohare included information about how magnetic field affected the dipole with a torque in the dipole as sources of B field section. Then I realized that splitting up the dipole interaction into 2 sections one for creation of the field and the other for interaction may have hurt the article. I liked what Brews Ohare did there. Maybe it is more important to organize both dipole sections together (creation of B and interaction with B) and do the same for both magnetic field and currents sections, etc.

Issue 2: In my opinion we need to keep permanent magnet section near the very top and separate from magnetic dipoles. My main reason for this is that all articles have to start with the simple and familiar before going to the more complicated and unfamiliar.

Issue 3: In my opinion we need to keep the magnetic field lines and visualizing them section near the very top just after the discussion of the permanent magnets (including compasses). In particular the section on magnetic fields alwas comes in loops sets up a lot of the understanding that is needed for later. Plus people should be fairly familiar with seeing field lines.

Issue 4: Consistency with the level of complication. What level of detail are we looking for here? Right now we have the Landau-Lifshitz-Gilber equation but not the Biot-Sarvart Law nor Maxwell's equation. If we included every equation at the level of L-L-G would we have enough room for the article to be qualitative? In my opinion an article like magnetic field needs to be simple with links to the more in depth stuff. Alternatively every section could start off simple then work toward the complex. Having the article with a repeating patern of easy to complex, easy to complex sounds like a way to lose everyone, though. Then again I don't claim to be an expert in that matter.

Issue 5: When to use magnetic field and when to use magnetic B-field is a question that Brews Ohare brought to my attention by his edits. After some thought I am convinced that we should use magnetic field (implying both B and H) for any source of magnetic fields, BUT I think we should use the magnetic-B field for any affect. The magnetic-H field is largely a theoretical construct that has no effect on anything. It cannot be directly measured typically. It is the magnetic B-field that pushes stuff around.

Proposal: We replace elementary sources of B and elementary effects of B by something like:

The magnetic field and permanent magnets

Permanent Magnets as sources of magnetic fields

Affects of the magnetic B field on permanent magnets

Visualizing the magnetic field

Magnetic B-field lines

Magnetic B-field always form closed loops

Magnetic Monopoles

The magnetic field and moving charges

Moving charges and currents as sources of magnetic field

Affect of magnetic B-field on moving charges and currents

The magnetic field and magnetic dipoles

Magnetic dipoles as sources of magnetic field

Affect of magnetic B-field on magnetic dipoles

Magnetic torque on a dipole due to a B-field

Magnetic force on a dipole due to a non-uniform B-field

Electrodynamics: The interaction of the magnetic and electric fields

Changing electric field creates a magnetic field

Changing magnetic field creates an electric field

I hope this gives a good indication of my thoughts at least so that we can come up with a good solution and really make this article shine. TStein (talk) 21:25, 3 April 2009 (UTC)

The quandary of treating sources and then effects is not satisfactorily resolved. The earlier organization with separate treatment of "sources" separately from "effects" worked better. The magnetic dipole is a bit of a quandary, as you point out, because the gyromagnetic ratio is best understood in terms of "effect", and is indispensable in understanding the fundamentals of magnetic dipoles. Although the combination of the two topics under "sources" breaks the logic of the separation, maybe both topics can be retained under "magnetic dipole" by making "magnetic dipole" the last of the sources and using this section as a segue to the section on "effects"? I think that would work best. Brews ohare (talk) 14:48, 7 April 2009 (UTC)

To be honest, I could probably be pushed either way when it comes to the reorganization. The major reason I pushed the button on it, though, was that I think it will allow for a lot of removal of duplicate material. The third reason is more stylistic. I hate lists of stuff in articles. It is boring to read and interrupts the flow. As a reader I don't want to be told something; I want to be showed something. It was a pleasure to get rid of the sections saying 'There are four ...'. The reorganization will also allow us to start from the easy and familiar and build toward the hard and unfamiliar. It is more of a spiral approach. TStein (talk) 15:49, 7 April 2009 (UTC)

Or, maybe the introduction should include a sketch of the article's organization referring to specific subsections, which seems to be Wiki policy regarding introductions anyway? Brews ohare (talk) 14:56, 7 April 2009 (UTC)

The introduction needs a major rewrite no matter what we do. Overall the reorganization will be more work, but I think it will be worth it. TStein (talk) 15:49, 7 April 2009 (UTC)

