Are magnetic lines of force real ?

[smithpa9]

Are magnetic lines of force "real" ?

You know the standard demonstration showing iron filings line up on a piece of paper over a bar magnet to illustrate the magnetic "lines of force?"

I'm curious if the line of iron filings really represent places on that plane that are "special" (more "special" than the empty places between them and the next line), and if so, what's so special about them? Or do they just indicate the general shape and direction of the field.

Specifically, I'm curious if that experiement with iron filings was repeated several times (with the paper and magnet in exactly the same places), would the iron filings line up in the same place on the paper every time? Or would they just line up generally in the same shape, curving from one pole to the other -- with a line in one experiement just as likely to lie halfway between the places where two lines had fallen on the previous experiment as it would be to fall on exactly the same curve.

Thanks !

 

[HallsofIvy]

There are no "empty" spaces between lines of magnetic force. If the filings had fallen a little differently, the lines you see would lie in different places. They would, however, still point in the same directions. That is the sense in which they are "real".

 

[JohnDubYa]

The iron filings separate from each other because their ends become slightly magnetized, with the same polarity facing the same direction. Since the ends that are closest to each other have the same polarity, the two filings repel slightly.

 

[pmb_phy]

There are no "real" lines of force. Fields lines are simply a mathematical construct used for visualization only. Only the allignment, i.e. direction, of the filings are measurable.

There are cases where thinking in terms of field lines can get you into trouble and people have made mistakes in the physics literature because they took this field line thing too seriously.

Pete

 

[zoobyshoe]

There are no "real" lines of force. Fields lines are simply a mathematical construct used for visualization only. Only the allignment, i.e. direction, of the filings are measurable.

There are cases where thinking in terms of field lines can get you into trouble and people have made mistakes in the physics literature because they took this field line thing too seriously.

Pete

So, it sounds like people are saying that any resemblence to authentic lines is actually an illusion created by the means of detecting the directions: the iron filings.

Although any given point in the field has a direction associated with it, is it actually suspected, believed, or has it been proven that the field is monocoque in structure?

What trouble does it lead to to suppose the lines are real? In what situations is that a liability?

 

[Janitor]

In answer to zoobyshoe, it seems to me that taking the lines-of-force view too seriously would give the impression that the field is null in the space between lines, though in fact the field would not be null.

 

[zoobyshoe]

In answer to zoobyshoe, it seems to me that taking the lines-of-force view too seriously would give the impression that the field is null in the space between lines, though in fact the field would not be null.

Yes, I can see that being a problem if someone were to attempt to base something practical on the spaces being null. Not sure where that might come up though.

 

[robphy]

In answer to zoobyshoe, it seems to me that taking the lines-of-force view too seriously would give the impression that the field is null in the space between lines, though in fact the field would not be null.

The "density of field lines" is a measure of the strength of the field in that region.

 

[Janitor]

Fair enough, as long as that definition is kept in mind.

 

[kuengb]

The biggest problem with field lines for me is that they are supposed to be an explanation for the magnetic/electric force itself (something like strings who pull the charges), but two seconds later people say that they are arranged the way they are because there is a repelling force between them. The snake bites its own tail.

As pmb says, field lines are good for visualization. They show the two properties of the E-/B-field in a vacuum:
1. It is source-free, divE=divB=0 (that is, field lines are continuous and do never end/begin in empty space)
2. It has locally a potential which fulfills the Laplace equation (which gives the typical arrangement of the lines, intuitionally something like "minimized tension")

 

[zoobyshoe]

The biggest problem with field lines for me is that they are supposed to be an explanation for the magnetic/electric force itself (something like strings who pull the charges),

Strings who pull the charges? I haven't run into anything like this.

but two seconds later people say that they are arranged the way they are because there is a repelling force between them. The snake bites its own tail.

I have been wondering about this "repelling force". It looks like they repell each other laterally. However,in the case of the electric lines, I've read this effect characterized as the result of the fact they always enter or leave a source as perpendiculary as possible. This, by itself, could explain why electric field lines appear to be bowing away from each other as if by repulsion. In fact they may be constrained to entering or leaving the charged object at right angles to it.

Don't know what "The snake bites it own tail, " means.

