Talk:Magnetic field

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Definition[edit]

Why is there no mentioning on how B relates to H? — Preceding unsigned comment added by 186.77.145.193 (talk) 02:09, 21 May 2013 (UTC)


The current definition is incorrect and confuses readers. The references are poor sources, and the information is not recognised by anyone I've worked with in magnetic research.

It would be better to remove the definition box with made-up alternatives, and leave the text as:

"There are two magnetic terms, H and B. In a vacuum they are indistinguishable, differing only by a multiplicative constant that depends on the physical units. Inside a material they are different (see H and B inside and outside of magnetic materials). The term magnetic field is historically reserved for H and magnetic flux density for B." — Preceding unsigned comment added by 84.92.38.86 (talkcontribs) 09:33, 12 May 2013

I don't have time to evaluate the situation (and it is a long time since I have thought about the subject), but I know that a lot of rubbish has been written on the topic, so you may well be correct. However, the text "There are two magnetic terms, H and B" is not quite right because a term is not magnetic. Not sure how to fix that at the moment, but I wanted to say thanks for the edit, and please add a space then four tilde characters to sign a comment on a talk page (see the links at your talk). Johnuniq (talk) 11:14, 12 May 2013 (UTC)
"Inside a material they are different" – this statement would have to be qualified, since it assumes the macroscopic definitions (see Maxwell's_equations#"Microscopic" versus "macroscopic"). They remain the same (up to a change in units) in the microscopic case), and this should not be ignored. Only the microscopic case can be made truly rigorous. Just saying: don't lose sight of the microscopic definition in this article. — Quondum 11:43, 12 May 2013 (UTC)
I reworded the paragraph a bit. It's somewhat confusing to say there are "two magnetic fields". Zueignung (talk) 07:07, 22 May 2013 (UTC)

I have made an edit in the definition panel, but I feel that the entire article should be revised in relation to my comments which follow. As a professional electrical engineer educated in the U of London, I am constantly perplexed by the conflation and confusion of B & H which I frequently see in discussions of magnetism which originate in the US, and by the almost universal practice there of refering to B as 'magnetic field'. In the SI (which is the only system of units I know anything about) B & H are clearly distinguished, and are in no sense the same. They have different units (Teslas, Amps/metre) and are dimensionally different; they are given distinguishing names - 'magnetic flux density' and 'magnetic field strength'. They are in fact different quantities related by this equation: B = u*H where u is the permeability of the material (or free space) in which the quanties are measured. u has units Henries/metre, and is not dimensionless. As well as being the proper name for H, the term 'magnetic field' is commonly used in layman's language to refer to a region of space, or of a material, which is magnetised. It seems to me that it is useful for a phycisist or engineer to use this latter, general, meaning of the term, but it is necessary to be careful to distinguish B from H by using the correct distinguishing terminology (flux density and field strength) when only one of these quantities is specifically intended. The present article is marred by an uncritical use of the phrase 'magnetic field' which, it seems, can refer indiscriminately to B or H or to an unquantified region of magnetisation . The article would be much improved if it were re-worded so that these concepts are properly distinguished. — Preceding unsigned comment added by G4oep (talkcontribs) 08:43, 31 August 2013 (UTC)

Have you read Section 2, "Definitions, units, and measurement", and note 4 yet? The uses of the phrase 'magnetic field' are clearly discussed and supported by citations. It is not the job of an encyclopedia to say that a widely used practice is incorrect. Note especially Purcell's comment that it is only the names that cause confusion, not the symbols. Meanwhile, I have never encountered your alternate definition of 'magnetic field' as a region of space that is magnetized; do you have references to support that usage? RockMagnetist (talk) 17:38, 31 August 2013 (UTC)
I have removed the dubious comment about space. If you have references to support it, I suggest adding the definition to the body of the article before changing the lead. Also, it seemed at best misleading in the second paragraph to specify that H is produced by moving charges, implying that B is not. RockMagnetist (talk) 17:47, 31 August 2013 (UTC)
I agree with RockMagnetist. Keep in mind, G4oep, that WP articles should be written as much as possible for a general readership, not just engineers. The introduction in particular is required to be comprehensible to ordinary readers. It seems reasonable to me for the introduction to simply say that the term "magnetic field" is used for both B and H, and defer to the body of the article the confusing explanation of why there are two vector fields, measured in different units, which both represent the same magnetic field. That is an accurate description of the usage: in scientific literature, when there is no possibility of confusion, either B or H is often loosely called the "magnetic field". But I agree that B and H could be better distinguished in the body text. --ChetvornoTALK 20:46, 31 August 2013 (UTC)
I also support the reversion of the confusing language about "magnetized space" that was added to the introduction, but I sympathize with G4oep's intent. It's very difficult to write a definition of magnetic field for laypeople that also satisfies engineers. My personal opinion is the existing intro is pretty good, but could have a little more explanation (maybe one more sentence) for general readers. --ChetvornoTALK 20:46, 31 August 2013 (UTC)