Regarding Issue 5; isn't usage of B and H well enough described in footnote 1? Does the Purcell-Gerloch discussion agree with yours? Brews ohare (talk) 14:59, 7 April 2009 (UTC)

I agree with the spirit of Purcell and Gerloch in the first foot note. On the other hand, currents and magnets are sources not only of the magnetic field (B) but also of the H field. We need a term to describe both fields together I think. Further, there is so much confusion in the literature, that a little redundancy would help hammer home when the magnetic field is a magnetic B-field. In any case, eventually we need to settle on what to call it and apply that through out the article. What do you think of 'magnetic B-field', 'H-field', and 'magnetic field' for when it could be either. Of the textbooks I have Griffiths uses magnetic field B or plain B and H, Jackson uses 'B', 'magnetic field B', 'H' and 'magnetic field quantity H', for whatever that is worth. TStein (talk) 15:49, 7 April 2009 (UTC)

It seems the article often used the constructions "magnetic B-field" and "magnetic H-field"; I have tried to eliminate the word "magnetic" in these combinations, as it seems to contribute nothing. Brews ohare (talk) 17:18, 7 April 2009 (UTC)

The History of B and H

Are we then going to leave it that Poisson's great approach relating to B and H, which worked so well, only worked by coincidence even though it was wrong? That's what it says in the main article. What about wording it more neutrally, something along the lines of, 'despite the success of Poisson's approach, it is no longer accepted in modern physics, and no alternative approach has yet taken it's place?'.

As regards Maxwell, there are a few inaccuracies in the article. You will have a hard job finding any references to electric charge in Maxwell's 1861 paper. The closest that he comes to it is with 'density of free electricity'. Maxwell tends to work in terms of force per unit volume, with the volume term incorporated into a kind of density term. The density term corresponds reasonably closely with the modern day charge to mass ratio, and so his electromotive force terms tend to correspond to 'electric field'.David Tombe (talk) 12:29, 17 April 2009 (UTC)

The tone for Poisson's approach definitely needs to be toned down a little. Tone it down if you wish. It is completely wrong, though, to say that there is no alternate approach that has taken its place. The modern approach is more involved and not nearly as easy to explain as Poisson's theory. I tried to outline some of it in the history and throughout the article.

I am not near an expert on Mawell's papers. Feel free to correct it. My only concern is that the history be useful for understanding E&M today. Physics history in my opinion is not as much about the past as it is about present understanding. TStein (talk) 19:32, 17 April 2009 (UTC)

I'll try. But from what I can see, the modern explanation is nothing more than a mathematical definition. I could probably fix up the bits about what Maxwell said and then leave it for somebody else to do the rest. David Tombe (talk) 14:12, 18 April 2009 (UTC)

Presently, the history section reads too objectively, I think. Rather than, "this approach was wrong," we should suggest that modern science offers a more complete alternative theory. The current theory appears to be valid, but we certainly don't know that it's "correct." --Jeff Wheeler (talk) 03:56, 23 October 2009 (UTC)

It's not that it's too "objective", it's that it's too "judgmental". Sounds like we're spitting on Poisson's grave! :-) --Steve (talk) 05:38, 23 October 2009 (UTC)

Faraday's law

This topic is a delicate one, as reading the talk pages at Faraday's law of induction will illustrate. The main problem is that Faraday's law in terms of flux density covers both motional and transformer EMFs, and of course both have enormous practical use in motors and generators. However, the Maxwell relation using curl E that is one of Maxwell's equations does not include motional EMF, and so is not equivalent to the first law, although often referred to as Faraday's law nonetheless.

Including the motional EMF makes for a a complicated equation. One has several options here: (i) put in the gory details (ii) leave out all the equations and stick to words (iii) keep the flux density equation and dump the partial DE using curl (iv) try to cover the matter at several levels by putting in enough verbiage to cover things for the casual reader, but retaining the mathematical expression.

Further revisions of the section Magnetic_field#Electric_force_due_to_a_changing_B-field may be advisable, but it requires some care Brews ohare (talk) 06:25, 3 May 2009 (UTC)

The way I see it, we can either deal with every petty 'controversy' about naming conventions or we can write a great article. I will do both of course when I can. But, I won't sacrifice a well written article for some foolish debate about semantics.