1. It is source-free, divE=divB=0 (that is, field lines are continuous and do never end/begin in empty space)

I have seen one phenomenon that may constitute an exception to this, which is the phenomenon of solar flares. I saw a film on TV of a great, massive loop of magnetic flux glowing with the plasma coursing through it, that expanded off the surface of the sun growing longer and longer until it reached some apparent breaking point and snapped apart into two massive glowing filaments that slowing shrank back into the sun.

 

[HallsofIvy]

"The biggest problem with field lines for me is that they are supposed to be an explanation for the magnetic/electric force itself (something like strings who pull the charges)"

I have NEVER seen field lines used as an "explanation" for force. Could you cite a textbook that does?

 

[kuengb]

I have NEVER seen field lines used as an "explanation" for force. Could you cite a textbook that does?

No. Thank God no.

IF you look at field lines as something with physical reality, i. e. the field consisting of those lines, whatever they are, you HAVE to link them somehow with the force, don't you? If not, they have absolutely no physical meaning.

I have been wondering about this "repelling force". It looks like they repell each other laterally.

That's the point: It looks like. That's what I meant with the "snake biting its own tail" -- sorry I didn't know this expression isn't used in English :flush: , but I hope you get the image: In order to explain how the field looks, you introduce another force acting on...well, on what? There is no charge there. Or is the force between the lines again the EM-force? If yes, where are its field lines?

 

[zoobyshoe]

I find this quote from a Nasa educational site pretty thought provoking:

"To Faraday field lines were mainly a method of displaying the structure of the magnetic force. In space research, however, they have a much broader significance, because electrons and ions tend to stay attached to them, like beads on a wire, even becoming trapped when conditions are right. Because of this attachment, they define an `easy direction' in the rarefied gas of space, like the grain in a piece of wood, a direction in which ions and electrons, as well as electric currents (and certain radio-type waves), can easily move; in contrast, motion from one line to another is more difficult."

Magnetism
Address:http://www-istp.gsfc.nasa.gov/Education/Imagnet.html
This suggests that they are something more than abstract, potential paths. For something to become "attached to" or "trapped" implies the existence of something real enough to interact with ions, electrons, and electric currents. That something would have to be at least as real as they are to behave as the "easy direction" for them. That is the suggestion I percieve, here.

 

[robphy]

This article http://www-tfp.physik.uni-karlsruhe.de/~didaktik/publication/repr_of_fields_ajp.pdf looks like an interesting discussion of field lines... although I'm not quite sure about the author's suggestion that the "density of field lines" (by itself) is not a measure of the local field strength.

From a more abstract point of view [differential forms: http://www.ucolick.org/~burke/forms/draw.ps ], it seems that "field lines" may be better interpreted by "flux [density] tubes".

 

[zoobyshoe]

Thanks for the links, robphy, but the primitive webtv system by which I access the net has no provisions for reading pdf files, or any non-html material for that matter. I appreciate your effort, though.

 

[robphy]

Thanks for the links, robphy, but the primitive webtv system by which I access the net has no provisions for reading pdf files, or any non-html material for that matter. I appreciate your effort, though.

This URL (gotten by pasting the URL in google's search box) is ugly, but it might work
http://www.google.com/search?q=http...n/repr_of_fields_ajp.pdf&ie=UTF-8&hl=en&meta=
...then "View as HTML". Unfortunately, the diagrams don't show up.

 

[zoobyshoe]

The "view as html" option never works for me either. Trying it just now, I got what seems to be one page of text superimposed on another. Thanks for giving it a try, though.

 

[zoobyshoe]

I just ran across this film that reacts to magnetic fields so that you can see something like MRI slices of them:

Address:http://www.wondermagnet.com/viewingfilm.html

A whole page of interesting pictures. The images it creates remind me more of images of stress patterns in solids, than the classic iron filing lines.

 

[sol2]

In response to the iron filings.

If one looked at http://superstringtheory.com/forum/dualboard/messages11/354.html , it would become apparent would it not, that such coordinates are being described, so that from this, a consistancy is born out of seeking higher dimensional understanding?

The space in between those field lines are just more coordinates that have not been described, yet exist. In this context each filing is describing a coordination point there would have some assocaition here then in relation to the field and the filing?

Using a array of twelves magnets( a quarter inch wide by 1" by 3" the gauss strength was calculated)I laid them out in relation to a circle and fixed points, and quickly discern the differences between the N and S and utilized the em consideration not either N or S facing the center).

