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Magnet Back and Forth.

  1. Feb 23, 2014 #1
    If i move a magnet back and forth in free space, will it radiate?

    If it does not then why?
     
  2. jcsd
  3. Feb 23, 2014 #2

    Vanadium 50

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  4. Feb 23, 2014 #3
    Why is it not used for practical purposes? Is it because it cannot produce high energy radiation?
     
  5. Feb 23, 2014 #4

    davenn

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    you got a vague answer because you asked a vague question

    there is a magnetic field out around a magnet regardless of if it is moving or not

    its used for many many different things ... electric motors just to name one


    what hi energy radiation ? ... a magnet produces a magnetic field

    cheers
    Dave
     
  6. Feb 23, 2014 #5
    But that is due to change in flux linkage, imagine a magnet in absolute free space. If i were to

    then move it back and forth, will it emit radiation just like an antenna?
     
  7. Feb 23, 2014 #6
    Move a magnet past a coil of wire (or move a coil past a magnet - the choice is yours) and a current flows in the wire. This is the basis of electricity production... essentially all thermal power stations and wind farms generate electricity using this process (solar PV panels don't).

    Energy taken from a moving magnet is used for practical purposes all over the world (but in case you're thinking you'd like to get some of that free energy, remember you're only taking out at an absolute maximum the same amount of energy you put in to move the magnet in the first place.)

    Far from this not being used for practical purposes, I'm honestly having trouble thinking of a phenomenon in the modern world put to more frequent practical use.
     
  8. Feb 23, 2014 #7
    ok, just read your latest post ... slightly different question then.
    But now I don't get it - you're thinking of something like, say, the earth - a big magnet moving in space? It has a magnetic field around it that moves with the earth. It doesn't "emit radiation" due to its magnetic core any more than it emits gravitational "radiation". Maybe on a quantum level you're thinking of force carrying particles that we haven't yet identified? Can you explain what kind of practical use you're talking about... propelling a spaceship by jiggling a magnet about or something?
     
  9. Feb 23, 2014 #8
    Okay, i think the question has been a little vague i guess. Let me rephrase.

    --An electron, if moved back and forth, emits radiation. This is the basis of antenna theory.

    --In contrast, any neutral metal will not radiate if moved back and forth because it is neutral and

    hence does not produce a magnetic field.

    --Now, if a magnet, which although has neutral charge on the whole, but has a magnetic field,

    is moved back and forth, will it radiate?
     
  10. Feb 23, 2014 #9
    I'm not really in my field (no pun intended) but I'd answer that antenna theory starts with an isolated negative charge (if we're using an electron) with a net negative charge spreading out into the universe. A magnetic pole is never isolated - you only ever (so far) see dipole magnets. By analogy you couldn't create an antenna by shaking a piece of net-charge-neutral metal. Happy to be out-argued.
     
  11. Feb 23, 2014 #10
    Yes, it will, as Vanadium 50 said in the first reply.

    The frequency of the radiation will be equal to the frequency of the oscillation of the magnet, of order 1 Hz. I don't think there is any practical use for such low-frequency radiation. Furthermore the total radiated power will be extremely low.
     
  12. Feb 24, 2014 #11
    Thank you everyone :)
     
  13. Feb 24, 2014 #12

    davenn

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    Electrons oscillate back and forward in a conductor when an AC current is applied
    the acceleration of that charge generates an electromagnetic field which will be radiated

    again, the only thing being radiated is a magnetic field


    there is NO RF radiation

    there's only a magnetic field, not an electromagnetic field because there is no AC current flowing to generate one

    Dave
     
    Last edited: Feb 24, 2014
  14. Feb 24, 2014 #13

    sophiecentaur

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    But the fact that there is a varying magnetic field must imply a varying induced electric field, surely.
    ∇E = -dB/dt
    any change that is 'forced' onto one field (as is waggling a magnet around) will induce a change in the other.

    It would be interesting to know whether anyone has gone to the trouble of spinning one of these new fangled super magnets at very high speed (100s of kHz would not be impossible) to see if the resulting RF field can be detected at a distance.
     
  15. Feb 24, 2014 #14

    Vanadium 50

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    There is a changing magnetic dipole, so there is RF (assuming you call a few Hz RF) radiation. As sophiecentaur points out, the changing magnetic field induces an electric field.

    The power radiated is:

    [tex]<P> = \frac{\mu_0 d_0^2 \omega^4}{12 \pi c^3}[/tex]

    Where the magnetic dipole is ##d = d_0 \cos(\omega t)##.

    Note that for a bar magnet being shaken this is very small. Everything in the numerator is small, and everything in the denominator is big.
     
  16. Feb 24, 2014 #15

    Vanadium 50

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    Plugging in numbers, I get something like 10-37 watts radiated by shaking a bar magnet back and forth.
     
  17. Feb 24, 2014 #16

    davenn

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    OK
    I see where you are coming from
    So you are saying that the field outside the magnet is then cutting back through the magnet as the magnet moves within the field and thus generating a current which in turn is generating an EM emission

    and OK so its freakin' tiny, nothing like what the OP was indicating or hoping for :wink:

    Dave
     
  18. Feb 25, 2014 #17

    No i guess, the magnet's movement induces closed electric loops in space everywhere, that is

    what Maxwell's equation suggests ∇XE = -∂B/∂t. The fact that if a magnet moves back and forth

    , there will be flux change everywhere, and hence an electric field given by ∇XE = -∂B/∂t.

    Also consequently, there will be a magnetic field due to the induced electric field, it will be

    ∇XH = -∂D/∂t . Where J = 0. So everything self propels to create RF.

    Am i right?
     
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