You seem to be saying that the flux law is the one that should be called Faraday's Law, while Griffith's is very clear that only the curl equation is to be called Faraday's law (page 303). The argument is pointless semantics, though, for two reasons. First, from Faraday's point of view, they are both Faraday's laws since he developed both together and saw them both as a consequence of one field, the vector potential A. (He did not use that term of course, but he envisioned it similar to a stored momentum.) Second, the distinction exists only from the point of view of E and B when relativity is not accounted for. From the perspective of the potentials they are both the same equation. For by using A and taking the total time derivative the convective part accounts for the motional EMF while the partial time derivative portion accounts for time varying magnetic field part. It is only because of undergraduate E&Ms obsession with the force fields that we have this 'controversy' at all.

This article is way too long as it is and yet there is still a lot of stuff missing. There is no section about the vector potential and no real discussion about the difference between magnetostatics and magnetodynamics; the history section while too long already is woefully incomplete; Ampere's law is given short shrift, Maxwell's equations are not listed fully, and Maxwell's equations in matter is left out entirely; there is no real discussion about what the H field looks like in a magnet; the Biot-Savart law is not spelled out (that was my fault); the physical meaning of the curl and divergence (and gradient) is not made clear enough for such an introductory article. The list goes on and on.

Worse, All kinds of fields and quantities such as (D, E, rho, H_eff, A, m, various integrals, etc.) are appearing out of thin air with absolutely no explanation. All of it is to satisfy people who already understand the material already and don't need to read the article at all.

I apologize if it appears that I am dumping on you here. I appreciate your effort, even though at times we have completely different goals.

What I suggest we do is get rid of the middle stuff you added and add a footnote if you so desire explaining there is a 'controversy'. Otherwise we are going to need to add paragraphs explaining what the vector potential is and the full lorentz force and all kinds of other unneccesary things just to explain what you added. All of that stuff belongs in the Faraday's law article of course; we just don't have enough room in magnetic field. TStein (talk) 04:27, 4 May 2009 (UTC)

There is no "controversy" here. The distinction between motional and transformer EMF is real and well-documented. It prompted some of EInstein's thought about relativity and formed the preamble to one of his famous papers. It has been remarked upon by Feynman (see the Faraday's law of induction article). It has very noticeable practical consequences.

Two usages of "Faraday's law" are in the literature. This kind of multiple usage occurs all through physics, and often seems to lead to sides being taken about who is "right". The real point is that Faraday's law of induction is a significant topic and covers two phenomena while the Maxwell-Faraday equation covers one of the two. So what do you want to do? I'd say cover both.

I think your statement : "The way I see it, we can either deal with every petty 'controversy' about naming conventions or we can write a great article." is wide of the mark. Brews ohare (talk) 21:43, 4 May 2009 (UTC)

I have attempted to simplify this discussion using some of your suggestions. Brews ohare (talk) 22:52, 4 May 2009 (UTC)

Article length

This article seems rather long. Suggest shortening. For example history of B and H to be moved mostly to sub article. Other sections towards the bottom of the article that already have subarticles should be shortened to a paragraph. What do you think? Daniel.Cardenas (talk) 04:57, 28 May 2009 (UTC)

Hack away Daniel. The worst that will happen is that one of us will revert it ;). I expanded the history section to try and elaborate why there is so much confusion about B and H. It probably is big enough to place on its own page, though. (Good luck coming up with a summary for this page that every one can agree on.) The main reason I have not broken the history section out is that it has a good chance of being ignored afterwards. It will get more attention here.

There are a lot of other details in this article that you can take a hatchet to as well. Some sections are more detailed then the main articles they are trying to summarize as well (such as the discussion about dipoles); these details need to be summarized and moved to their appropriate articles, IMO. Even sub articles often deserve 2 paragraphs of summary, though. TStein (talk) 08:16, 28 May 2009 (UTC)

I am debating on how to move the history of B vs H section out. There already is a History of electromagnetism which is too qualitative and general for this I think. Perhaps I could start a History of electromagnetic field or History of electromagnetic theory (History of magnetic field seems to small of a topic to me.) The main problem is that the physics history section is a mess and I would almost have to fix all kinds of other problems before I created this page so that I can link it properly to other articles. Ideas and help would be appreciated. TStein (talk) 17:40, 17 June 2009 (UTC)

Changing subjects (Orders of magnitude)

I think I saw on wikipedia a comparison of different magnetic field strengths. Does someone know where that is at? Should we put it in this article? Thx, Daniel.Cardenas (talk) 04:57, 28 May 2009 (UTC)

