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I How do magnetic "return paths" really work?

  1. Apr 21, 2016 #1
    While reading about how magnetic flux works, I've come across two concepts that seem to contradict each other:

    Concept A: http://www.emfs.info/what/adding/ adding magnetic fields is simply vector addition. In other words, magnetic fields are always acting independently, and the sum of flux at any given location in space the just the sum total of independently behaving magnetic fields at that point in space.

    Concept B: putting magnets or permeable materials close together alters the "return path" of their magnetic flux lines. For example, a "closed core" transformer versus a "shell core", where the idea behind the shell is to bring the magnetic return path of the coil closer to the coil. This would suggest that the magnet flux of any given atom is not a fixed shape, but that it's shape can altered by other magnetic fields, which contradicts the idea that a combination of magnetic fields is simple vector addition.

    Are magnetic lines of flux really "shaped" by the other magnetic things around them, or do they always maintain a particular shape no matter what, and the concept of guiding "returns paths" merely an abstraction?
     
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  3. Apr 21, 2016 #2

    berkeman

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    Magnetic fields are attracted to ferrous metals, and magnets are generally made from ferrous metals. So when you bring the two magnets near each other, that will cause a distortion in their fields because the magnets are made from ferrous metals. It's not the fields interacting with each other, it's the fields being distorted by the material nearby.
     
  4. Apr 21, 2016 #3
    How can combining magnetic fields be simple vector addition if their fields are distorted? You'd no longer have "field A + field B", but "somehow distorted field A + somehow distorted field B". Does that make sense?
     
  5. Apr 21, 2016 #4

    berkeman

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  6. Apr 21, 2016 #5

    berkeman

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    You can combine them if the source is not a ferrous metal. For example, you can combine the B-fields generated by two current-carrying wires with simple vector addition. (As long as the wires are copper or some other non-ferrous metal).
     
  7. Apr 21, 2016 #6
    Do you have a source for this? The source I linked above did not mention caveats about magnetic flux lines only vector summing in non ferrous mediums.

    I've read that overlapping magnetic field obey the super position principle:
    https://en.wikipedia.org/wiki/Superposition_principle
    Furthermore, when a ferrous material is exposed to a magnetic field, it becomes a magnet also, so AFAIK, at that point you're rally just talking about yet another magnet being added to the equation, vector summing in the same manner as the permanent magnet that caused the ferrous material to magnetize.
     
  8. Apr 21, 2016 #7

    berkeman

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    BTW, I should mention that I was referring to static B-fields. If they are AC fields, they can induce currents in the other nearby conductors, which will change the overall combined B-fields.
    Not necessarily. It depends on the material. The wider the hysteresis curve for the material, the more residual magnetization you will have. But some materials have pretty skinny hysteresis curves...

    http://www.intechopen.com/source/html/48734/media/image28.jpeg
    image28.jpe
     
  9. Apr 21, 2016 #8
    AFAIK, the hysteresis curve is caused by the (un)willingness of the magnetic domains in the ferrous material to align, and/or re-align later, as is dictated by the particulars of the physical structure at the atomic level, but once they align, it is essentially a proper magnet from that point onward.

    If it's true that the concept of a guided magnet return path is an abstract concept, this would be big news to me, as I've never seen it stated as such.
     
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