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Archive 1 |
Let's say a charged particle moves from point A to point B. It was initially at rest at A, then it suddenly accelerates, moves to B, then decelerates suddenly to stop at B. The electric field around the charge has to change to become centered at its new position. But it cannot change instantaneously throughout all of space because doing so would send information at a speed which is faster than light. So an electromagnetic wave is emitted which has the effect, after it dissipates, of recentering the electric field.
Now, if a charge is moving inertially at a given velocity and then suddenly accelerates to a new velocity but then stays moving inertially at that given velocity, then an electromagnetic wave is transmitted. This is called bremsstrahlung. This is what happens to electrons encountering the ionosphere: they decelerate and produce polar auroras.
If an charged particle is at rest, then it is surrounded by an electrostatic field which is symmetric in every direction. But what if the charge is moving inertially at a constant velocity? Then from its own frame of reference, it is surrounded by an electric field which is static, the same in every direction, without the presence of any magnetic field. But seen from a different frame of reference, the charge is moving, and the electric field has to move along with it, and this is where the problem sets in. If the charge moves along the x-axis, then let's say it moves from x0 towards the "right" to x1 during a unit of time. Point x2 lies directly in the particle's path. The electric field will point, say, away from the proton, and during the unit of time its magnitude will increase. This increase of the electric field at point x2 will cause a circular magnetic field line to be induced, which can be thought of as the rotation of a "magnetic fluid", and the "angular momentum" vector of this rotation points in the direction of the proton's velocity. Then, say there is a point x-1 behind the moving proton. It has an electric field vector pointing in the negative direction, and in one unit of time its magnitude decreases. This means that the electric field vector at point x-1 has increased in the positive direction, thereby inducing -- again -- a circular rotation of the magnetic fluid around it, such that the angular momentum vector of the rotation points in the direction of the proton's velocity.
Therefore the proton, when moving at constant velocity, is surrounded not only by a spherically-symmetric electric field which moves along with it, but also by a cylindrically-symmetric magnetic field whose axis of symmetry parallel to the velocity vector of the proton. Both of these fields decrease in magnitude inversely with the square of the distance. But what does this imply? At any point not directly in the proton's path, there is going to be a magnetic field vector which is perpendicular to the electric field vector. This means that there is going to be a Poynting vector at that point in space.
Points which lie on a transversal plane (perpendicular to the velocity) which includes the charge itself, will have Poynting vectors parallel to the velocity vector, and pointing in the same direction: forwards. Points on transversal planes which are in front of the charge will have Poynting vectors which will also be pointing not only forwards but also inwards towards the path of the charge. Points behing the charge will have Poynting vectors which point not only forwards but also away from the charge's path.
Poynting vectors are associated with electromagnetic waves: they indicate the direction of propagation and the power (rate of energy movement). A proton moving with constant velocity would have Poynting vectors which point in directions tangent to concentric spheres whose centers are the proton. These spheres move along with the proton. These Poynting vectors also all belong to planes which pass through the proton's path. The magnitudes of these Poynting vectors would vary as the fourth power of distance (?). What all these vectors do is to move energy in the direction of the proton. The electric field surrounding a static charge stores energy: this energy is proportional to the magnitude of the electric field vectors. When a charge moves at constant speed, the electric field around it has to move along, somehow, to keep up with its source. This implies a movement of electric energy, and the speed of this movement is power: hence the Poynting vectors.
But if these Poynting vectors describe E-M waves then these must be very unorthodox E-M waves, because they appear to move on the surfaces of spheres, starting all from a single point behind the charge and converging all again at a single antipodal point in front of the charge. Besides, the E and M fields moving along with the particle do not appear to oscillate at all, as would be expected of E-M waves.
Then, what happens if like charges are strung along a line which extends infinitely. If these charges move along their line, then they collectively form a current, even though the charge density remains constant. The constant charge density extending in an infinite line implies a cylindrically-symmetric electric field centered around the line, but the moving charges also imply a cylindrically-symmetric magnetic field. The magnetic field lines are circles whose centers are points on the line of the current. This can be seen to be a consolidation of the one-particle case, but is now equivalent to Ampère's law. Now the Poynting vectors are all parallel to the direction of the current, no matter where they are located in space. This would appear to imply that there are E-M waves moving parallel to the current; but then again, there are no oscillations of the E-M field. Perhaps these E-M waves are "degenerate", or "frozen". By the way, this is an example of a magnetostatic field. -- AugPi 10:12, 3 Apr 2004 (UTC)
It is nice to see a fresh treatment of a well-known subject. To balance this point of view, the two-fluid analogy should also treat the relationship of the two fluids to each other; we should see how B turns into E and vice versa, in an eternal harmonic motion. The fluid analogy, in fairness to Maxwell's equations, should encompass this.
LePage had a nice model as well. It is not fluids, but the encyclopedia, if it exposes the fluid model, needs to set LePage's viewpoint out as well, for balance.
Also, Faraday's contribution, that the fluids were somehow viscous or rubbery and hence exerted force on the charges.
Also, what about the freeway analogy, which uses the irrotational nature of Maxwell's equations: ie the travel of cars on a freeway obeys Maxwell's equations as well! (covariant motion)
Also, what about Purcell's viewpoint that an electromagnetic field is 'something that crackles'. 169.207.89.249 19:37, 3 Jan 2004 (UTC)
It seems clear that as a front of electrical fluid propogated away from a positive source charge, its velocity of propogation would decrease, which in turn would induce a magnetic field. Is this correct? As someone who doesn't understand EM concepts very well, I think the entry needs to explore in more depth the implications of the fluid analogy.
The fluid analogy does not work in this sense: that objects immersed in a moving fluid (e.g. a river) tend to be pushed by that fluid in such a way that the velocity of the object aligns with the velocity of the fluid. Once the velocities are aligned, the fluid's motion should vanish from the object's point of view.
However, the force of an electric field on a charged particle is , and this force is independent of the velocity of the particle, which means that the particle will accelerate continually in the direction of the field. If the field is the velocity field of a fluid then the fluid would be causing the object to accelerate continually in the direction of the fluid's motion, to the point that the object's speed becomes way larger than the fluid it is immersed in. This is paradoxical.
