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Changing magnetic field

  1. Sep 24, 2008 #1
    if a changing magnetic field creates a changing electric field and a changing field electric field creates a changing magnetic field, then it will result in EM radiation..
    so if i move or shake a magnet,will that result in EM radiation?
  2. jcsd
  3. Sep 24, 2008 #2


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    sure. at a very low frequency (and with pretty low intensity). if you set up a coil of wire in the room and connect it to a very sensitive voltmeter, as you wave the magnet around, you will see the voltage change also. might be pico-volts and the 60 Hz hum that permeates the air around us (50 Hz in Europe) might totally swamp out your EM wave.
  4. Sep 25, 2008 #3
    so if we move the magnet accordingly,we will get EM radiation at visible light frequency...so is this a magnetic torch because light is produced because of magnet? is there any relation between moving the magnet and the frequency of radiation produced because of that
  5. Sep 25, 2008 #4


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    "accordingly" means you have to move it back and forth about 500 trillion times per second. and you won't be able to move it very far in a quadrillionth second before you have to reverse direction and move it in the opposite manner. doesn't matter so much if it's a magnet or a "static" charge, the two interactions are fundamentally the same.

    imagine that you and i are standing some distance apart and facing each other. you're holding a positive charge and i am holding a negative charge and that we both are restricting our charges so they cannot move toward each other but they can move up and down and left and right (just not forward or backward). so i move my charge up a meter. since your charge is attracted to mine, your charge also wants to move up a meter and you allow that. then i move it down and your charge follows it down. now i move it to my right (your left) and your charge moves toward your left. then to my left (your right) and your charge follows it.

    now i move my charge up and down repeatedly and your charge follows it up and down. that is an electromagnetic wave that originated with me moving my charge around and that wave moved toward you (at the speed of propagation of E&M waves which is "c") and causes your charge to move correspondingly. in a very real sense, my moving charge is a "transmitting antenna" and your moving charge is a "receiving antenna". if, somehow, i could move my charge up and down a million times per second, you could tune your AM radio to 1000 kHz and hear a signal (a silent carrier). if i could move it up and down 100 million times per second, you could tune it in with your FM radio just between the 99.9 and 100.1 settings (provided no other stations were close by). if i could move it up and down 500 trillion times per second, you would see it as a blur of orange colored light. now i can't move it up and down an entire meter 500 trillion times per second because the speed of that movement would exceed c. but i can have a whole pile of like charges and move them up and down maybe 10 microns at a frequency of 500 trillion Hz. that is what happens in a transmitting antenna or something that emits visible light. charges are moving and that causes other charges to move. but they don't react instantaneously (as observed by a third party that is equi-distant to you and i).

    that is what light is (from a wave-property perspective, no mention of photons here) and it required no medium for these waves to travel. they just are there because unlike charges attract and like charges repel (that's the fundamental physics) - there need be no medium in between for that to happen.
  6. Sep 26, 2008 #5
    Thanks rbj..A very clear explanation for guys like me..
  7. Sep 26, 2008 #6
    Hi there. (First post here.)
    Thanks for the interesting description.
    I think I would take issue with that last speculation though. Perhaps we aren't clear on what (if any) medium that these forces affect, but I think we also don't know the mechanism by which this "information" is transmitted through space.
    If the Large Hadron Collider discovers new dimensions, perhaps we will get some clues.
  8. Sep 27, 2008 #7
    I would also take issue. I do not know if there have been any achievable experiments with regards to the pervasive "virtual particle" aspect of "space' and it's relationship with photon/EMF propagation for example, but that would be interesting.

    In any event, great posts by rbj and JerryT.
  9. Sep 27, 2008 #8


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    People often consider the Casimir effect experimental evidence for this.
  10. Sep 27, 2008 #9
    1. Ask yourself: if you get in motion electron (with magnetic moment), is there EM radiation?
    2. If this motion is the ground state of H atom, is there EM radiation?
    3. If this motion isn't the ground state of H atom, is there EM radiation?

    So may be You should get more information about circumstances ?
    As said by aincient Rome citizens in Latin: Rebus dictantibus
  11. Sep 30, 2008 #10

    I am confused..
  12. Oct 1, 2008 #11
    is there any relation for this changing magnetic field dphi/dt and frequency of the emitted electromagnetic radiation...i think these two should be directly proportional..

    one another doubt..is this electromagnetic radiation is emitted from magnet,if so,will there be any change in the energy of the magnet..
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