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Magnetic 'lines'

  1. Jun 1, 2009 #1
    Hi. I was thinking about magnetism, and remembered the classic experiment to show magnets working. Basically, you put a magnet under a piece of paper, and drop iron filings on the paper. The filings all line up along lines that curve between the poles. My question is, what are those lines? The first time I saw this experiment, I expected all the filings to just bunch up where the magnet is, why do they 'choose' those certain positions?
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  3. Jun 1, 2009 #2
    I think the answer to that question is less obvious than some people might think, especially if they were to respond with "Those are the field lines." (??)

    Here's what I think: the filings have a certain amount of friction with paper, which is what prevents them from bunching up the way you describe - I agree that is what they would do in the absence of any friction (or other) opposing force. But - even though they cannot easily slide across the paper, they can more easily rotate. Each little filing piece will become magnetized and develop N and S poles, and thus experience a torque from the external field. This torque will go to zero when the filing rotates so that it is aligned with the direction of the magnetic field vector at that point, so that alignment is an equilibrium configuration. When you have a whole bunch of filings, they all become magnetized and the effect is magnified, that is, they'll line up head-to-tail and give the appearance of lines.

    I have no idea if this is what's really happening, but it's my best guess! ;-)
  4. Jun 2, 2009 #3


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    As belliott4488 said it's just like that. The fillings become tiny magnets any they arrange acording to one another and the main magnet field lines ( something like that). nothing to add...

    Just that for the field lines to become more obvious at greater distances, lets say an entire A4 sheet, use about 8 magnets stacked on top of each other.
  5. Jun 2, 2009 #4
    I'm not sure I made myself well understood. I mean: What are field lines? I've been sort of thinking of them like contours in the field (as in, they depict lines of equal field strength), but that doesn't explain why the filings line up on them. I've got a feeling this may have to be a mathematical answer, with no practical equivalent.
  6. Jun 2, 2009 #5


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    They don't. The "lines" you sometimes see are not in any way related to field lines.
    Field lines are just mathematical constructs similar to contour lines on maps, they are useful for illustrating the shape/strength of a field but they are not real (they are also arbitrary, you can use as many lines as you want since the information about the field strength is in the density of lines)

    I must admit I don't have a good answer to why you sometimes see lines; although I am pretty sure it is a "many-body" effect where each individual filing is affected by the field of all other filings; presumably there is some critical distance over which this happens (related to the magnetization of the filings and the friction of the surface) which in turn will cause the lines to "clump" together in lines (with the number if filings/unit area being roughly proportional to the local field strength).
    I.e. it is a stochastic process but it is affected by some collective tendencies (I am sure there is a mathematical term for this)
    But note that this is just an educated guess.
  7. Jun 2, 2009 #6


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    My guess is that the lining up of iron filings is akin to the lining up of the magnetic domains within a ferrite bulk but the difference is that the iron filings can move to a certain degree (restricted by friction). The applied magnetic field will induce filings to reorient themselves to line up their own magnetic domains in relation to the applied field. So if we line up the filings head to toe, it is less energetic then if they are side by side. At side by side, you have like poles in the filing's domains lining up which causes a repulsive force between the filings. Perhaps it would even cause separation of the filings further demarcating these lines.

    N N
    | |
    | |
    S S

    If they clump along a line, they will line up the induced domains along opposite poles and thus reinforce the formation of a line from the mutual attraction.


    This would be my guess as to why we see these lines form, they are another minimization of the system's energy.
  8. Jun 3, 2009 #7
    Ah, I get it. They're not field lines, but because each iron filing is magnetized in the field, they all line up end to end. Thanks for the replies.
  9. Jun 3, 2009 #8
    Yes. And they don't clump up in the middle because the paper exerts a frictional force on them. They can rotate much more easily than they can slide. However, you will find that you usually have to shake the paper a little bit to get them to line up at all. In that case the friction is large enough to stop them rotating as well, so by shaking the paper the iron fillings jump up and down a bit, and they can then rotate when they are free of the paper.

    The field lines are just a mathematical construct that helps us visualize (for example) magnetic fields. They represent lines of force; the magnetic force is parallel to the field lines at all points.
  10. Jun 4, 2009 #9
    Actually those lines of forces are a resultant of the M.F at that point.

    There are magnetic sources also...like the bar magnet's opposite charge for example.

    So these lines of forces are made as a resultant considering all other source magnetic fields in the universe.

    So when a line curves...its cause of some other magnet nearby it.
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