From the continually accelerating object's point of view, if its speed has already surpassed the speed of the fluid, then the fluid is moving backwards, so the field should be pointing in the direction opposite to the direction in which the object keeps accelerating. This means that that the object should stop accelerating and begin decelerating, until its speed aligns with the speed of the electric fluid.
An alternative interpretation would be that the field is not actually a velocity field, but a density field of photonic fluid, which is constantly moving at the same speed: the speed of light, independent of the speed of the observer (the charged object). Photonic fluid never changes speed but can change net direction and the intensity of its net movement in that direction. This interpretation would have to be verified by someone who knows QED. -- AugPi 02:48, 24 Mar 2004 (UTC).
I just noticed this page listed on the "Peer Review" page, and thought I might add some comments. All these of course, are to be understood as prefixed with "IMHO". The article seems to have a lot of discussion on the fluid analogy and other analogies, so my comments are regarding this aspect.
To summarize : There is no point pushing the fluid analogy this far - it seems to be hiding more relevant ideas.
[[User:AmarChandra| Amar | Talk]] 17:18, Jun 30, 2004 (UTC)
I think the fluid analogy in this article hurts more than helps. Most people have a strong intuitive sense of how a fluid should behave, and EM fields do not behave like fluids. They behave like force fields. The article currently relies so heavily on the fluid analogy that I think some people will be confused into thinking that there actually is a ethereal fluid of some kind that makes these processes work, which is not true at all (as far as we know).
I think a better approach would be to help people form an intuition about what a force field (vector field) is. This article is crying out for diagrams; two or three pictures would greatly enhance any verbal explanation. I will try to contribute more specific ideas later as time permits. -- Beland 04:04, 4 Jul 2004 (UTC)
Some comments:
Hope it helps... Pcarbonn 20:13, 13 Jul 2004 (UTC)
The article contains a lot of useful and important information. The presentation could be improved, however; for example, the notation is undefined in places (although it's obvious to those familiar with the notation). Also, there really should be a section on the maths describing the EM field (which should discuss the various formulations, e.g. vector field structure, as compared to the tensor field approach, quantum formulation etc...). MP (talk) 13:45, 4 December 2005 (UTC)
I have rewritten the article to give more focus on the em field itself, rather than the fluid interpretations (which are not that important). However, as a courtesy to the work of other editors, I have moved the bulk of the fluid interpretation work to a new article: hydrodynamic interpretation of the electromagnetic field. Some of the new sections that I've added, for example, 'Relation to and comparison with other physical fields' and 'Everyday applications' still need to be expanded. I also believe that the 'See also' list is too lengthy and only the most directly relevant links to the em field should be kept. MP (talk) 11:16, 4 April 2006 (UTC)
Here's an idea. Perhaps the extremely long list of links in the 'See also' section can be incorporated into the 'Applications' section. MP (talk) 09:20, 23 April 2006 (UTC)
Woaw ! Hold your horses! The EM field is 'an abstract mathematical field whose sources are charged elementary particles' - that statement is loaded wih inconsistencies, one of which is that if it's a mathematical field, then you can't mix that with physical concepts such as elementary particles. The EM is physically REAL and not just a mathematical concept - we only use the maths to formulate the theory precisely and test the theory against experimental results. If the EM field was a purely mathematical thing, then you can't really explain much in physics (such as the stablility of particles etc...). MP (talk) 12:30, 11 May 2006 (UTC)
I don't know what are these Maxwell-Hertz relations, which you refers to in the article ; are they the expression of Maxwell equations in the free space (with and ) ?
Almeo 10:01, 25 May 2006 (UTC)
I've copied the table from fundamental interaction and placed it in this article in the relevant section. The table gives a brief summary of comparing the 4 forces, but I'd like more details of the EMField (as compared to the other fields) in the form of a short description, just like the (incomplete) one I've given for EM and gravitation. I think there should also be some comparison of the field equations (not necessarily stated explicitly) - maybe the table could be extended to accommodate this. Just a few ideas. Comments appreciated. Thanks. MP (talk) 21:18, 10 June 2006 (UTC)
If you've been keeping track of this page, I'm sure you'll notice the somewhat largish edit I made. The biggest (but not the only) part of it was the addition of two examples showing how the EM field tensor transforms under a Lorentz transformation. I feel that the examples can help enlighten people, but I do have my concerns, so I thought I'd voice them here. Firstly, is the component by component listing of the computation too much or too long? Should the terms that are zero straight out be left out? Or should the whole of that be removed, and left as a so called exercise for the reader? Or is it fine as it is? Secondly, should the examples be there at all? On the one hand, I feel they help illustrate how fields can change due to a Lorentz transformation. On the other hand, people might not care, or might want it in a separate article or something. So, thoughts on that? I guess there's a third thing, which has to do with my addition to the section on the tensor formulation vis-a-vis Maxwell's equations. Basically, I just briefly added in some comments saying which of the traditional equations came from where. Should this be expanded upon to show explicitly how Maxwell's equations come out? Anyway, let me know what you think. Muchas gracias. DAG 09:14, 16 June 2006 (UTC)
I made two more edits. The first beefed up the section on the vector formulation somewhat, adding some discussion on the compatibility of Maxwell's equations with special relativity and such. The second, more substantial, was that I added a section on the potential formulation of Maxwell's equations. I felt that, given the inclusion of the other two formulations, this one deserved a place. My one big concern is kind of structural, in that towards the end I put a lot of equations inside a paragraph where they kind of break it up visually. But I felt (at the time at least) that that was the best way to include that info. Any ideas or comments on that? Or anything else? DAG 08:10, 17 June 2006 (UTC)
I don't suppose I could ask why User:Sinniko for all intents and purposes the article was more or less reverted to how it existed at the end of June 5 (specifically more or less to the edit on 12:51, 5 June 2006 by 217.250.90.165, with the only difference being the apparent addition of a space)? I'll admit some of what I added was long and such, which is why I asked for comments and suggestions here, but I also think that some of the stuff was worth having in there, at least briefly (which I'll admit I am anything but). If you're reading this Sinniko, could you explain what you thought was wrong with it so that we can move forward and continue improving it taking your thoughts into consideration? Thanks. DAG 10:49, 17 June 2006 (UTC)
"Indeed, it is believed by many that reconciling certain apparent coincidences in how two different observers moving relative to each other can explain the same effect with different explanations" seems to be an incomplete sentence.
