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Can an electric field change faster than light?

  1. Apr 7, 2012 #1
    I found the thread "Can magnetic field expand faster than light?" but no thread on electric fields.

    For example:


    If, in the above setup, the dielectrics used in capacitors were physically very long (the distance between plates very high) and the capacitors were charged up to high potential in a way that closing the switch would create a HV peak measurable at the resistor, is it possible that this peak could appear faster than light travels the distance between capacitor plates?
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  3. Apr 7, 2012 #2


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    Light consists of electric and magnetic fields so the speed of light is hardly likely to be different. April 1st was several days ago.
  4. Apr 7, 2012 #3
    This thread is not an april fools joke.

    In a capacitor, nothing has to travel the distance between the plates (and nothing does, assuming a perfect dielectric). Is there a reason why the speed of light would be a limit?
  5. Apr 7, 2012 #4

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    Electric fields and magnetic fields are both electromagnetic fields.
    Both expand at most as fast as the speed of light in vacuum.
  6. Apr 7, 2012 #5


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    An EM wave consists of changing E and H fields. The rate of propagation of a change in field IS the speed of the EM wave. How could it be different?
    Why are people obsessed with the dielectric in a capacitor? Capacitors can have a vacuum between the plates. Any material between will SLOW DOWN the wave speed - that's what the refractive index is about.
  7. Apr 7, 2012 #6
    Excuse my ignorance. I just don't quite understand this. And all this EM wave talk is making it much more confusing. There's no EM wave in a capacitor, right?

    I can understand this very well for magnetic field that starts from one point and expands, but electric field lies on the sides of the dielectric and does not propagate through it. If the charge is changing on one side.. it has to change on the other side too? Otherwise the field can be canceled out and that wouldn't make any sense either.

    Do I just lack knowledge of time to understand this?
  8. Apr 7, 2012 #7
    Ever hear of Maxwell's equations?
    A changing electric field produces a magnetic field, and vice versa. Ergo, an EM "wave".

    "Equation Four" here ought to suffice.....
    Read through from "Equation Four" to the bottom (last two eqns. are for the resultant EM wave.)
    Simplistic, but you will get the idea.

    Last edited: Apr 7, 2012
  9. Apr 8, 2012 #8
    A charged particle does not know it has to move differently unless the force on it changes, i.e. the electric field changes. Light in itself is the information that the electric and magnetic field has changed. When you wiggle an atom back and forth, the force felt by a stationary particle some distance away changes, because of the inverse square law. When the electric field changes, it creates a wavefront traveling at the speed of light with the information of a changed electric (and magnetic) field and that is what light is. Electrons do not know what the charge is on the other plate or anything else, they just act once the information about the changed field reaches them and that's what light is.

    So an electric field cannot change faster than light, because light is how a particle knows the field has changed.
  10. Apr 8, 2012 #9


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    It may help to think of the scale involved. The separation between capacitor plates is very small, compared with the wavelength of typical RF signals so you can't think of 'whole wavelengths' between them. The voltage on one plate will lag behind that on the other plate but this lag represents only a small portion of an actual wave.
  11. Apr 8, 2012 #10
    Thank you for the replies.
  12. Apr 8, 2012 #11


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    While the fields can't change faster than light, Maxwell's equations does show a parameter that can be FTL and can be used to generate EM waves of a curious nature.

  13. Apr 10, 2012 #12


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    the reason that light (at whatever frequency) propagates at this apparently constant speed (we call "c") is because the electromagnetic interaction propagates at that speed.

    it is a fundamental property of nature that these ostensibly "instantaneous" fundamental forces all propagate at this speed which is sometimes called the "speed limit" of the universe.
  14. Apr 10, 2012 #13


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    I'm supposing this includes gravity, too?

    Does the observation that it acts at the speed of "c" (but does it?) constitute good evidence that gravity is also, at root, an electromagnetic interaction?

    Respectfully submitted,
  15. Apr 10, 2012 #14

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    Yes, changes in gravity also expand at the speed of light.
    But disappointingly gravity really is not an electromagnetic interaction.
    (We're still waiting for the grand unification theory.)
  16. Apr 11, 2012 #15


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    Why are you "disappointed" about this? It's something we are 'stuck with' just the same as the length of a day and taxes. I guess that, somewhere along the line, when a Unified Theory arrives, it will take us somewhere beyond this and possibly suggest applications where the commonality can be made use of - possibly for FTL, antigrav etc.
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