O.K. found it here: Orders of magnitude (magnetic field). Might be good if we make it more prominent in the article. The common folk would like to know things like the strength of earths magnetic field which isn't in this article or the earth magnetic field article. Daniel.Cardenas (talk) 05:01, 28 May 2009

(UTC)

 Done Check it out. I am a little worried that it should go someplace more obvious and up front. On the other hand I like keeping the introduction as simple as possible. Let me know what you think. TStein (talk) 17:27, 17 June 2009 (UTC)

Differential form of the magnetic field

Why isn't there a differential form of the magnetic field such as:

$d{\vec {H}}={\frac {Id{\vec {l}}\times {\vec {a_{r}}}}{4\pi R^{2}}}$

For a line current? I think those let you see very easily the connection to the rotational form in Maxwell's equations. —Preceding unsigned comment added by 169.198.254.6 (talk) 14:38, 12 June 2009 (UTC)

 Done I am not quite sure what you had in mind. I figured at the least, though Biot-Savart law should be included explicitly. I still don't see the obvious connection to the rotational form in Maxwell's equations. (The Biot-Savart law is the solution of both Ampere's law and divB = 0 with the boundary conditions that I goes to zero at infinity faster then 1/r^2, but that does not seem obvious to me.) If you could explain it then it might be very useful. TStein (talk) 17:34, 17 June 2009 (UTC)

You're probably thinking of the Biot-Savart law, which is discussed and linked a lot in this article, albeit without writing down the formula. --Steve (talk) 17:23, 12 June 2009 (UTC)

As Steve mentioned this is Biot-Savart law, but in a notation that I am not that familiar with. (In particular using a_r for a unit vector in the R direction seems strange.) It is also misleading in the differential form since the Biot-Savart law is only technically correct for the line integral over the whole loop even for a steady current. See page 119 and 438 in Griffith's E&M or the general equation for the magnetic field of a point charge where there is an extra term even when a = 0. (I don't mind sacrificing TC for a reasonable pedagogical gain, but I don't see that gain here.)

It is not obvious to me how this is easily connected to the 'rotational form' in Maxwell's equations. (I am assuming you mean Ampere's law by the term 'rotational form in Mawell's equations'.) To me at this point it is only a useful equation for calculating the magnetic field and not one that tells me anything particularly useful about B that cannot be stated straight-out better. That could easily be due to a lack of imagination on my part and I am looking forward to you enlightening me. TStein (talk) 18:54, 12 June 2009 (UTC)

Definition of B

I removed the following because I could find nothing to support it:

The magnitude of B is defined (in SI units) in terms of the voltage induced per unit area on a current carrying loop in a uniform magnetic field normal to the loop when the magnetic field is reduced to zero in a unit amount of time.

The best I could find is that (conceptually) the Weber is defined by the flux law, while Tesla is defined conceptually as a Weber/square meter. In practice, the unit of Tesla is typically determined using properties that are way beyond the scope of the article. The B fields has many potential definitions based on its effects and it seem superfluous to chose one that is not even used in practice to set the standard for how to measure the unit.

Are there any 'official' definitions of B or H out there? TStein (talk) 21:36, 12 June 2009 (UTC)

Offer something to the lay person, delay technical details

The first thing the reader is told is that a magnetic field is a vector field. Not only is this meaningless to most readers but a) appears to avoid the task of defining the topic and b) sounds a lot like mistaking the map for the territory. Some of you may be so immersed in the theory that you have difficulty explaining the phenomena in plain language. This is not anywhere near my area of expertise but how about something like this: "Magnetic fields are a cloud of forces that surround magnetic materials and electric currents. These forces can be detected by their interaction with the magnetic fields of other magnetic materials and electric currents."? Jojalozzo (talk) 19:52, 8 July 2009 (UTC)

The enterprising lay reader who can get past the first sentence, will find "dipole" introduced in the second without any explanation, not even a responsibility-avoiding link. The only people who are likely to understand this introduction already understand enough to skip it. This article would be much improved if the intro gave non-experts a good idea of what is known without them having to refer to other articles or reference materials. Jojalozzo (talk) 20:08, 8 July 2009 (UTC)

Suggest you start tossing out some ideas for intro sentence(s). Daniel.Cardenas (talk) 22:25, 8 July 2009 (UTC)

You didn't like: "Magnetic fields are a cloud of forces that surround magnetic materials and electric currents. These forces can be detected by their interaction with the magnetic fields of other magnetic materials and electric currents."? Or are you asking for more? :-) Jojalozzo (talk) 22:48, 8 July 2009 (UTC)

