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Electromagnetic wave

  1. Oct 21, 2012 #1
    I can't represent me mentally what is an electromagnetic wave.

    I know that a mechanical wave is the propagation of a disturbance in a material medium and mentally I can see what it is. But electromagnetic ....

    Last year, I did electromagnetism. We studied Maxwell equations and we saw a variation of the field E => B field variation but we never spoke of electromagnetic waves.

    what transport an electromagnetic wave? Energy without matter like a mechanical wave? should we see this as a particle (wave-particle duality)?

    i'm currently studying quantum mechanics. But before I get into the wave functions, it is best to know what we are talking about before ...

    Thank you for your answers which I hope will steer me well
     
  2. jcsd
  3. Oct 21, 2012 #2
    If you know about the electric field and the magnetic field, you know that neither is made of matter, yet they can both transmit a force.

    In fact, matter (solid objects) actually interact with each other, not by actual contact of the atoms, but though the repulsive electrostatic force between their orbiting electrons.

    So matter isn't more 'real' than electric/magnetic fields.

    It's difficult to get a grasp on these ideas, the best you can do is to build mental images of models in the 'real' world that behave a bit like the thing you are trying to study. That's particularly hard with em waves because there is nothing quite like them in ordinary experience. The idea of a 'transverse' wave is hard to grasp - the nearest you can get is the motion of waves on a string - but that's only one-dimensional.

    An electromagnetic wave forms when there is a sudden change in an electric or magnetic field. The change of the E field gives rise to a B field nearby As the B field rises and collapses, it gives rise to a new E field which generates another B field and so on. There is no friction involved so the disturbance propagates outwards like ripples on a pond.

    Quantum mechanical waves are even more difficult to picture because they vibrate in the complex plane (whatever that means!) Don't confuse QM waves with EM waves. A lot of people make that mistake. Apart from both being waves, they are as chalk and cheese. They have nothing to do with each other.
     
  4. Oct 21, 2012 #3

    sophiecentaur

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    Remember, if you want to set up a magnetic or electric field, that takes Energy. It takes time to build up. There will be a delay in the field you set up 'here' and the resulting field you will get 'over there'. That delay will give you a wave behaviour when you start with an alternating field 'here' and look at the way the wave 'over there' varies with time.
    As it happens, the magnetic and electric fields are at right angles to the direction that the wave eventually travels (after it has settled down) and, also, the two fields vary 'in phase' with each other.
    You don't need a medium for an electric or magnetic field to exist - hence EM waves can propagate through empty space.
     
  5. Oct 21, 2012 #4
    Well you do need the (empty) space. Whatever that means.

    :wink:

    Oh and welcome to Physics Forums Cryptocatron. Are you some kind of robot feline?

    :biggrin:
     
  6. Oct 21, 2012 #5

    sophiecentaur

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    OMG
    It's the way he tells 'em!.
     
  7. Oct 22, 2012 #6
    So, does that mean that the 60 Hz alternating electric field in my home wiring is emitting EM radiation into the matter and space in my home?
     
  8. Oct 22, 2012 #7

    davenn

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    yup indeed,

    the radiated RF is easily picked up on a AM radio amongst other things

    Dave
     
  9. Oct 22, 2012 #8
    But the AM radio band is in the KHz range. How can 60 Hz be picked up by the antenna?
     
  10. Oct 22, 2012 #9

    davenn

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    because there are lots of harmonics that go higher up the RF spectrum and the radio can pic up some of those harmonics.
    thats the main reason

    another reason would be that because the radiated RF of the 60Hz is so strong, the sensitive receiver in the radio just gets overloaded with the signal when the radio is brought near the power wires.

    Dave
     
  11. Oct 22, 2012 #10
    That makes sense.

    I'm guessing that's why electric current can conduct to the ground, since that 60 Hz electric field is transmitting into the earth. Hmm...so if I am touching the ground, then I am also part of that alternating field. No wonder you get shocked if you touch the hot wire!
     
  12. Oct 22, 2012 #11

    Drakkith

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    No, that is purely due to the electrical circuit. The EM radiation is not transferring a current. In an AC power circuit in a house the live wire comes in from the power lines and runs back out to them, completing the circuit. If there is a short or you grab a live wire and short yourself, that's when the current gets out of the wire and into something it shouldn't be in.
     
  13. Oct 22, 2012 #12
    Right, but isn't the EM field necessary to create the potential difference (voltage) so that the current can flow? The EM field in the hot wire is out of phase with the EM field in the neutral wire/ground, which creates the potential difference allowing current to flow.
     
  14. Oct 22, 2012 #13

    davenn

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    No, its the flowing current between a potential difference that produces a radiated EM field

    Think of a lightning strike .... Huge potential difference builds up between the cloud and the earth. no EM field present. Then the lightning discharges and the current flows, a HUGE em field is created that radiates for many 100's of km's

    Dave
     
  15. Oct 22, 2012 #14
    So which came first, the chicken or the egg?:tongue:

    In order to get current to flow, you must first have an electric field. The way I learned it, the generator produces an electric field (EMF) by its turning through a magnetic field. This alternating electric field is transmitted through wires, which produces the electric current that flows into your home. Now I'm guessing that alternating EMF is different from the electromagnetic wave produced by the subsequent current? So maybe the alternating EMF is more confined to the conducting media whereas the EM waves radiate everywhere?

    Lightning is different because there is no alternating field (which you need for a EM wave), but a direct electric force field is built between earth and the cloud. As you said, that is not an EM field.
     
    Last edited: Oct 22, 2012
  16. Oct 22, 2012 #15

    Drakkith

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    He means there is no changing field to produce the EM waves until the current flows.
     
  17. Oct 22, 2012 #16

    davenn

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    thanks Drakkith :)

    yes to clarify ...
    you can have a large ELECTRIC field build up thats NOT and EM field
    for an electromagnetic field to form there must be an oscillation....
    in a lighhtning strike it is many microsecond pulses back and forward between the cloud and the ground

    in an electronic circuit it can be the rising and collapsing of the electric and magnetic fields around a coil ( inductor) as a DC voltage is applied and then removed or an AC voltage is applied that has a positive and negative cycling of its voltage

    cheers
    Dave
     
  18. Oct 22, 2012 #17

    davenn

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    incorrect, its no different .... see my previous post :)

    Dave
     
  19. Oct 22, 2012 #18
    Okay..I got it now. I misread your post #13.

    So, now the issue that still remains for me is the difference between the alternating field (EMF) produced by the generator/transformer that creates the current and the EM waves that are generated after the current is created.
     
  20. Oct 22, 2012 #19

    davenn

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    OK Im wondering if you dont have a clear understanding of what an EMF is ?

    I like Jim Hardy's explanation on this forum earlier in the year......

    An EMF is not a field

    Dave
     
  21. Oct 22, 2012 #20
    Okay, from what I've just read, EMF is not a force, either! The confusion being that EMF can stand for "electromotive force" or "electromagnetic field" Sheesh.

    It was always clear to me that it was an electric force, the kind produced when a wire moves through a magnetic field in a generator. I thought that the electrons in the wire moved in response to this electric "force." I guess I incorrectly believed that this was a force. I guess I got it wrong! So what makes the electrons move if there is no force?!
     
    Last edited: Oct 22, 2012
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