It took me a long while before I realized that all our descriptions of physical forces are not actually descriptions of the forces, but descriptions of the math and logic of the theory. To use the tried and possibly true analogy; the theory is the map, and not the terrain. Thus when we say that EM is this or that, we're not actually talking about EM itself, but something which is three degrees of separation away from it.
And this is important, and more importantly, not obvious for a layman. No teacher I've ever has even hinted at this. For a student seeking to understand nature, this is a huge obstacle.
I'll let it be an excercise for the reader to draw their conclusions from this. And I'm hoping "damn troll" ain't one of them. I'm quite serious. -- Ceriel Nosforit 20:49, 19 July 2006 (UTC)
http://www.niehs.nih.gov/emfrapid/booklet/basics.htm This web page contains lots of information about emf, can well include some of it in the wiki, and in such a way as it is as easy to understand as in that article without annoying anyone more expeacned who is after equations and such?
I made an edit refering to http://www.niehs.nih.gov/emfrapid/booklet/basics.htm Whereby it states that electric feilds are made from current and magnetic fields from voltege, the article had it the other way around, however It may still be wrong as the "moving" and "not moving" bits may now be the wrong way around? Could someone look to see if this is the case?
Is this article a bit to simerlar http://en.wikipedia.org/wiki/Electromagnetism possibly needs a merge?
I see this was moved to the bottem of the page? Alan2here 18:49, 21 January 2007 (UTC) (sorry I forgot to sign the first time)
This article, In my opinion, is too difficult to comprehend for users of wikipedia who have not learned advanced physics and maths. For example, just reading the first paragraph confuses me even more than before I read this article. Also, the mass majority of the population would like a brief description of the electromagnetic field in something close to layman's terms. I understand that this may not be crutial, but it will certainly help. I thank whoever reads this for their time and attention in this matter.
Some readers will arrive at this page because they are interested in learning about the possible health risks associated with exposure to electromagnetic fields, an issue that has been the subject of voluminous and inconclusive research. This page should contain a (non-technical) overview of the research results.
The following paragraph from this section seems questionable:
One of the most common places EMFs can be found is near power lines which have both voltage and current running through them. Power = voltage times current, or, P = VI. Therefore if power needs to be increased, in order to ensure proper health and safety, the current should be changed accordingly rather than the voltage in order to decrease the danger of EMF caused by increased voltage.
Since the main advantage of AC current in power lines is to reduce power loss by decreasing the transmitted current, this suggested solution seems a bit silly. Probably should be deleted as "original research". I would do it myself but decided to ask first since EMF health edits may be potentially controversial. -- New 20:36, 19 September 2007 (UTC)
Okay, I deleted the offending paragraph. Also cleaned up the next paragraph which appeared to suggest a stronger stance by NIOSH than justified by the citation. New 21:50, 2 November 2007 (UTC)
Someone please look at the articles http://en.wikipedia.org/wiki/Electromagnetism and http://en.wikipedia.org/wiki/Electromagnetic_force with merge in mind. Im shure we don't need thee verry simerlar articles. Alan2here 15:59, 20 November 2006 (UTC)
I have made an attempt at better organising the material in the article. Given the plethora of maths in the article (much of which is unnecessary for the general reader to wade through, as it can be found elsewhere and is not really needed to understand the basics as presented in the article), I have decided to create a new article - Mathematical descriptions of the electromagnetic field - where the maths can be discussed in as much detail as one wishes. In the present article, I propose that a general overview of the mathematics of the EM field be given - mention vector fields, potentials, tensor fields, lagrangians, and the quantum stuff and indicate the purposes of the different approaches (e.g. vector field approach was historically the first one, then SR came along and now we use tensors, with QFT etc...).
I strongly suggest that the Lorentz force law be given a separate section, as it describes the basic (classical) approach to describing electromagnetic interactions. MP (talk) 18:58, 27 January 2007 (UTC)
The issue of vandalism and reversion has reared it's ugly head again. The 'Revision as of 04:51, April 18, 2007 (edit)' was vandalism but the revert 'Revision as of 13:52, April 18, 2007 (edit) (undo)' wasn't that great. Let's try to revert correctly. Thanks. :) MP ( talk• contribs) 20:26, 3 November 2007 (UTC)
"The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction."
This is contradicted by several observations:
1. Observable space is a finite size, but the Sloan Great Wall had time to form in a Universe that formed 14 billion years ago?
2. Beyond the weakest radiation we see (CMB), it's just dark
3. Hubble redshift
It seems the best way to reconcile all these observations is to suggest that the EM field does NOT in fact stretch across infinity.
It very clearly begins to weaken (Hubble redshift) and weaken (CMB) until it just dies out. —Preceding unsigned comment added by 98.145.87.121 ( talk) 16:45, 31 May 2008 (UTC)
I hear terms like electromagnetism and electromagnetic fields spewn forth unintelligibly by various individuals subscribing to bizarre spiritual philosophies. Maybe that's worth a mention in a small section of the article, describing perhaps certain colloquial uses of the term? This would of course have to be done by somebody with much less POV than me.
Also, what's with the "skip down to my topic" thing going on at the top of this talk page? Mbarbier ( talk) 17:01, 7 June 2008 (UTC)
I added the following text to the introduction:
It was reverted with the, quite correct, comment:
Strictly speaking this is correct; however there is a tendency to refer to what is essentially a magnetostatic field, possibly varying, as electromagnetic; I think the article should make the distinction clear in the introduction. Personally I wouldn't be (and wasn't) too concerned about niceties which can be explained in the appropriate part of th body. I'm thinking of statement such as "computer disc drives are subject to erasure by electromagnetic fields". In particular "electromagnetic" is often used inappropriately where electromagnets are involved. It would be useful for anyone who follows a link to this article to have the distinction clearly made in the first screenful. In practice there is a very clear distinction between a ray of light illuminating a tape and a magnet damaging a tape if it is very close (falling off as inverse cube of distance). Pol098 ( talk) 00:28, 30 August 2008 (UTC)
The section "External links" is not visible on display, and I suspect that the problem is related to the reference tags, but I do not know how to correct the problem. -- Wavelength ( talk) 02:42, 2 May 2010 (UTC) The categories also are not visible on display, and I mistakenly said in my edit summary that the article had no category. -- Wavelength ( talk) 02:59, 2 May 2010 (UTC)
The lede says that electromagnetic fields are "sometimes incorrectly [called] EMF". However, there's no citation for why EMF is supposedly wrong, and several official sources use the abbrevation EMF, such as:
So I'm going to edit that EMF is an acceptable abbreviation for electromagnetic fields. If I am in error somehow then the article should have an explanation as to why, since reputable bodies are using EMF as an abbreviation. MichaelBluejay ( talk) 06:30, 24 May 2010 (UTC)
I'm not entirely certain why the section is labelled as Health and Safety. The body of research on the topic indicates that there is no link between health problems and EM Fields, by and large. I certainly think that it should address things like MRIs. However, it would be more apt to describe it as Public Concern over Health and Safety, then further expanded to describe how all research on the topic, and all reports by major health organizations have found no link between the two.
