Electromagnetic Waves through parallel plates

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Discussion Overview

The discussion centers around the behavior of electromagnetic waves as they interact with parallel plates, akin to a capacitor. Participants explore various aspects of wave induction, potential differences, and the implications of wavelength in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a potential is induced at specific points (A, B, C) and if these potentials differ, suggesting that the potential at A may not equal that at B.
  • Another participant argues that electromagnetic waves do not have a physical extent and should be visualized differently, emphasizing the importance of wavelength in determining the interaction with the plates.
  • A participant attempts to clarify the relationship between wavelength and plate distance, questioning if blue light would induce effects on the plates if the sizes are appropriate.
  • There is a discussion about the misconception that EM waves have physical extents, with a suggestion to visualize them as concentric circles rather than sine waves.
  • One participant mentions an experiment involving cheese to measure microwave wavelengths, indicating a misunderstanding of the results and the physics involved.
  • Another participant introduces the concept of waveguides, noting that boundary conditions are crucial for understanding wave behavior in confined spaces like a microwave cavity.
  • There is a reference to standing waves in microwave cavities, with a participant questioning whether this is a naive perspective or a misconception.
  • One participant expresses that standing waves may be misleading, as they only occur under specific conditions related to wavelength and plate distance, suggesting that induction patterns can be complex for arbitrary wavelengths.

Areas of Agreement / Disagreement

Participants express differing views on the nature of electromagnetic waves and their interaction with parallel plates. There is no consensus on the implications of standing waves or the correct visualization of EM waves, indicating ongoing debate and uncertainty.

Contextual Notes

Limitations include the dependence on specific definitions of wavelength and plate distance, as well as unresolved questions regarding the nature of wave induction and potential differences at various points.

Samson4
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Quick question about electromagnetic waves traveling 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?
 

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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
 
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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?
 
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

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.
 
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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.
 
Samson4 said:
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.

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.

Zz.
 
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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

Dave
 
ZapperZ said:
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

https://www.physicsforums.com/threads/standing-waves-in-a-microwave.437867/

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?
 
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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