I feel that there is a difference in usage between British and US users of the concepts being discussed here, and I would like, if possible, to adjust what is written in the main article so that it makes sense to both. I feel that it would be a great advance in clarity, if ambiguity about the meaning of 'magnetic field' could be removed in the main article. If we agree to call B 'magnetic field' then what do we call H ? To me it seems unsatisfactory, and something that should be strenuously avoided, that the term should be used in scientific discourse with 2 distinct meanings. The meanings can be distinguished, and in fact are distinguished in the main article, but not consistently so. It is this inconsistency that I would like to see removed. I can only make sense of the idea that 'magnetic field' refers to both B and H by recourse to popular language, which has no need of the distinction. My Concise Oxford Dictionary (COD), Clarendon Press, Oxford, 1990, defines 'Magnetic field' as 'a region of variable force around magnets, magnetic materials, or current carrying conductors'. To my mind this 'region' (I previously mentioned 'space') could be in a vacuum or in some material, though the layman would probably expect it to be in air. I recognise this definition as the meaning used in lay speech, and it is to this that I referred previously; the COD is my reference for this. I would propose that this definition (or one like it) is put forward as a general, lay, definition of an unquantified magnetic field, but that 'magnetic field', and 'magnetic flux density' be used as terms to refer to H & B respectively as quantifiable concepts (vectors)in scientific discourse. B is not called 'magnetic field' in the UK; that term is reserved for either H or the layman's concept of the magnetic field, as given by the COD. I have been brought up with texbooks which give such definitions as "The vector H is known as the magnetic field", and "The vector B is referred to ... as the magnetic flux density " (both quotations from Electromagnetism for Electronic Engineers, R.G. Carter, Von Nostrand Rheinholt, 1986 - I am confident that other references could be found). Of course my comments are those of one who lives and works in the UK, and I am aware that others speak and write differently, but it seems to me that the usage I am proposing has the advantage of clarity and lack of ambiguity.— Preceding unsigned comment added by G4oep (talkcontribs) 09:06, 1 September 2013

Removing the ambiguity from the article will not remove the ambiguity from scientific discourse; it will simply make the article less helpful by pretending the ambiguity doesn't exist. I also doubt this is a UK vs. US/Canada/NZ/whatever issue. From a few minutes' poking around, it seems that David Tong's QFT lecture notes refer to B as "the magnetic field", as does The Quantum Theory of Light by Rodney Loudon (who was educated at Oxford, is now emeritus professor at Essex, and is so staunchly British that he insists on writing . for the divergence instead of ·). If anything, I suspect this is a pure vs. applied science issue. The practice of referring to H as "the magnetic field" is something I usually see in texts on electromagnetics (i.e., electromagnetism applied to waveguides, antennas, etc.). Zueignung (talk) 18:38, 1 September 2013 (UTC)
If the COD defines magnetic field as a region, then it may be appropriate to give the definition a brief mention - but it is based on a misunderstanding of the mathematical concept of a field, and may just be a distraction. It would be interesting to see if the greater Oxford dictionary provides quotes for this usage. RockMagnetist (talk) 15:44, 2 September 2013 (UTC)
I looked at the Greater OED, and it was quite interesting. The relevant definition is definition 15a of FIELD, and there is a quote by Maxwell saying "The electric field is the portion of space in the neighborhood of electrified bodies, considered with reference to electric phenomena." It's hard to say what he means by that. The main dictionary definition is followed by this statement: "The concept of fields arose out of the work of Faraday and Maxwell. Fields came to be regarded as having an existence independent of the space they occupy." RockMagnetist (talk) 16:40, 2 September 2013 (UTC)
This reply is sidestepping the issue. There is no reason why the ambiguity cannot be removed from the language of the article, as long as the terminology and the fact of the ambiguity in "general" use is adequately addressed. There is no reason for the article to perpetuate the confusion; the editors are free to use unambiguous terminology (especially as such terminology does exist and is presumably standard pretty much everywhere except in the US) while making it clear to readers what terminology is generally in use and notable. It is not uncommon to say "in this article, xxx refers to yyy"; the editors can simply elect which terminology serves to convey the concepts with the least confusion, while clarifying what terminology is "usually seen", nor does it have to follow it (especially if this is WP:UNDUE). — Quondum 04:44, 2 September 2013 (UTC)
What specific usage in the article would you change? --ChetvornoTALK 06:22, 2 September 2013 (UTC)