I was just playing around to see if the field would show any strangeness between the n and s and Em fixture I gave it when aligning one side or the other to the center. I alternated these features from zero to 360 degrees moving clockwise. EM, S,N, EM,S N,EM,S, N, EM until I completed the circle

I took photographs about 20 years ago that are amongst my files someplace. I wanted to see the field dynamcis of such a arrangement.

So at this point looking back I wonder then about the relationship of Gaussian coordinates here and the relationship between N and south from a magnetized filing point of view. How would we see the alignment from such considerations in a dynamical field?

If we are to consider magnetic resonance capabilties then the signature constituents would become self discriptive in such a field already existing?

What also lead to what might effect these fields was the issue of the faraday cage as a point of consideration. This lead to the diamagnetics properties of crystals in that field and eventually lead to questions on the principals of superconductors.

 

[jammieg]

I think what Smith is getting at is why are there lines anyway, shoudn't it be a nice neat homogenous field? What could possibly cause such a thing?

 

[zoobyshoe]

it seems that "field lines" may be better interpreted by "flux [density] tubes".

I tried googling different things, and when I put in "magnetic flux tubes" got many sites about the solar flares I mentioned earlier. Here's one:

Research Interests: Active Regions
Address:http://sprg.ssl.berkeley.edu/~abbett/flux.html

I couldn't make a lot of sense out of it since I didn't find a basic explanation of these "flux tubes" and how they relate to the average magnetic field, but it seems to be the phenomenon by which the solar plasmas are contained such that they can form those massive loops.

There are obviously many different research teams studying this - many, many different papers published about every concievable aspect of these flux tubes.

 

[Loren Booda]

Wouldn't quantized magnetism manifest discrete "field lines"?

 

[zoobyshoe]

Wouldn't quantized magnetism manifest discrete "field lines"?

Sounds logical to me but I have no idea if anyone approaches it that way. Could be there's a good reason not to. Could be there's nothing suggesting this would be fruitfull. First time I've encountered the notion.

 

[zoobyshoe]

** ** ** **
This article speaks of the magnetic field lines of the sun breaking and reconnecting:

"It has long been thought that flares and their turbulent lives are due to strong magnetic fields that interact with the plasma. Such magnetic fields break and reconnect in a process called magnetic reconnection. Oppositely directed magnetic lines are brought together into a strong "eddy," which physically pushes away the gas. In this process, magnetic energy is converted to kinetic and thermal energy that are released into space.

"Besides solar flares, magnetic reconnection is at the heart of other spectacular events in the solar system, some of which affect us earthlings via the Earth's magnetosphere. The Earth's magnetic field is constantly perturbed by the magnetic field of the sun (called the "solar wind"). Solar magnetic activity, including flares, can eject high-energy charged particles into space. When these particles reach earth, they can disrupt power grids and communications systems and threaten spacecraft and satellites. Northern lights (the aurora borealis) are also considered a product of magnetic reconnection.


Full article:** **
Magnetic Reconnection, Solar Flares, and You - March/April
Address:http://www.pnl.gov/er_news/04_98/art1.htm

 

[sol2]

I think what Smith is getting at is why are there lines anyway, shoudn't it be a nice neat homogenous field? What could possibly cause such a thing?

I mean imagine this space becoming ever smaller in regards to the greater energy determination? In Inverse Square law not only can we recognize the strength of the field generated in regards to EM consideration, but of gravitational ones as well.

So how would we look at this?

http://cerncourier.com/objects/2000/cernnews5_3-00.gif


Because we understand this dynamical movement in plasmatic features as supersymmetical consideration, one would have to understand how gravity moves to supergravity. If we understand the gravity field can have its differences( dimensional relationship?) then how we look at the Q<--->Q measure becomes a interesting relation in terms of understanding the metric in a different way?

Some might even like the http://wc0.worldcrossing.com/WebX?14@235.D6CLcH8KeYI.0@.1dde61c6/4 Do not forget to scroll down past heading and read Kaku's statement after linked post.

 

[robphy]

I just recalled some good presentations of "flux [density] tubes"
http://www.ee.byu.edu/ee/forms/
http://www.lgep.supelec.fr/mse/perso/ab/IEEEJapan2.pdf [see p. 10]