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How can magnetic field lines snap ?

  1. Apr 13, 2010 #1
  2. jcsd
  3. Apr 14, 2010 #2
    Re: How can magnetic field lines "snap"?

    I'm going to be sorry for taking this on, but here goes:

    First off magnetic field lines are bogus and don't exist! They are just a way of imagining fields that came from looking at iron filings but really don't have mathematical validity although one CAN "plot" a field creating drawings of "lines" but all the talk about counting "lines" etc. is really nonsense.

    So now that I've said that, lets talk about counting lines! :smile: Lines really are just a pictorial representation of a magnetic field. They show the directions of it in different locations and if you have a number of them they can sort of be related to intensity.

    Well, if that's true then how do they "snap"? Well first off, magnetic fields in free space do not "snap"! A "snap" implies something. It implies positive feedback! Toggle switches "snap". Positive feedback generates instability. Hence like a "flip-flop" if you have two stable states, as you edge from one to the other you reach a point where positive feedback takes over and forces the transition rapidly.

    Now when it comes to aurora borealis the key is that one is dealing with magnetic fields in a plasma. Plasmas are charged particles. Moving charged particles are currents. Currents create magnetic fields. Hence a LOT of VERY strange things happen in plasmas! And that sets up the possibility of positive feedback situations. The TOTAL magnetic field you view is actually TWO (or more) magnetic fields. The original one from the sun and/or earth plus the one generated by charge motion in the plasma.

    The bottom line is that you find a situation where there are two stable configurations separated by a positive feedback situation. Then as one is slightly moved toward the other the feedback takes over and it "snaps" to the new stable configuration. In this case that configuration produces charge acceleration resulting in the aurora. But without the plasma there are no positive feedback situations and no "snap" will occur.

    The "lines" don't really release "their" stored energy when they "snap". What you have is two different stable magnetic field configurations. One has a different energy than the other. When you "snap" from one to the other obviously that difference in energy is either absorbed or released.

    Does that help?
  4. Apr 14, 2010 #3


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    Science Advisor

    Re: How can magnetic field lines "snap"?

    Ok, I think I understand what you are saying above ... the lines are just a way of pictorially representing the magnetic flux density, right? So here's my goofy question for you ... if it is just a continuous field (and I agree that it is), then why does the iron filings experiment to "show the field lines" of a bar magnet work? Is there some sort of spontaneous symmetry breaking that goes on to create spatial regions with higher and lower magnetization? Would the same effect be observed in zero gravity using ferrofluid to show the 3-D "pattern of lines"?

    @Mods: if this is too much of a diversion from the OP's question, let me know and I'll start another thread.
  5. Apr 14, 2010 #4
    Re: How can magnetic field lines "snap"?

    The magnetic field should be evenly distributed. The simplest explanation would just be that the iron filings are creating their own smaller magnetic fields and attracting to each other, creating gaps between them that actually have nothing to do with the larger field.
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