Is this acceptable ? In lay language, a magnetic field is ‘a region of variable force around magnets, magnetic materials, or current carrying conductors’ (COD as reference)'. This meaning is also used by members of the scientific community to refer to a general unquantified concept which can be differentiated into two distinct quantified mathematical descriptions of the magnetic influence of electric currents and magnetic materials. These are denoted by the symbols B and H. B refers to magnetic flux density, and H to magnetic field strength, but sometimes ‘magnetic field strength’ can refer to magnetic flux density (B). B and H are related by this equation: B = μH where μ is the permeability (reference needed) of the material (or free space) in which B and H are measured. B and H at any given point are specified by both a direction and a magnitude (or strength); as such they are vector fields.[nb 1] In the SI system, magnetic flux density is defined in terms of Faraday’s Law of electromagnetic induction (Wikki entry on Weber flux unit as reference). Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. The units of H are amperes/metre (A/m) which reflects this dependence of magnetic field strength on electric current. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.G4oep (talk) —Preceding undated comment added 07:44, 4 September 2013 (UTC)

This is for the introduction? --ChetvornoTALK 21:48, 4 September 2013 (UTC)
Chetvorno - by 'introduction' do you mean 'lead'? RockMagnetist (talk) 23:55, 4 September 2013 (UTC)
Yes; I'm asking, is G4oep proposing this for the first paragraph of the article? If he is, my feeling is that the first sentence is a slight improvement on the current one, but the rest of it is too technical. The second sentence: "This meaning is also used by members of the scientific community to refer to a general unquantified concept which can be differentiated into two distinct quantified mathematical descriptions of the magnetic influence of electric currents and magnetic materials." is way to stuffy and wordy. It won't resolve any "ambiguity" or "confusion" about the term magnetic field, because most readers will give up long before they reach the end of the sentence. The lead should introduce B and H and give their units and most common names (flux density and field strength), but it doesn't need to give alternate names. I don't think the equation B = μH needs to be in the intro. In sum, I like the intro better as it is, except for adding the units of B and H. --ChetvornoTALK 02:58, 5 September 2013 (UTC)
I am still skeptical that "magnetic flux density" is the most common name for B, at least in pure physics. In my experience, B is referred to predominantly as the "magnetic field". This is the case with Purcell's and Griffiths's E&M books (though notably not Jackson's) as well as Sakurai's Modern Quantum Mechanics. Purcell and Griffiths both go out of their way to mention that B is, in practice, not usually referred to as the "magnetic flux density", but rather as the "magnetic field". Zueignung (talk) 05:02, 5 September 2013 (UTC)
Even Jackson starts by calling B the magnetic field, then on the following page introduces H and says "The two new field quantities E and H, usually called the electric displacement and magnetic field (B is then called the magnetic induction) ..." RockMagnetist (talk) 16:16, 5 September 2013 (UTC)
Then what do we call H? It's most often called "magnetic field strength". I don't really care what they are called - as Purcell said the only confusion is with the names; the mathematics is perfectly clear. We have a good table in the article giving the different terms used for B and H. --ChetvornoTALK 10:43, 5 September 2013 (UTC)
I usually hear it called "H" or the "H-field". This more or less follows what Griffiths has to say about it, which is that "H has no sensible name: just call it 'H‍' ". "Magnetic field strength" sounds like you're talking about the absolute value |B| (as in, "the atoms are subjected to a strong magnetic field"). In Purcell's chapter on magnetism in matter, he refers to everything as much as possible as "the B-field" or "the H-field", which I think is probably the right way to go for the bulk of this article, simply so that readers are not constantly doing double-takes at unfamiliar or jarring nomenclature. Zueignung (talk) 14:07, 5 September 2013 (UTC)
I agree with Zueignung that the terms "B-field" and "H-field" would be simpler to use, since there's no ambiguity in terminology, there is a direct reference to which vector fields are used, and they're easier to write. As Chetvorno says, there is already a clear table for alternative names, and the lead should have the common names anyway. M∧Ŝc2ħεИτlk 14:27, 5 September 2013 (UTC)
Yes, I agree, that would be clearer. --ChetvornoTALK 14:37, 5 September 2013 (UTC)
How do other people feel about the issue G4oep raised, the language in the introduction that refers to both B and H as the "magnetic field"? --ChetvornoTALK 14:54, 5 September 2013 (UTC)
I think that any definition of the magnetic field as a region should be avoided, even when talking about lay language. As I described above, the support for this definition in the OED is very weak, and scientists always draw a distinction between fields and the space they permeate. I also think that extensive discussions of nomenclature should be avoided in the lead, because its purpose is to summarize the whole article. RockMagnetist (talk) 16:24, 5 September 2013 (UTC)
I agree with RockMagnetist on the specific points mentioned. Description as a "region" is archaic and the COD/OED should not be considered a reliable source on matters of a scientific nature, even from a lay perspective. From a modern perspective it is simply an incorrect description: a field might occur in a region (along with other fields), but it is not that region. Would you describe a person as a region in space? The textbooks that I learned from at school level took pains to specify distinct terms for H and B. The terminology issue cannot be sidestepped in the article, but it can be minimized in the lead. The reason is that this article will be sought out by students precisely to resolve issues of terminology. This suggests that we should devote a section to discussion the confusion of the terminology (in addition to the existing table of alternatives), and to state the naming policy adopted in remainder of the article. Then we should use something unambiguous for the remainder for the body of the article, e.g. B-field and H-field as Maschen says. I don't like the lack of uniform established terminology, but using one of several meanings of the same term would be POV. I suggest that we never use the term "magnetic field" when specifically one of the fields is meant, but when the ambiguity is irrelevant, it is fine. — Quondum 21:45, 5 September 2013 (UTC)
There already is a section (2.1) for discussions of the terminology. RockMagnetist (talk) 23:55, 5 September 2013 (UTC)
I disagree. We should not conflate use of terminology with definition of the concepts. It should be split out into a separate (sub?)section. — Quondum 02:26, 6 September 2013 (UTC)