While the CDC and the WHO have issued precautionary guidelines, they also state no evidence suggesting a connection.
WHO -- http://www.who.int/mediacentre/factsheets/fs304/en/index.html
NIH -- http://www.ncbi.nlm.nih.gov/pubmed/14628308
CDC -- http://www.cdc.gov/niosh/emf2.html
This position is also held by other organizations:
FCC/EPA -- http://www.osha.gov/SLTC/radiofrequencyradiation/epa_990430.html
AEEI -- http://www.aeei.gov.sk.ca/health-effects-and-exposure-guidelines-overview
I could go on, as there is a wealth of data on the topic. The section should be re-formatted as an explanation of the public concern and reasons, versus the wealth of scientific research on the topic, indicating that precautionary guidelines have been issued based on public concerns, not on evidence that there is any evidence suggesting a real health concern.
It should also have an explanation of the nocebo effect's effect on the research.
71.238.163.251 ( talk) 05:55, 17 December 2010 (UTC)
I propose that Flux density be merged into Electromagnetic field. I think that the content in the Flux density article can easily be explained in the context of Electromagnetic field , and the Electromagnetic field article is of a reasonable size in which the merging of Flux density will not cause any problems as far as article size or undue weight is concerned. Do not confuse Flux Density (different capitalization), which already redirects to Electromagnetic field. Chris the speller yack 01:13, 15 October 2011 (UTC)
Hi, I am new here, so maybe I miss something. According to me, the quantities H and D were never introduced in this page, beside the boundary conditions are not clearly explained (what "current free" and "charge free" mean ?). One possibility is to introduce the macroscopic Maxwell's equations (in a medium) in parallel with microscopic ones (in the vacuum). But this will be redundant with the Maxwell's equations page where this job is correctly done. I think this page is aimed to discuss general aspects of Electromagnetism without entering too much in theoretical frames. For instance I have seen that a fluid analogy was discussed as well as health issues. On my side I am working on a Galilean electromagnetism page that is much more consistent then the fluid analogy (but can be somehow linked to it), see Draft:Galilean electromagnetism. The two formalism Galilean and Relativist (Lorentz invariance)can help to disconnected the page from a specific mathematical frame. What do you think ? -- Henri BONDAR ( talk) 11:15, 4 February 2016 (UTC)
In other words is it Newtons per second or what exactly? I am curious. -- Ben Houston 03:43, 21 August 2006 (UTC)
[E]=V/m
[B]=T
Cheers -- 141.33.192.198 15:07, 19 December 2006 (UTC)
si 81.199.168.46 ( talk) 08:50, 6 October 2023 (UTC)
According to the article, an EM field is generated by moving charges. But doesn’t photon also gives rise to an EM field? — Kri ( talk) 14:44, 31 December 2023 (UTC)
As far as I can tell, the purpose of this article is to discuss classical electromagnetic fields. In any case it makes no sense to mix up quantum and classical in the way that was done. So I just cut out the quantum parts I could find. Johnjbarton ( talk) 02:13, 2 January 2024 (UTC)
I think the sections "Interference" and "Health and Safety" are issues for Electromagnetic radiation. Of course radio waves are time varying EM fields, but I would not expect to see these topics in a textbook discussion of EM fields until the issue of EM radiation is considered. Johnjbarton ( talk) 22:15, 6 January 2024 (UTC)
What's the point of it? Why is the article devoting so much space to a peculiar way of describing one aspect of electromagnetism in awkward bullet-point form? It's like a fragment of 2002 Wikipedia that nobody bothered to remove. XOR'easter ( talk) 23:53, 6 January 2024 (UTC)
I think the entire content of the Structure section and the figure should be removed in favor of a section on static field structure, eg Faraday iron filings like view. Johnjbarton ( talk) 23:03, 7 January 2024 (UTC)
(I should have learned by now).
We have Electrostatics and electric field and electromagnetic radiation and Classical electromagnetism.
Seems to me this article needs to be merged with one of the above. Or maybe converted into "Effects of electromagnetic radiation". Johnjbarton ( talk) 23:08, 7 January 2024 (UTC)
The intro has this sentence:
It ends with a reference supporting this claim.
The reference has an image including an antenna. I think the problem here is that only way to change the electric field is to move charges which create magnetic fields. The electric/magnetic field split is artificial there is only a solution to the field equations, not two things. So we can say they generate each other or we can say they are both generated by moving charges, but we can't decide because no physical experiment can distinguish these points of view. (I don't have a ref however).
@ Bill field pulse Do you have a reference for your point of view? Johnjbarton ( talk) 19:32, 8 January 2024 (UTC)
I had very deliberately used {{ sfn}}'s for the references to Purcell–Morin and to Feynman–Leighton–Sands, because both were being cited multiple times for different pages, and {{ rp}} would make for a lot of cluttered superscripts in the main text. Now Purcell–Morin is being cited with {{ rp}} and Feynman–Leighton–Sands is being cited with {{ sfn}}. Can we settle on a single way to do it so that this doesn't happen? XOR'easter ( talk) 18:38, 12 January 2024 (UTC)
<ref>...</ref>
is not too bad – sometimes a footnote pops up two levels (a harvard-style ref and then a full ref) and others just one level (a full ref). The reference section ends up as a mix of short and full, followed by a separate list of full refs. Does anyone object if I migrate the full refs to {{
sfnp}} as well? —
Quondum
13:58, 13 January 2024 (UTC)
rp
template is still a recommended solution. sfnp is not even mentioned.
