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Electromagnetic Waves through parallel plates

  1. Apr 21, 2015 #1
    Quick question about electromagnetic waves travelling through what's basically a capacitor. In the picture I have labelled areas that I'm curious about.

    1. Is there a potential induced at A?
    2. If yes, is it localized? I suspect the potential would be different at A and B.
    3. Is it correct to assume C will be at an equal but opposite potential of A?
    4. In the next half cycle, will B be at the potential of A or C?
    5. Would an AC current flow if we closed D?

    Attached Files:

  2. jcsd
  3. Apr 21, 2015 #2
    I think the first thing that should be pointed out is, electromagnetic waves don't have a physical extent as you have in your picture. Yes, they are transversal waves, but not transversal in space; they are transversal in their own field. Which means, they don't "touch" the plates like you have there. The way you should visualize it is by that classic "stone dropped into the pond" concentric circles.
    Regarding your question though, it will mostly be a question of wavelength. If the wavelength of the wave is very small (e.g. in the range of visible light), it will simply pass through the plates without any influence on them. If the wavelength is high, i.e. on the order of the distance of the plates or higher, they will likely induce into the plates. The reason for this is due to diffraction: http://en.wikipedia.org/wiki/Diffraction
    Last edited: Apr 21, 2015
  4. Apr 21, 2015 #3
    I tried to demonstrate that the wavelength is equal to the distance of the plates but I see I did a bad job. If say blue light travel through a space of the required size, it would induce on the plates?

    If the plates are of required size, no matter how tiny this might be, what happens in the questions above?
  5. Apr 21, 2015 #4
    Hmm, I think *I* didn't do a good job at explaining what I meant :D

    What I mean is, I think your question mostly stems from the misconception that EM waves are like the sine wave in your picture, where the troughs and valleys have a physical extent perpendicular to their movement of travel.
    If you want a visualization, you should use this type:

    http://www.presentation-process.com/images/concentric-circles-powerpoint.jpg [Broken]

    Imagine the center of those circles to be really far away from your plates. What would enter the plates then would be almost completely parallel "wave fronts". With that in mind, you'll see that both plates will be induced equally at a certain distance into the plate.
    Last edited by a moderator: May 7, 2017
  6. Apr 21, 2015 #5
    Ahh now I see. Have you seen the experiment where a guy uses cheese to measure the wavelength of his microwave? I just didn't understand the results and came up with the wrong perspective.
  7. Apr 21, 2015 #6


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    Unfortunately, the physics is more involved than what you think. This is where the idea of "waveguides" has to come in, because the boundary conditions of the "confinement" is now very important. For your microwave example, the most naive picture one can give you is that idea that there is a standing wave inside the microwave cavity, causing different amount of power at different locations inside the microwave oven.

  8. Apr 21, 2015 #7


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    Thanks Zz for bring up waveguides, if you hadn't, I would have

    Samson, do some googling on RF propagation in waveguides ... it can become a very complex subject

  9. Apr 22, 2015 #8

    I keep reading that there are standing waves. Did you mean that it's a naive way to look at it or it's a misconception?
  10. Apr 22, 2015 #9
    I think the idea of standing waves here is a bit misleading, since standing waves will only occur when there are specific relationships between the wavelength of the EM wave and the distance of the plates.
    For an arbitrary wavelength, standing waves won't occur, and the induction patterns will be complex and erratic.
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