There is an incoherent conflation in the definition used in the introduction. The first sentence says "A magnetic field is a mathematical description..." Then the second paragraph says "Magnetic fields are produced by moving electric charges..." If these were both true statements then one could combine them and say "A magnetic field is a mathematical description produced by moving electric charges." But this is an incoherent conflation because electric charges are concrete physical objects and descriptions are abstract mathematical ideas. If a magnetic field is a mathematical description then it is a description produced by human minds, not by electrons. I agree that fields in physics are abstract mathematical descriptions of some concrete physical thing that exists in space. In the case of a wind vector field, the field is a description, and the things being described are moving gas molecules within a defined area. The described things and the description itself should not be and cannot coherently be conflated. So can we say that a magnetic field is a description of moving electrons within a defined area? — gcsnelgar 20:50, 20 April 2014

No, a magnetic field is the vector force field generated by the moving charges, that is the intent of the phrase "mathematical description". The essence of the idea is that it is a condition of space that is viewed as separate from what generated it. For example, when you look at a star, your eye is responding to oscillating electric and magnetic fields emitted by an electron in the star, perhaps thousands of years ago. The electron which created the magnetic field could have been annihilated by another particle long ago, or the star could have collapsed into a black hole. It would be pretty silly to have to refer back to the hypothetical motion of an electron thousands of years ago to define the magnetic effects. Instead the light wave is described in terms of an oscillation in the electric and magnetic field, travelling through space. --ChetvornoTALK 22:07, 20 April 2014 (UTC)
While I disagree with the idea that the field is the description, a good point is being made. In effect, the language of the first sentence is wrong. —Quondum 22:11, 20 April 2014 (UTC)
I think the previous sentence was adequately comprehensible. The lead sentence now is pretty much a circular definition that gives little information: "A magnetic field is the magnetic influence." --ChetvornoTALK 22:53, 20 April 2014 (UTC)

The Earth and the planets[edit]

All popular theory to explain the magnetic field of a planet are wrong, because there is no known theory able to explain the obvious connection with the rotation, including why the direction of rotation and field approximately match. The rotation of the planet is obviously the cause of the magnetic field. Popular theory not even explain the polarity of the field. Despite the lack of evidence, outlandish theories about the magnetic pole reversals are common. In fact, the polarity is clear from the rotation and can therefore cannot change.

The correct explanation is actually surprisingly easy. The cause is simply the electric current generated directly by the rotation of the planets, and the charge separation of free ions by gravity. Positively charged ions are usually heavier than negative charged ions and are therefore shifted to the rotation axis.

We first consider only the negative charges, the electrons, of the Earth, neglecting the positive charges of the atomic nuclei. In 24 hours the total charge of the electrons crosses once through a cross section of the Earth. The mass of the Earth is at first approximation two times the mass of the protons, since there are about as many neutrons almost as heavy.

Then the current caused by the rotation of the electrons around the axis of the Earth within 24 hours is easy to calculate.