Johnjbarton (
talk)
17:14, 14 January 2024 (UTC)
![]() | This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 |
Let's say a charged particle moves from point A to point B. It was initially at rest at A, then it suddenly accelerates, moves to B, then decelerates suddenly to stop at B. The electric field around the charge has to change to become centered at its new position. But it cannot change instantaneously throughout all of space because doing so would send information at a speed which is faster than light. So an electromagnetic wave is emitted which has the effect, after it dissipates, of recentering the electric field.
Now, if a charge is moving inertially at a given velocity and then suddenly accelerates to a new velocity but then stays moving inertially at that given velocity, then an electromagnetic wave is transmitted. This is called bremsstrahlung. This is what happens to electrons encountering the ionosphere: they decelerate and produce polar auroras.
If an charged particle is at rest, then it is surrounded by an electrostatic field which is symmetric in every direction. But what if the charge is moving inertially at a constant velocity? Then from its own frame of reference, it is surrounded by an electric field which is static, the same in every direction, without the presence of any magnetic field. But seen from a different frame of reference, the charge is moving, and the electric field has to move along with it, and this is where the problem sets in. If the charge moves along the x-axis, then let's say it moves from x0 towards the "right" to x1 during a unit of time. Point x2 lies directly in the particle's path. The electric field will point, say, away from the proton, and during the unit of time its magnitude will increase. This increase of the electric field at point x2 will cause a circular magnetic field line to be induced, which can be thought of as the rotation of a "magnetic fluid", and the "angular momentum" vector of this rotation points in the direction of the proton's velocity. Then, say there is a point x-1 behind the moving proton. It has an electric field vector pointing in the negative direction, and in one unit of time its magnitude decreases. This means that the electric field vector at point x-1 has increased in the positive direction, thereby inducing -- again -- a circular rotation of the magnetic fluid around it, such that the angular momentum vector of the rotation points in the direction of the proton's velocity.
Therefore the proton, when moving at constant velocity, is surrounded not only by a spherically-symmetric electric field which moves along with it, but also by a cylindrically-symmetric magnetic field whose axis of symmetry parallel to the velocity vector of the proton. Both of these fields decrease in magnitude inversely with the square of the distance. But what does this imply? At any point not directly in the proton's path, there is going to be a magnetic field vector which is perpendicular to the electric field vector. This means that there is going to be a Poynting vector at that point in space.
Points which lie on a transversal plane (perpendicular to the velocity) which includes the charge itself, will have Poynting vectors parallel to the velocity vector, and pointing in the same direction: forwards. Points on transversal planes which are in front of the charge will have Poynting vectors which will also be pointing not only forwards but also inwards towards the path of the charge. Points behing the charge will have Poynting vectors which point not only forwards but also away from the charge's path.
Poynting vectors are associated with electromagnetic waves: they indicate the direction of propagation and the power (rate of energy movement). A proton moving with constant velocity would have Poynting vectors which point in directions tangent to concentric spheres whose centers are the proton. These spheres move along with the proton. These Poynting vectors also all belong to planes which pass through the proton's path. The magnitudes of these Poynting vectors would vary as the fourth power of distance (?). What all these vectors do is to move energy in the direction of the proton. The electric field surrounding a static charge stores energy: this energy is proportional to the magnitude of the electric field vectors. When a charge moves at constant speed, the electric field around it has to move along, somehow, to keep up with its source. This implies a movement of electric energy, and the speed of this movement is power: hence the Poynting vectors.
But if these Poynting vectors describe E-M waves then these must be very unorthodox E-M waves, because they appear to move on the surfaces of spheres, starting all from a single point behind the charge and converging all again at a single antipodal point in front of the charge. Besides, the E and M fields moving along with the particle do not appear to oscillate at all, as would be expected of E-M waves.
Then, what happens if like charges are strung along a line which extends infinitely. If these charges move along their line, then they collectively form a current, even though the charge density remains constant. The constant charge density extending in an infinite line implies a cylindrically-symmetric electric field centered around the line, but the moving charges also imply a cylindrically-symmetric magnetic field. The magnetic field lines are circles whose centers are points on the line of the current. This can be seen to be a consolidation of the one-particle case, but is now equivalent to Ampère's law. Now the Poynting vectors are all parallel to the direction of the current, no matter where they are located in space. This would appear to imply that there are E-M waves moving parallel to the current; but then again, there are no oscillations of the E-M field. Perhaps these E-M waves are "degenerate", or "frozen". By the way, this is an example of a magnetostatic field. -- AugPi 10:12, 3 Apr 2004 (UTC)
It is nice to see a fresh treatment of a well-known subject. To balance this point of view, the two-fluid analogy should also treat the relationship of the two fluids to each other; we should see how B turns into E and vice versa, in an eternal harmonic motion. The fluid analogy, in fairness to Maxwell's equations, should encompass this.
LePage had a nice model as well. It is not fluids, but the encyclopedia, if it exposes the fluid model, needs to set LePage's viewpoint out as well, for balance.
Also, Faraday's contribution, that the fluids were somehow viscous or rubbery and hence exerted force on the charges.
Also, what about the freeway analogy, which uses the irrotational nature of Maxwell's equations: ie the travel of cars on a freeway obeys Maxwell's equations as well! (covariant motion)
Also, what about Purcell's viewpoint that an electromagnetic field is 'something that crackles'. 169.207.89.249 19:37, 3 Jan 2004 (UTC)
It seems clear that as a front of electrical fluid propogated away from a positive source charge, its velocity of propogation would decrease, which in turn would induce a magnetic field. Is this correct? As someone who doesn't understand EM concepts very well, I think the entry needs to explore in more depth the implications of the fluid analogy.
The fluid analogy does not work in this sense: that objects immersed in a moving fluid (e.g. a river) tend to be pushed by that fluid in such a way that the velocity of the object aligns with the velocity of the fluid. Once the velocities are aligned, the fluid's motion should vanish from the object's point of view.
However, the force of an electric field on a charged particle is , and this force is independent of the velocity of the particle, which means that the particle will accelerate continually in the direction of the field. If the field is the velocity field of a fluid then the fluid would be causing the object to accelerate continually in the direction of the fluid's motion, to the point that the object's speed becomes way larger than the fluid it is immersed in. This is paradoxical.
