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B Instantaneous collapse of magnetic field?

  1. Aug 18, 2015 #1
    Let's suppose you had an electromagnet generating a magnetic field. That field would expand at the speed of light.

    However, once upon a time, I was told that if this electromagnet were turned off, the field would completely instantaneously collapse, instead of collapsing at the speed of light, and thus the magnetic field would cease to expand (however weak it may be).

    This seems sketchy to me. Someone please explain.
  2. jcsd
  3. Aug 18, 2015 #2


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    Sounds to me like saying that if the sun magically disappeared, we would stop feeling its gravity instantaneously.
  4. Aug 18, 2015 #3
    Precisely why this seems sketchy to me.
  5. Aug 18, 2015 #4


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    I think "sketchy" is giving it too much credit.
  6. Aug 18, 2015 #5
    It would not collapse instantly. It could collapse very quickly, but not faster than the speed of light.

    It's very complicated, but the short answer is that electromagnetic effects are mediated by photons (light, radio, etc). These may be real or "virtual", but they all move at the speed of light because they are light.

    There is also the practical matter of the casual effect to stop the flow of charge getting to the coil and propigating around an d through it at, at most, the speed of light. Then you'd have to stop the electrons instantly by instantly extracting all of the energy you put in there to originally build-up the magnetic field.

    Nope, that wouldn't happen.

    Video about what magnetic fields really are.
  7. Aug 18, 2015 #6
    Mind you, this came an alleged hot-shot egghead, in regards to my inquiry about a system that would hypothetically allow you to craft flying teapots to go along with your flying saucers--save for this instantaneously-collapsing conundrum. (Though, I shan't yet go too deep in these details, lest I give a poor first impression. o.- )
  8. Aug 19, 2015 #7
    A change of heart, I may as well:

    Suppose you had two electromagnets bound to the same vessel.

    . . . . {a} . . . . {b} . . . .

    [a] first turns on and very quickly (assuming such a machine could be made), turning OFF before/just-as the field reaches . so:

    . .(+( . {a} . )-) . {b} . . . .

    Hypothetically, nothing happens yet. But now, just as the magnetic field of [a] reaches , turns on, as to interact with the field of [a] :
    (assuming the field does not instantaneously collapse when the electromagnet is turned OFF)

    (+( . . {a} . . )-)({b}) . . .

    Now here is where we have the quandary. Hypothetically, from the perspective of {b}. {a} is turned on and producing a magnetic field to interact with, to push from or pull to. However, from the perspective of {a} at this time, {b} is off and is presenting no such field to interact with, resulting in a 1-sided interaction and hypothetically locomotion.

    (if the system were optimized, the polarity of {a} would reverse just as its initial field reaches {b} instead of turning OFF, and I hope you can see where it goes from there . . .)
    Last edited: Aug 19, 2015
  9. Aug 19, 2015 #8
    Blargh forums. It should be fixed and readable now.
  10. Aug 19, 2015 #9


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    Welcome to the PF.

    We do not discuss or debunk crackpottery here. Please re-read the PF rules that you agreed to when you joined here (see Info at the top of the page). Thank you. :smile:
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