\left( 
\frac{ mass \, of \, Earth} {2 \times mass \, of \, proton} 
\right) \times \left( \frac{charge \, of \, electron}{24 h } \right)

= -3.3 \times {10} ^ {27} \, A

Since the Earth is electrically neutral overall, a current of exactly equal magnitude is produced by positive charges. The magnetic fields generated by both currents cancel each other almost completely. But, since the positively charged ions, however, are slightly shifted by gravity towards Earth's axis, the remaining effect of the negative charges causes Earth's magnetic field.

Franz Scheerer (Olbers) (talk) 17:47, 29 October 2013 (UTC)

This isn't the proper place for this; theories on how the Earth's magnetic field is generated should go in Dynamo theory, or Earth's magnetic field. However, if this is an unpublished theory, it doesn't belong in Wikipedia, which requires that all content be supported by published reliable sources. Maybe a better place for your idea is in Wikibooks? It doesn't require sources, and you could write a whole article about it. --ChetvornoTALK 19:54, 29 October 2013 (UTC)
You're too kind. "Despite the lack of evidence" is outrageous given the extensive geological evidence for reversals. Moreover the proposed theory would predict a similar magnetic field for Venus and Mars, but Earth's magnetic field is more than a thousand times stronger than that of either of them. Vaughan Pratt (talk) 03:58, 10 January 2014 (UTC)
Ok, I again thought about it today. Indeed I'm convinced the reason for the magnetic field is as easy as I wrote some time ago. One may say, the (main) reason are free electric charges, namely ions in a liquid, and vertical charge separation due to gravity. The more heavy ions are seperated from the less heavy in direction to the rotation axis. Yes, it is so simple, the ions closer to the axis cause a field of lower magnitude. The direction of the field depends on weather the positive or the negative ions are more heavy. In gerneral, the more heavy ions are mainly positive metallic ions. Though in sea water the situation is different.

To explain the magnetic field of the planets my theory fits very well. Venus rotates very slowly and Mars has few liquid ions (too cold). 178.201.250.13 (talk) 10:15, 5 February 2014 (UTC)

Magnetic component of EM radiation[edit]

The article Electromagnetic radiation says nothing about the appropriate units for the magnetic component of EMR, while this article says nothing about EMR. Where can a Wikipedia reader go to learn about the appropriate SI unit for the magnetic component of EMR (teslas?), without the distraction in this article of trying to keep the distinction between B and H straight? Surely somewhere in Wikipedia there is the simple equation E = cB for EMR that would clear this up. Vaughan Pratt (talk) 23:20, 9 January 2014 (UTC)

There ought to be a discussion of units in Electromagnetic_radiation#Derivation_from_electromagnetic_theory and in Electromagnetic wave equation; you certainly shouldn't need to hunt as far afield as this article (where there is no reason that I can think of for a discussion of radiation). The electromagnetic radiation article claims that the relation E = cB "can be seen immediately from the Poynting vector" - the derivation is in Poynting_vector#In_plane_waves, and to me at least it isn't quite so immediate. RockMagnetist (talk) 23:42, 9 January 2014 (UTC)

Right, there it is, thanks. I was looking for it in the derivation, didn't think to look for it in the discussion paragraphs.

The bit about "can be seen immediately from the Poynting vector" makes no sense. First, the Poynting vector is derived from the assumption E = cB, second I don't see how one can get E/B = c from S = E × B --- there's a missing factor of B2 there. Without an explanation I would think that bit should be deleted. Vaughan Pratt (talk) 03:35, 10 January 2014 (UTC)

On a second look, it appears that the Poynting vector is irrelevant - but the result is correct (I found it in a few different references). The key is that the wave is approximately planar in the far field, and then you need to use Maxwell's equations to derive the relationship. They haven't done that in any of the three pages. If I can find the time, I'll add it. RockMagnetist (talk) 06:19, 10 January 2014 (UTC)

Re: Recent changes to introduction[edit]

I just wanted to explain why I reverted your edits, Jwkeohane. The intro, and what should be said about B and H, has been the subject of an enormous amount of discussion on this page; see "Definition" above, as well as the archives. The current intro is the result of a consensus that the detailed definitions and history of B and H, and the question of which is the "true" magnetic field, should be deferred to the body of the article. All the introduction should say is that they are both vector fields which describe the magnetic field. Maybe we could discuss your proposed changes here. --ChetvornoTALK 14:29, 17 February 2014 (UTC)

Chetvorno Thanks, but the prior version was totally misleading. I did think you were probably right about it needing to not dwell on the history so much, so I reverted them back, by moving the section to a footnote. It is important to realize that the introduction needs to be the best current physics definition, and not based on ideas that are 100 years old. (Jonathan Keohane (talk) 15:02, 17 February 2014 (UTC))