From the continually accelerating object's point of view, if its speed has already surpassed the speed of the fluid, then the fluid is moving backwards, so the field should be pointing in the direction opposite to the direction in which the object keeps accelerating. This means that that the object should stop accelerating and begin decelerating, until its speed aligns with the speed of the electric fluid.
An alternative interpretation would be that the field is not actually a velocity field, but a density field of photonic fluid, which is constantly moving at the same speed: the speed of light, independent of the speed of the observer (the charged object). Photonic fluid never changes speed but can change net direction and the intensity of its net movement in that direction. This interpretation would have to be verified by someone who knows QED. -- AugPi 02:48, 24 Mar 2004 (UTC).
I just noticed this page listed on the "Peer Review" page, and thought I might add some comments. All these of course, are to be understood as prefixed with "IMHO". The article seems to have a lot of discussion on the fluid analogy and other analogies, so my comments are regarding this aspect.
To summarize : There is no point pushing the fluid analogy this far - it seems to be hiding more relevant ideas.
[[User:AmarChandra| Amar | Talk]] 17:18, Jun 30, 2004 (UTC)
I think the fluid analogy in this article hurts more than helps. Most people have a strong intuitive sense of how a fluid should behave, and EM fields do not behave like fluids. They behave like force fields. The article currently relies so heavily on the fluid analogy that I think some people will be confused into thinking that there actually is a ethereal fluid of some kind that makes these processes work, which is not true at all (as far as we know).
I think a better approach would be to help people form an intuition about what a force field (vector field) is. This article is crying out for diagrams; two or three pictures would greatly enhance any verbal explanation. I will try to contribute more specific ideas later as time permits. -- Beland 04:04, 4 Jul 2004 (UTC)
Some comments:
Hope it helps... Pcarbonn 20:13, 13 Jul 2004 (UTC)
The article contains a lot of useful and important information. The presentation could be improved, however; for example, the notation is undefined in places (although it's obvious to those familiar with the notation). Also, there really should be a section on the maths describing the EM field (which should discuss the various formulations, e.g. vector field structure, as compared to the tensor field approach, quantum formulation etc...). MP (talk) 13:45, 4 December 2005 (UTC)
I have rewritten the article to give more focus on the em field itself, rather than the fluid interpretations (which are not that important). However, as a courtesy to the work of other editors, I have moved the bulk of the fluid interpretation work to a new article: hydrodynamic interpretation of the electromagnetic field. Some of the new sections that I've added, for example, 'Relation to and comparison with other physical fields' and 'Everyday applications' still need to be expanded. I also believe that the 'See also' list is too lengthy and only the most directly relevant links to the em field should be kept. MP (talk) 11:16, 4 April 2006 (UTC)
Here's an idea. Perhaps the extremely long list of links in the 'See also' section can be incorporated into the 'Applications' section. MP (talk) 09:20, 23 April 2006 (UTC)
Woaw ! Hold your horses! The EM field is 'an abstract mathematical field whose sources are charged elementary particles' - that statement is loaded wih inconsistencies, one of which is that if it's a mathematical field, then you can't mix that with physical concepts such as elementary particles. The EM is physically REAL and not just a mathematical concept - we only use the maths to formulate the theory precisely and test the theory against experimental results. If the EM field was a purely mathematical thing, then you can't really explain much in physics (such as the stablility of particles etc...). MP (talk) 12:30, 11 May 2006 (UTC)
I don't know what are these Maxwell-Hertz relations, which you refers to in the article ; are they the expression of Maxwell equations in the free space (with and ) ?
Almeo 10:01, 25 May 2006 (UTC)
I've copied the table from fundamental interaction and placed it in this article in the relevant section. The table gives a brief summary of comparing the 4 forces, but I'd like more details of the EMField (as compared to the other fields) in the form of a short description, just like the (incomplete) one I've given for EM and gravitation. I think there should also be some comparison of the field equations (not necessarily stated explicitly) - maybe the table could be extended to accommodate this. Just a few ideas. Comments appreciated. Thanks. MP (talk) 21:18, 10 June 2006 (UTC)
If you've been keeping track of this page, I'm sure you'll notice the somewhat largish edit I made. The biggest (but not the only) part of it was the addition of two examples showing how the EM field tensor transforms under a Lorentz transformation. I feel that the examples can help enlighten people, but I do have my concerns, so I thought I'd voice them here. Firstly, is the component by component listing of the computation too much or too long? Should the terms that are zero straight out be left out? Or should the whole of that be removed, and left as a so called exercise for the reader? Or is it fine as it is? Secondly, should the examples be there at all? On the one hand, I feel they help illustrate how fields can change due to a Lorentz transformation. On the other hand, people might not care, or might want it in a separate article or something. So, thoughts on that? I guess there's a third thing, which has to do with my addition to the section on the tensor formulation vis-a-vis Maxwell's equations. Basically, I just briefly added in some comments saying which of the traditional equations came from where. Should this be expanded upon to show explicitly how Maxwell's equations come out? Anyway, let me know what you think. Muchas gracias. DAG 09:14, 16 June 2006 (UTC)
I made two more edits. The first beefed up the section on the vector formulation somewhat, adding some discussion on the compatibility of Maxwell's equations with special relativity and such. The second, more substantial, was that I added a section on the potential formulation of Maxwell's equations. I felt that, given the inclusion of the other two formulations, this one deserved a place. My one big concern is kind of structural, in that towards the end I put a lot of equations inside a paragraph where they kind of break it up visually. But I felt (at the time at least) that that was the best way to include that info. Any ideas or comments on that? Or anything else? DAG 08:10, 17 June 2006 (UTC)
I don't suppose I could ask why User:Sinniko for all intents and purposes the article was more or less reverted to how it existed at the end of June 5 (specifically more or less to the edit on 12:51, 5 June 2006 by 217.250.90.165, with the only difference being the apparent addition of a space)? I'll admit some of what I added was long and such, which is why I asked for comments and suggestions here, but I also think that some of the stuff was worth having in there, at least briefly (which I'll admit I am anything but). If you're reading this Sinniko, could you explain what you thought was wrong with it so that we can move forward and continue improving it taking your thoughts into consideration? Thanks. DAG 10:49, 17 June 2006 (UTC)
"Indeed, it is believed by many that reconciling certain apparent coincidences in how two different observers moving relative to each other can explain the same effect with different explanations" seems to be an incomplete sentence.
