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Are magnetic lines of force real ?

  1. Jun 9, 2004 #1
    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 !
  2. jcsd
  3. Jun 9, 2004 #2


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    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".
  4. Jun 10, 2004 #3
    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.
  5. Jun 11, 2004 #4
    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.

  6. Jun 19, 2004 #5
    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?
  7. Jun 19, 2004 #6


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    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.
  8. Jun 19, 2004 #7
    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.
  9. Jun 19, 2004 #8


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    The "density of field lines" is a measure of the strength of the field in that region.
  10. Jun 19, 2004 #9


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    Fair enough, as long as that definition is kept in mind.
  11. Jun 20, 2004 #10
    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")
    Last edited: Jun 20, 2004
  12. Jun 22, 2004 #11
    Strings who pull the charges? I haven't run into anything like this.
    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.

    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 untill it reached some apparent breaking point and snapped apart into two massive glowing filaments that slowing shrank back into the sun.
  13. Jun 22, 2004 #12


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    "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?
  14. Jun 22, 2004 #13
    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.

    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?
  15. Jun 22, 2004 #14
    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."

    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.
  16. Jun 22, 2004 #15


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    This article http://www-tfp.physik.uni-karlsruhe.de/~didaktik/publication/repr_of_fields_ajp.pdf [Broken] 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".
    Last edited by a moderator: May 1, 2017
  17. Jun 22, 2004 #16
    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.
  18. Jun 22, 2004 #17


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    This URL (gotten by pasting the URL in google's search box) is ugly, but it might work
    ...then "View as HTML". Unfortunately, the diagrams don't show up.
  19. Jun 22, 2004 #18
    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.
  20. Jun 22, 2004 #19
    I just ran across this film that reacts to magnetic fields so that you can see something like MRI slices of them:


    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.
  21. Jun 22, 2004 #20
    In response to the iron filings.

    If one looked at http://superstringtheory.com/forum/dualboard/messages11/354.html [Broken], 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.
    Last edited by a moderator: May 1, 2017
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