How about: "Magnetic field is the term used in physics for the invisible forces in the space around a magnet which pull on iron and other magnets. It is a vector field which surrounds magnets and electric currents . . ." --Chetvorno^TALK 01:38, 9 July 2009 (UTC)

maybe "attracts or repels" instead of "pulls"? what about electric currents? finally, as I understand it, a vector field is a mathematical construct not a physical reality, so we use vector fields to describe magnetic fields but a magnetic field is not itself a vector field. Jojalozzo (talk) 02:27, 9 July 2009 (UTC)

My two cents: Don't forget that there are separate articles for magnet, magnetism, and magnetic field. Someone who doesn't know the first thing about magnetism should presumably be reading the magnetism article, not this one. "Magnetic field" is a technical aspect of magnetism (OK, not that technical, but still more technical than magnetism in general). So maybe put a note at the top:

For a general introduction to magnetism, see the article Magnetism.

Of course, this is in addition to having an accessible introduction to this article...I'm glad you're having that conversation. Unfortunately, the proposals so far have used the term "force" in a way that's inconsistent with how it's used in physics. The first sentence has to be both accessible to everyone and unobjectionable to nit-picking physicist readers. Is there a better word than "force"? I can't come up with any at the moment. Maybe it would work by putting "force" in scare-quotes, or using the phrase "loosely speaking" somewhere? I don't know. --Steve (talk) 02:47, 9 July 2009 (UTC)

How about "A magnetic field is the sum total of magnetic forces surrounding a magnetic object or electrical current. These forces can be detected by their interaction with the magnetic fields of other magnetic materials and electric currents." Jojalozzo (talk) 04:03, 9 July 2009 (UTC)

Jajalozzo, I prefer "is a vector field" to "is described by a vector field". It's like saying "my height is 1.7m" versus "my height is described by 1.7m". 1.7 is a number not a physical reality, right? But 1.7m is a physical height nonetheless! :-) --Steve (talk) 02:52, 9 July 2009 (UTC)

I agree --Chetvorno^TALK 03:52, 9 July 2009 (UTC)

1) The analogy with "my height is" seems weak since you are not trying to define your height. Saying a magnetic field is a vector field seems more like starting out an article about Steve with "Steve is a hominid" rather than something like "Steve is a notable Wikipedia editor who has been active since it's early days." 2) I'm not asking for the sentence to say a magnetic field is described by a vector field. I am saying that defining a magnetic field as a vector field is unhelpful. A vector field is a mathematical construct that can be used to describe a many things in addition to magnetic fields. The first sentence of the article is attempting to give a succinct definition of the topic, but instead sends the reader to another, more general article. Jojalozzo (talk) 04:03, 9 July 2009 (UTC)

I would love to make the first sentences more accessible and those terms used too early bothers me as well, but I have not seen a better alternative. One thing we have to keep in mind is that the lede section is not an introduction. The lede section has to do several (seemingly mutually exclusive) things:

• Be short
• Summarize what will be covered in article and what not
• Be accessible
• Disambiguate between other articles
• Entice reader to read on
• Be at least reasonably technically correct.

We should always strive for the third, but we have to be careful not to sacrifice too much of the first and the last. Using technical terms in the lede does not bother me too much provided that they are explained in the article and that they are directly related to the subject of the article. The term vector field fits both of these dipole probably does not.

All of the above examples, so far, have failed to be reasonably technically correct; in particular, they grossly misuse the term force. Just as important, the term vector field is so intimately tied with the concept of magnetic field that it demands to be included in the lede, IMO.

That being said, the first couple sentences are still the least of the problems that this article still has. It is still too bloated in spots and needs the H-field and magnetization section reorganized somehow, IMHO. I would love to also add a section on the H-field of a magnet, with comparisons between the H and the B fields shapes. I think it is good that we are working on improving the first sentences, but we need to do the same for the rest of the article.

TStein (talk) 06:43, 9 July 2009 (UTC)

The lede for Electromagnetic field: "The electromagnetic field is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field." Vector fields do not arise in that article until well into it, in a section entitled "Mathematical description."

Would the analogous lede here be: "A magnetic field is a physical field produced by magnets and electric currents and is detected by the force it exerts on moving electric charges and magnetic materials."?