It took me a long while before I realized that all our descriptions of physical forces are not actually descriptions of the forces, but descriptions of the math and logic of the theory. To use the tried and possibly true analogy; the theory is the map, and not the terrain. Thus when we say that EM is this or that, we're not actually talking about EM itself, but something which is three degrees of separation away from it.
And this is important, and more importantly, not obvious for a layman. No teacher I've ever has even hinted at this. For a student seeking to understand nature, this is a huge obstacle.
I'll let it be an excercise for the reader to draw their conclusions from this. And I'm hoping "damn troll" ain't one of them. I'm quite serious. -- Ceriel Nosforit 20:49, 19 July 2006 (UTC)
http://www.niehs.nih.gov/emfrapid/booklet/basics.htm This web page contains lots of information about emf, can well include some of it in the wiki, and in such a way as it is as easy to understand as in that article without annoying anyone more expeacned who is after equations and such?
I made an edit refering to http://www.niehs.nih.gov/emfrapid/booklet/basics.htm Whereby it states that electric feilds are made from current and magnetic fields from voltege, the article had it the other way around, however It may still be wrong as the "moving" and "not moving" bits may now be the wrong way around? Could someone look to see if this is the case?
Is this article a bit to simerlar http://en.wikipedia.org/wiki/Electromagnetism possibly needs a merge?
I see this was moved to the bottem of the page? Alan2here 18:49, 21 January 2007 (UTC) (sorry I forgot to sign the first time)
This article, In my opinion, is too difficult to comprehend for users of wikipedia who have not learned advanced physics and maths. For example, just reading the first paragraph confuses me even more than before I read this article. Also, the mass majority of the population would like a brief description of the electromagnetic field in something close to layman's terms. I understand that this may not be crutial, but it will certainly help. I thank whoever reads this for their time and attention in this matter.
Some readers will arrive at this page because they are interested in learning about the possible health risks associated with exposure to electromagnetic fields, an issue that has been the subject of voluminous and inconclusive research. This page should contain a (non-technical) overview of the research results.
The following paragraph from this section seems questionable:
One of the most common places EMFs can be found is near power lines which have both voltage and current running through them. Power = voltage times current, or, P = VI. Therefore if power needs to be increased, in order to ensure proper health and safety, the current should be changed accordingly rather than the voltage in order to decrease the danger of EMF caused by increased voltage.
Since the main advantage of AC current in power lines is to reduce power loss by decreasing the transmitted current, this suggested solution seems a bit silly. Probably should be deleted as "original research". I would do it myself but decided to ask first since EMF health edits may be potentially controversial. -- New 20:36, 19 September 2007 (UTC)
Okay, I deleted the offending paragraph. Also cleaned up the next paragraph which appeared to suggest a stronger stance by NIOSH than justified by the citation. New 21:50, 2 November 2007 (UTC)
Someone please look at the articles http://en.wikipedia.org/wiki/Electromagnetism and http://en.wikipedia.org/wiki/Electromagnetic_force with merge in mind. Im shure we don't need thee verry simerlar articles. Alan2here 15:59, 20 November 2006 (UTC)
I have made an attempt at better organising the material in the article. Given the plethora of maths in the article (much of which is unnecessary for the general reader to wade through, as it can be found elsewhere and is not really needed to understand the basics as presented in the article), I have decided to create a new article - Mathematical descriptions of the electromagnetic field - where the maths can be discussed in as much detail as one wishes. In the present article, I propose that a general overview of the mathematics of the EM field be given - mention vector fields, potentials, tensor fields, lagrangians, and the quantum stuff and indicate the purposes of the different approaches (e.g. vector field approach was historically the first one, then SR came along and now we use tensors, with QFT etc...).
I strongly suggest that the Lorentz force law be given a separate section, as it describes the basic (classical) approach to describing electromagnetic interactions. MP (talk) 18:58, 27 January 2007 (UTC)
The issue of vandalism and reversion has reared it's ugly head again. The 'Revision as of 04:51, April 18, 2007 (edit)' was vandalism but the revert 'Revision as of 13:52, April 18, 2007 (edit) (undo)' wasn't that great. Let's try to revert correctly. Thanks. :) MP ( talk• contribs) 20:26, 3 November 2007 (UTC)
"The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction."
This is contradicted by several observations:
1. Observable space is a finite size, but the Sloan Great Wall had time to form in a Universe that formed 14 billion years ago?
2. Beyond the weakest radiation we see (CMB), it's just dark
3. Hubble redshift
It seems the best way to reconcile all these observations is to suggest that the EM field does NOT in fact stretch across infinity.
It very clearly begins to weaken (Hubble redshift) and weaken (CMB) until it just dies out. —Preceding unsigned comment added by 98.145.87.121 ( talk) 16:45, 31 May 2008 (UTC)
I hear terms like electromagnetism and electromagnetic fields spewn forth unintelligibly by various individuals subscribing to bizarre spiritual philosophies. Maybe that's worth a mention in a small section of the article, describing perhaps certain colloquial uses of the term? This would of course have to be done by somebody with much less POV than me.
Also, what's with the "skip down to my topic" thing going on at the top of this talk page? Mbarbier ( talk) 17:01, 7 June 2008 (UTC)
I added the following text to the introduction:
It was reverted with the, quite correct, comment:
Strictly speaking this is correct; however there is a tendency to refer to what is essentially a magnetostatic field, possibly varying, as electromagnetic; I think the article should make the distinction clear in the introduction. Personally I wouldn't be (and wasn't) too concerned about niceties which can be explained in the appropriate part of th body. I'm thinking of statement such as "computer disc drives are subject to erasure by electromagnetic fields". In particular "electromagnetic" is often used inappropriately where electromagnets are involved. It would be useful for anyone who follows a link to this article to have the distinction clearly made in the first screenful. In practice there is a very clear distinction between a ray of light illuminating a tape and a magnet damaging a tape if it is very close (falling off as inverse cube of distance). Pol098 ( talk) 00:28, 30 August 2008 (UTC)
The section "External links" is not visible on display, and I suspect that the problem is related to the reference tags, but I do not know how to correct the problem. -- Wavelength ( talk) 02:42, 2 May 2010 (UTC) The categories also are not visible on display, and I mistakenly said in my edit summary that the article had no category. -- Wavelength ( talk) 02:59, 2 May 2010 (UTC)
The lede says that electromagnetic fields are "sometimes incorrectly [called] EMF". However, there's no citation for why EMF is supposedly wrong, and several official sources use the abbrevation EMF, such as:
So I'm going to edit that EMF is an acceptable abbreviation for electromagnetic fields. If I am in error somehow then the article should have an explanation as to why, since reputable bodies are using EMF as an abbreviation. MichaelBluejay ( talk) 06:30, 24 May 2010 (UTC)
I'm not entirely certain why the section is labelled as Health and Safety. The body of research on the topic indicates that there is no link between health problems and EM Fields, by and large. I certainly think that it should address things like MRIs. However, it would be more apt to describe it as Public Concern over Health and Safety, then further expanded to describe how all research on the topic, and all reports by major health organizations have found no link between the two.