Your version is still misleading because it gives the impression that B and H are an anachronistic relic left over from a 19th century debate. You have to remember that the intro will be read by nontechnical people. It is confusing and unnecessary to go into the history of B and H in the intro. In modern terms, B and H are simply different mathematical models of the magnetic field, which are both equally useful. Also, your version does not make clear that B and H are vector fields, not just vectors.--ChetvornoTALK

Actually they are not equally valid. What I have is correct. All notation in science is historical, but some is left-over from conceptually wrong physics like the pole model and the aether. But in this case it is particular egregious, because they measure the same thing in the lab. Thus, the only reason we still use H is because of pre-relativistic physics. This might be a good place to link to the history of the aether. Non-technical folks should not be confused by B and H, and realize that there is no need for H at all anymore. The more fundamental quantity is B, and H only exists for historical reasons. (Jonathan Keohane (talk) 19:00, 17 February 2014 (UTC))

It is also extremely misleading to general readers to say that "...magnetic fields and electric fields are one and the same..." That can promote the common misconception that the attraction of a magnet and a charged object are the same thing, the same field. The relation of electric and magnetic fields through Lorentz transformations should be left to the body of the article. The original version said much more clearly they are "two interrelated aspects" of the electromagnetic field tensor. --ChetvornoTALK 15:54, 17 February 2014 (UTC)

You are technically right here, but mentioning tensors is way too advanced for the student wanting a quick, and correct, overview. So, my goal was to simplify the point, which is simply that depending on your reference frame, you will observed something as an electric or a magnetic field -- this is really a combination of the two. This is what induction is -- keep that in mind. Remember, Maxwell's equations are relativistic, and Einstein's brilliance was not to make some sort of Maxwell Stress Tensor, but rather to make the conceptual leap that Electrodynamics is more fundamental than Newtonian mechanics.

Your point about the misconception is totally correct! The thing to do is to figure out some nice way to make that point clear. Moving charged particles, do act like magnets, and magnets attract because of a gradient of the field -- so that is a hard concept. I think that someone, but probably not me, should write a nice early section on forces on magnets. (Jonathan Keohane (talk) 19:00, 17 February 2014 (UTC))

