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External Electric Field of a Pulse Travelling in a Conductor

  1. Jun 24, 2014 #1
    If I send a pulse down a conducting wire, does the pulse emit an electric field?
     
  2. jcsd
  3. Jun 24, 2014 #2

    berkeman

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    Welcome to the PF.

    Your terminology is not quite right. In order for a pulse to propagate down a transmission line, there has to be a return path. See the animation at the top right of the wikipedia page on Transmission Lines:

    http://en.wikipedia.org/wiki/Transmission_line

    When you have a 2-conductor transmission line, you can make a pulse propagate down it by putting a step input voltage into it (like at the left of the figure). That voltage pulse propagates down the transmission line, with an electric field between the 2 conductors, and a current pulse induced in the 2 wires. The current does cause a magnetic B-field to be generated around each wire, but the electric E-field is mainly contained in the space between the two wires.
     
  4. Jun 24, 2014 #3
    Thank you for the welcome :) -you answered my question succinctly, despite my poor terminology.
     
  5. Jun 24, 2014 #4

    Baluncore

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    To launch a pulse of energy along a wire you will need some reference, another side to the system.
    A pulse of energy will travel at close to the speed of light.

    An electric field will appear between the wire and the reference, the other side of the system or other parts of the same wire.
    A magnetic field will form about the wire due to the current in the pulse.
    The impedance of the wire line will be the ratio of the electric to the magnetic field.

    So yes, an electric field will be emitted by the pulse as it travels along the wire.
    Relative field magnitude and distribution will be determined by the geometry of the wire and it's surroundings.
     
  6. Jun 24, 2014 #5
    Fascinating.

    This forum is awesome!

    Thanks again :)
     
  7. Jun 27, 2014 #6

    sophiecentaur

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    The nearest thing to what you seem to have in mind is the Goubau Line, which consists of a single conductor transmission line. See this link.

    You can look upon it as a coaxial line with the outer at 'infinity'. You can launch a pulse at one end with a tapered horn and receive it at the other with a similar horn. The wave that is launched is tied to the line on its journey. Ideally, no power is actually radiated (within limits of course).
    I remember it being considered, in my department, as a way of getting signals up to a balloon mounted transmitter for transmitter site surveys. Afaik, it was not actually used but it was an interesting idea.
     
  8. Jun 29, 2014 #7

    Baluncore

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    It should be noted here that only very low frequency current has sufficient time to diffuse into and so flow in a conductor.
    A pulse travels at close to the speed of light on, or in the surface skin of the conductor, as an EM surface guided wave.

    So when a pulse travels along a Conductor it is an External Electric Field.
    There is no pulse of current deeper inside the conductor.

    G-Lines and now E-Lines:
    Adventures in Engineering - E-Line data transmission via single conductor power wire.
    http://mackys.livejournal.com/935479.html [Broken]
    Which leads to a 1.2Mbyte pdf.
    Introduction to the Propagating Wave on a Single Conductor
    http://www.corridor.biz/FullArticle.pdf
    Re: Balloons, see; Illustration 14:
     
    Last edited by a moderator: May 6, 2017
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