While the CDC and the WHO have issued precautionary guidelines, they also state no evidence suggesting a connection.
WHO -- http://www.who.int/mediacentre/factsheets/fs304/en/index.html
NIH -- http://www.ncbi.nlm.nih.gov/pubmed/14628308
CDC -- http://www.cdc.gov/niosh/emf2.html
This position is also held by other organizations:
FCC/EPA -- http://www.osha.gov/SLTC/radiofrequencyradiation/epa_990430.html
AEEI -- http://www.aeei.gov.sk.ca/health-effects-and-exposure-guidelines-overview
I could go on, as there is a wealth of data on the topic. The section should be re-formatted as an explanation of the public concern and reasons, versus the wealth of scientific research on the topic, indicating that precautionary guidelines have been issued based on public concerns, not on evidence that there is any evidence suggesting a real health concern.
It should also have an explanation of the nocebo effect's effect on the research.
71.238.163.251 ( talk) 05:55, 17 December 2010 (UTC)
I propose that Flux density be merged into Electromagnetic field. I think that the content in the Flux density article can easily be explained in the context of Electromagnetic field , and the Electromagnetic field article is of a reasonable size in which the merging of Flux density will not cause any problems as far as article size or undue weight is concerned. Do not confuse Flux Density (different capitalization), which already redirects to Electromagnetic field. Chris the speller yack 01:13, 15 October 2011 (UTC)
Hi, I am new here, so maybe I miss something. According to me, the quantities H and D were never introduced in this page, beside the boundary conditions are not clearly explained (what "current free" and "charge free" mean ?). One possibility is to introduce the macroscopic Maxwell's equations (in a medium) in parallel with microscopic ones (in the vacuum). But this will be redundant with the Maxwell's equations page where this job is correctly done. I think this page is aimed to discuss general aspects of Electromagnetism without entering too much in theoretical frames. For instance I have seen that a fluid analogy was discussed as well as health issues. On my side I am working on a Galilean electromagnetism page that is much more consistent then the fluid analogy (but can be somehow linked to it), see Draft:Galilean electromagnetism. The two formalism Galilean and Relativist (Lorentz invariance)can help to disconnected the page from a specific mathematical frame. What do you think ? -- Henri BONDAR ( talk) 11:15, 4 February 2016 (UTC)
In other words is it Newtons per second or what exactly? I am curious. -- Ben Houston 03:43, 21 August 2006 (UTC)
[E]=V/m
[B]=T
Cheers -- 141.33.192.198 15:07, 19 December 2006 (UTC)
si 81.199.168.46 ( talk) 08:50, 6 October 2023 (UTC)
According to the article, an EM field is generated by moving charges. But doesn’t photon also gives rise to an EM field? — Kri ( talk) 14:44, 31 December 2023 (UTC)
As far as I can tell, the purpose of this article is to discuss classical electromagnetic fields. In any case it makes no sense to mix up quantum and classical in the way that was done. So I just cut out the quantum parts I could find. Johnjbarton ( talk) 02:13, 2 January 2024 (UTC)
I think the sections "Interference" and "Health and Safety" are issues for Electromagnetic radiation. Of course radio waves are time varying EM fields, but I would not expect to see these topics in a textbook discussion of EM fields until the issue of EM radiation is considered. Johnjbarton ( talk) 22:15, 6 January 2024 (UTC)
What's the point of it? Why is the article devoting so much space to a peculiar way of describing one aspect of electromagnetism in awkward bullet-point form? It's like a fragment of 2002 Wikipedia that nobody bothered to remove. XOR'easter ( talk) 23:53, 6 January 2024 (UTC)
I think the entire content of the Structure section and the figure should be removed in favor of a section on static field structure, eg Faraday iron filings like view. Johnjbarton ( talk) 23:03, 7 January 2024 (UTC)
(I should have learned by now).
We have Electrostatics and electric field and electromagnetic radiation and Classical electromagnetism.
Seems to me this article needs to be merged with one of the above. Or maybe converted into "Effects of electromagnetic radiation". Johnjbarton ( talk) 23:08, 7 January 2024 (UTC)
The intro has this sentence:
It ends with a reference supporting this claim.
The reference has an image including an antenna. I think the problem here is that only way to change the electric field is to move charges which create magnetic fields. The electric/magnetic field split is artificial there is only a solution to the field equations, not two things. So we can say they generate each other or we can say they are both generated by moving charges, but we can't decide because no physical experiment can distinguish these points of view. (I don't have a ref however).
@ Bill field pulse Do you have a reference for your point of view? Johnjbarton ( talk) 19:32, 8 January 2024 (UTC)
I had very deliberately used {{ sfn}}'s for the references to Purcell–Morin and to Feynman–Leighton–Sands, because both were being cited multiple times for different pages, and {{ rp}} would make for a lot of cluttered superscripts in the main text. Now Purcell–Morin is being cited with {{ rp}} and Feynman–Leighton–Sands is being cited with {{ sfn}}. Can we settle on a single way to do it so that this doesn't happen? XOR'easter ( talk) 18:38, 12 January 2024 (UTC)
<ref>...</ref>
is not too bad – sometimes a footnote pops up two levels (a harvard-style ref and then a full ref) and others just one level (a full ref). The reference section ends up as a mix of short and full, followed by a separate list of full refs. Does anyone object if I migrate the full refs to {{
sfnp}} as well? —
Quondum
13:58, 13 January 2024 (UTC)
rp
template is still a recommended solution. sfnp is not even mentioned.
Johnjbarton (
talk)
17:14, 14 January 2024 (UTC)