Does anybody else have an opinion about the new wording? --ChetvornoTALK 19:29, 17 February 2014 (UTC)
I have not gone through everything in detail, but I don't think what we currently get taught at school should be considered anachronistic. In the macroscopic Maxwell's equations, it remains useful to distinguish B and H, and the lead should not sidestep the concept of a magnetic field inside matter. The historical confusion arises not from the fact that there is a misinterpretation or that there is really only one field, but is rather due to the dual use of the phrase to describe fields that have been given different definitions and units. In any event, no space should be given in the lead to the matter of confusion of terminology, other than to note that the term may have dual use. On the "one and the same", this is far worse than use of the word "tensor". Ideally, it should indicate that the magnetic field is an observational aspect of a single field, the electromagnetic field (which is a bivector field, not a vector field, but leave this for the body). —Quondum 21:26, 17 February 2014 (UTC)
My comments echo some of the above:
  • The statement "it is a vector" is technically wrong: vectors and vector fields are not the same.
  • The statement "Thus, physicists often refer to the magnetic field by their symbols B and H." makes no sense since the magnetic field is a single entity: B and H are vector fields to describe it.
  • "B refers to magnetic flux density" is not necessarily common usage because B is also commonly called "the magnetic field" (some editors above support this). Consequently "The term magnetic field is somewhat confusing" is not really "confusing", it is a physical entity, the names for the vector fields B and H could be called a number of things (which is the confusion) which is clarified in the Definition section.
  • The history section in the lead is irrelevant since the lead should say what the topic is, and not the development leading up to it, which could be what the subject is not! This is better suited to its own "history" section. Often, the historical development is not always the best way to present a subject because it can introduce misconceptions which were not understood in the past, but are now.
  • "magnetic fields and electric fields are one and the same, depending on one's reference frame" is simply unclear, as said above.
  • "They are produced by the charge" is also unclear and not true in general, again said above: a charge at rest relative to an observer will appear to produce an electric field and not a magnetic field - magnetic fields arise by relative motion between an observers and charges, for the purposes of a typical reader, the statement "moving charges" is probably enough.
  • The mention of the electromagnetic tensor is important to mention in the lead after mentioning B (and H) because it is part of the extensively-used EM field tensor in theoretical physics (any application of special and general relativity, quantum field theory). There is no need to digress into details, this is just a pointer.
So I support the previous version of the lead. M∧Ŝc2ħεИτlk 23:15, 18 February 2014 (UTC)
That's three editors supporting the original version. Reverted changes. --ChetvornoTALK 00:02, 19 February 2014 (UTC)
I too agree with the revert, and I would like to add that B and H do not measure the same thing in a magnetic material, the difference being the magnetization. Far from being a historic relic, H is used more than B in the theory of magnetic materials. The question of whether B or H is the "true" magnetic field is one of these pointless controversies that keeps resurfacing. Also, a version of the pole model is extremely useful in magnetostatics. It's only wrong if you interpret the poles as actual physical poles. RockMagnetist (talk) 00:39, 19 February 2014 (UTC)
I absolutely agree with RockMagnetist. Jonathan Keohane's statement above that there is no need for H anymore, and that it is only a leftover from pre-relativistic physics was completely mistaken (and the relation between B and H has nothing to do with relativity). If he expressed that point of view in an electromagnetics lab, he'd be laughed out of the room. --ChetvornoTALK 01:21, 19 February 2014 (UTC)
I don't want to rehash a "pointless controversy", and I don't agree with Jonathan Keohane, but isn't there a significant truth he was trying to get at? The H model is not "useless" or a leftover as he claimed, but isn't it a less fundamental model than the Ampereic current B model? In the H model, the effects of magnetic materials are represented by a magnetization vector field M which is "smooth" on an atomic scale and just represents the "average" density of dipoles, so the H field can't represent the strong, locally varying magnetic fields between individual molecules, can it? If, instead, tiny circular "bound" currents j representing the individual dipoles (the electrons' spins) are included on the source side of Maxwell's equations, it will accurately give the B field on an atomic level. On an atomic scale, there is no H field, is there, only B? Or is that inaccurate? I'm not suggesting any changes to the article, I just want to understand; it's been a long time since I took electromagnetics. --ChetvornoTALK 12:30, 19 February 2014 (UTC)
@Chetvorno: I'm afraid you're wrong on several counts. At an atomic scale, there are only point charges in a vacuum, so H and B are identical. If classical analogues are used, the magnetic field of the dipoles can be derived using microscopic currents, as you describe, or as the limit of two monopoles approaching each other. The physical models underlying these calculations are equally untrue. You're right that M comes out of a smoothing process, and therefore the difference between H and B. There may be a better argument for saying that the magnetic vector potential and magnetic scalar potential are the real fundamentals - consider the Aharonov–Bohm effect. RockMagnetist (talk) 17:12, 19 February 2014 (UTC)
I think that there are good philosophic reasons for distinguishing between B and H, even if they are related by a constant of proportionality. (I'll ignore the practical reasons, which are very persuasive.) These are essentially the same as the reason for the distinction between inertia and mass. When one fails to distinguish between concepts at this level, one has more difficulty framing questions properly. Merging B and H into the same concept also assumes that our theories are an exact description of physics, which we should never do (e.g. they'd presumably be distinct if the photon had a small rest mass, translating into a field that does not exactly fit the inverse square law). To be more explicit: H is defined in terms of the source charge and current distribution, whereas B is defined in terms of the force felt by a test charge. How the two are related is a matter of empirical observation, and of course descriptive theories. —Quondum 18:39, 19 February 2014 (UTC)
@Quondum: I'm not sure if you think I'm advocating a merger of B and H. Quite the opposite, actually. RockMagnetist (talk) 19:34, 19 February 2014 (UTC)
@RockMagnetist: No, I did not think that (I apologize if it seemed that I was responding to you; maybe I should not have indented). I actually have difficulty following your post since its referents are not clear to me, and couldn't quite work out what you were disagreeing with. I might disagree about the smoothing concept, since it seems that what is significant is what charge and current is classified as bound, not the small-scale structure. The "macroscopic" equations apply without modification to any scale ... until there is a movement from one classification to the other, e.g. in ionization. —Quondum 22:12, 19 February 2014 (UTC)


Hi Everyone, I think that you-all have convinced me that there needs to be a revision, simply because from a more practical point of view, rather than theoretical. But what everyone has to realize is that the previous version was both unclear and wrong, so yes we need something better than either my version or the one before.

First, the intro needs to give a clear definition of the magnetic field. And all the debate by people who know this stuff, but from different backgrounds, only underscores it. I teach physics, and am writing a textbook on this stuff, so I come from a pedagogical and historical point of view. The biggest problem student have is with the distinction between cause and effect, and we need to make that clear. So here are are few things we need to keep in mind:

1. B is defined based on the effects of a magnetic field. Thus, if one wants to measure a magnetic field, with an unknown source, the only kind of magnetic field one can measure is B. This was not understood in the 19th century, and the reason was that they assumed that the universe was filled with the aether, so it was assumed that we were inside a linear medium -- this was not true. (There is a nice wikipedia article on the history of aether theories.)