If the consensus is that the concept of a vector field must play a part in the lede so be it, but I don't see that it contributes much at that point in the article or that it is required as prelude to what comes after. Likewise I do not think that either dipoles or energy density are critical to any of the TStein's helpful list of lede purposes. Jojalozzo (talk) 23:43, 9 July 2009 (UTC)

My personal feeling is that 'vector field' is needed and that replacing the term 'vector field' with 'physical field' is silly. The term 'vector field' may be gibberish to 'laymen', but the term 'physical field' is gibberish to both the layman and the physicist. The only difference is that it is easier for the 'layman' to gloss over 'physical field'.

I saw a sentence in another article, magnetism I do believe, that describes the vector field nature of the magnetic field quite simply and in relatively few words. I debated replacing the sentence in the lede. I did not at the time because I wanted to keep the lede focused on magnetic field and not other issues and because I was lazy. This may be a solution to this problem.

You will get no argument from me about removing the term dipole; although, I think that the effect of a magnetic field causing magnets to rotate needs to be in the lede. Perhaps you could replace 'dipole' with 'a small magnet'. This will come at a cost of ambiguity: what is small? how do we define the direction that a small magnet points? What will probably happen after you do this is that someone will fix this ambiguity by adding additional sentences. Then someone else will replace all of those sentences back with dipole leaving us back where we started. That is the only reason I haven't replaced it.

I am partial to including the term energy density. I can't imagine the term being that scary to a layman. Most people know both what energy and density means. It is not a major point, though; it could easily be removed if it bothers you that much. TStein (talk) 04:36, 10 July 2009 (UTC)

"Vector field" has to be in the lede; this is a technical subject and "vector field" is part of its definition. I suggest adding one sentence at the beginning to provide a nontechnical description, and leave the rest of the lede as it is. The nontechnical description is not so important. No prose description of a magnetic field comprehensible to nontechnical readers will have the rigor to satisfy technical readers, but that's ok; they will simply skip over it to the mathematical definition. TStein is right, we need to get on to the more important problems in the body of the article. --Chetvorno^TALK 05:23, 10 July 2009 (UTC)

Decided to be bold and start the process. Added sentence to lede. --Chetvorno^TALK 05:43, 10 July 2009 (UTC)

Not to complicate matters (or contradict myself), but the magnetic field is in truth a field (physics), and only in the classical physics approximation is it a vector field. (More accurately, it's a quantum field.) :-)

I'm very fond of the above suggestion, "A magnetic field is a physical field produced by magnets and electric currents and is detected by the force it exerts on moving electric charges and magnetic materials." An inaccurate use of the word "force" is to be avoided if possible, except as a last resort within scare-quotes. :-) --Steve (talk) 07:10, 11 July 2009 (UTC)

Your sentence is already in the lede, except that it has 'vector field' instead of 'physical field'. I think vector field is necessary to distinguish it from scalar fields like the electrostatic potential and tensor fields like elasticity or the stress-energy tensor. On the other hand, 'field' should be avoided in the nontechnical definition since nontech readers won't know what that is. --Chetvorno^TALK 09:20, 11 July 2009 (UTC)

I think that the type of mathematics required (scalar, vector, tensor) to describe the field should not be a requirement for the first (layperson's) sentence. How about just saying it's a field: "A magnetic field is a field which surrounds magnetic materials and electric currents, and is detected by the force it exerts on other magnetic materials and moving electric charges."? This is what was there before Chetvorno's bold proposal but without "vector".

Or to avoid redundancy: "Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges."

Bowing to the apparent consensus that vector field deserves prominence in the lede, perhaps the second sentence could be "They are described mathematically as vector fields." Jojalozzo (talk) 01:24, 12 July 2009 (UTC)

Since there seems so much interest in this sentence I thought I would propose a compromise before making the change.

"Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The magnetic field, at a given point, is specified by both a direction and a magnitude (or strength); as such it is a vector field."

This is essentially combines Jojalozzo compromise with a simplified sentence from magnet. The ref is added as an afterthought and is not the important. The first sentence is somewhat flawed technically in that magnetic fields are detected by more the just torques. Normally, I would be a little bolder, but with so many people interested in this, I thought I'd post it here first. TStein (talk) 05:24, 13 July 2009 (UTC)

Support. Nicely done. Jojalozzo (talk) 03:14, 14 July 2009 (UTC)

Good improvement. Thanks! Daniel.Cardenas (talk) 16:03, 25 July 2009 (UTC)