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ElectroMagnetic Waves

  1. Apr 15, 2005 #1
    This may sound dumb but...

    since light is classified as an electromagnetic wave, it has an E field and a B field. But doesn't this mean the E field will attract metal, and the B field will cause currents to flow???

    And since E = F/c = V/m does that mean the emr has a "voltage" or a "force"?
  2. jcsd
  3. Apr 15, 2005 #2


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    I think you meant to put E and B the other way around in your question. But yes, it's true. Radio waves are electromagnetic waves just like light waves but with a longer wavelength and lower frequency, and we detect them by the electric currents that the oscillating E and B fields produce in an antenna.

    Off the top of my head (haven't had enough coffee yet this morning), I can't think of any experiments that "directly" observe the E and B fields in light waves, but I wouldn't be surprised to learn that this has been done.
  4. Apr 15, 2005 #3


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    Er.... we put 1.3 GHz RF in our cavity and we have wall current losses, etc. directly due to the E and B field of the RF. So yes, this is something that is very obvious to us. It is why the ILC announced that the technology to be used in their accelerator will be the superconducting RF cavities. The whole purpose IS to reduce wall losses and get higher accelerating gradients.

  5. Apr 15, 2005 #4
    I plotted z=cos(x)+cos(y) in following interwals: x-direction: ]-infiniti, +infiniti[
    and y-direction: [-pi/2, +pi/2]. I got something like the picture in the attached file (the picture isn't really good (because we can only attache 100 kb in this forum).

    Now, the problem is actionally this one:
    I don't know how a three (x,y,z) dimensional electromagnetic oscillation in ground state (in free space) lookes like. Someone gave me a guess (marlon). So I thought of drawing one to look if I'm right. In vacuum there are many oscillation. So I picked one oscillation and plottet it. That's why the y-direction of the oscillation has a certain minimum (pi/2) and a maximum (pi/2) (Think of cos(pi/2)=0).

    Is that the right imagine of such an oscillation in ground state?

    I would be thankful if someone might answer this question.

    Attached Files:

  6. Apr 16, 2005 #5
    Can somebody help?
  7. Apr 16, 2005 #6


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    The electric and magnetic fields are vectors, so you can't draw an electromagnetic wave as a simple surface plot or line plot.

    Here's my attempt at an incomplete picture of a plane electromagnetic wave that travels parallel to the x-axis. It's a snapshot at one particular point in time. The red arrows indicate the magnitude and direction of E at various points in the xy-plane. The dashed blue lines indicate where E is zero.


    You have to complete this picture, mentally, as follows:

    1. Every point in the plane, not just the ones shown, has an arrow attached to it, with a magnitude and direction interpolating the pattern that I've sketched.

    2. Fill the space above and below the plane of the "paper" with copies of this diagram, stacked one on top of another so that all points in the three-dimensional space have arrows attached to them. The dashed blue lines become planes perpendicular to the x-axis.

    3. Make copies of all those diagrams and change the color of the arrows from red to (say) green. Rotate them 90 degrees around the x-axis so the arrows all point towards and away from you, and the y-axis points towards you (that is, it becomes the z-axis), and the x-axis is the same as the other x-axis. This is the magnetic field B that is associated with this wave. The planes where E is zero are also the planes where B is zero.

    4. As time passes, the magnitudes and directions of the E and B vectors at each point oscillate in such a way that the overall pattern (but not the arrows themselves!) marches from left to right or from right to left at speed [itex]c[/itex]. At all times, at each point, the red arrow points either up or down, or vanishes momentarily; and the green arrow points either towards or away from you, or vanishes momentarily.

    Attached Files:

    Last edited: Aug 10, 2014
  8. Apr 16, 2005 #7
    @jtbell: You are right. But I speak about QFT and the oscillation of the electromagnetic field in present of for example 3 photons.
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