2. H is defined based on the cause of the magnetic field. Thus, if I have a wire with a known current running through it, I know, by definition, what H is due to it, even I have no clue about something else.

3. Magnetic materials are not linear, and even the most basic observations need quantum mechanics to explain. In the 19th century they thought they understood magnetic materials better than they really did -- the pole model worked quite well. They are difficult and need their own pages.


So, I hope we can all agree, that what the introduction needs is a clear definition of both B and H, in a way that can easily be measured in the lab. I think that we need to give in the introduction simply #1 and #2 above.

Who wants to give it a whack?


That said, I am right about H having no place in the physics curriculum, unless it is put in historical context.

until the students take solid state. On the other hand, I am not an engineer, but the introduction needs to be simple, clear and right.

But someone reading a book or paper with H needs someplace to go that defines it correctly with a good modern working definition.

Consider the following scenario in a lab:

My point is simply that what is actually measured in magnetic materials are two things:

  1. The current and configuration of whatever is making the external magnetic field. By finding what the magnetic field would be, if it were not for the material involved, this makes a nice independent variable for the lab.
  2. The magnetic moment per volume of the material involved. For example there are some nice experiments at NIST that measure the torque on a known volume of material in an external magnetic field. In these experiments, H is now the independent variable and M is the dependent variable.
  3. And, of course, the magnetic field (B) outside of the material can also be measured.


B inside the material is inferred, so people really measure M but express it as B for historical reasons. (Jonathan Keohane (talk) 15:25, 21 February 2014 (UTC))

If you think H has no place outside a historical context, you are absolutely mistaken. You need to look at how B and H are used in modern engineering. Regardless, the introduction is not the place to address this issue. As a teacher, you should be able to appreciate the need to give an elementary grounding that general readers can understand, before getting into the more complicated stuff. --ChetvornoTALK 17:02, 21 February 2014 (UTC)
I agree with Chetvorno. In my experience, when people start talking about putting better definitions in a lead, it's a sign of trouble. A period of hairsplitting follows and the result is tortured prose that tries to do everything in the first sentence. Meanwhile the body of the article is neglected. I think the lead is pretty good - most of the text is devoted to the significance of magnetic fields. RockMagnetist (talk) 17:13, 21 February 2014 (UTC)
@Jwkeohane: I would like to add that if you are getting your ideas about B and H from textbooks on electromagnetism, you should realize that they tend to do a bad job on magnetism (and by association, H). That includes classics like Jackson's Classical Electrodynamics. RockMagnetist (talk) 17:19, 21 February 2014 (UTC)


This is really a fundamental thing. H is defined from Ampere's law -- that is just all to it. It is a nice way to characterize what the magnetic field is due to another source. Now, of course, if you define H in terms of B and M, then you run into the problem I addressed before, H is not directly measurable, but it is very helpful when characterizing something in magnetostatics. But, as with any fictitious concept, it needs not be in the introduction. Actually I would agree that most textbooks are not fully clear, and that most people do not fully understand cause and effect in electromagnetism. First principles guys -- don't confuse people by implying that H inside a material is somehow real -- this debate was settled a hundred years ago. There are no magnetic monopoles. There is no aether. And, so the introduction needs to define these in clear and simple manner. — Preceding unsigned comment added by Jwkeohane (talkcontribs) 18:00, 21 February 2014 (UTC)

You're not going to get anywhere with nonsensical claims that H is not real. Look in any of the following books, the first two of which are classic works in ferromagnetism, and see which they mention first, B or H:
  • Bozorth, Richard M. (2003). Ferromagnetism (Reissue of a [, ...] classic text. ed.). Hoboken, NJ: Wiley-Interscience. ISBN 978-0780310322. 
  • Jr, Sōshin Chikazumi ; English edition prepared with the assistance of C.D. Graham, (2009). Physics of ferromagnetism (2nd ed.). Oxford: Oxford University Press. ISBN 978-0199564811. 
  • Aharoni, Amikam (2000). Introduction to the theory of ferromagnetism (2. ed.). Oxford: Oxford University Press. ISBN 9780198508090. 
  • Spaldin, Nicola A. (2006). Magnetic materials : fundamentals and device applications (Repr. ed.). Cambridge: Cambridge Univ. Press. ISBN 978-0521016582. 
I don't know what debate you imagine was settled a hundred years ago, but this is not it. RockMagnetist (talk) 18:17, 21 February 2014 (UTC)
Your addition of units to the lead was an improvement, though. RockMagnetist (talk) 18:39, 21 February 2014 (UTC)