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Parallel Wires and Fields

  1. Dec 20, 2009 #1
    Hi All,

    As I understand it, a voltage impressed across a wire produces an electric field within the wire and in the direction of the wire which causes the movement of electrons.

    If two parallel wires carry a direct current, this field exists in both wires.

    Now when we talk about alternating current, along the length of the wire, the current increases and decreases thus inducing a magnetic field between the wires. The changing magnetic field induces an electric field from one wire to the other which alternates with the frequency of the current.

    Thus, when considering alternating current, do we actually have two fields, one within the wire pointing in the direction of the wire and another perpendicular to this direction going from one wire to the other?

    I'm trying to fully understand this in order to understand the functioning of waveguides.

    Thank you,

  2. jcsd
  3. Dec 20, 2009 #2


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    Science Advisor
    Gold Member

    Well, we can always decompose an electromagnetic wave any way you choose thanks to linear superposition. But that isn't very instructive in the case of waveguides with the exception of expanding out the standing waves into travelling waves. The currents are not what creates the electromagnetic waves, the electromagnetic waves are what creates the currents. With an AC signal, you create the wave at the beginning of the waveguide by applying a voltage across the waveguide that excites the wave. This can be done by charges, like the charges in the drain of a transistor, or it can be done by another wave, like with an antenna. As the wave propagates down the waveguide, the electric and magnetic fields of the wave induce the currents on the surface and volume of the waveguide. This conclusion can also be reached by considering the fact that AC currents do not have a net propagation of charges. It becomes difficult to explain how the power is moved in space by such currents.

    So in this sense, a waveguide is a simple device. As the name suggests, it is a device that has a specific geometry and material property that assists in directing electromagnetic waves along a desired direction. It simply guides the electromagnetic waves. The currents that exist in the waveguide are a result of the waves, and their excitation is dependent upon the type and frequency of the wave and how the wave propagates down the waveguide.

    Also, I should note that DC currents do produce magnetic fields.
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