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Are signals transmitted through electrical wires considered longitudinal waves (signals) or electromagnetic waves?

I think it would make sense for them to be longitudinal because the electrons are pushed then dragged back by the force of voltage or potential difference.

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  • \$\begingroup\$ Your title and your question seem to not fully agree. In the title you ask about logitudinal waves, in the question you ask if they are logitudinal waves or electromagnetic waves. If your logitudinal wave isn't electromagnetic, what is it? \$\endgroup\$ – Joren Vaes Sep 8 '17 at 8:24
  • \$\begingroup\$ I though the were transverse waves. \$\endgroup\$ – yoyo_fun Sep 8 '17 at 8:28
  • \$\begingroup\$ A wire is a 1-dimensional structure so it doesn't make sense to distinguish between longitdudinal and transversal waves. The essential difference of both is that longitudinal waves are waves of a scalar quantity and transversal waves are waves of a vector quantity. This becomes relevant only if waves propagate in space with more than 2 dimensions. \$\endgroup\$ – Curd Sep 8 '17 at 9:50
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    \$\begingroup\$ This question might be better suited to the physics forum.. \$\endgroup\$ – Trevor_G Sep 8 '17 at 11:35
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    \$\begingroup\$ I think the "longitudinal" waves you imagine are patterns of different electron density. These do exist but they aren't EM waves and they aren't a main property of conductors but insulators and semiconductors. Look up gunn diode for an electronic component which uses "the sound of electrons" to generate microwaves. \$\endgroup\$ – Janka Sep 9 '17 at 4:31
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Interesting question.

This is one of those questions that gives most EEs a bit of headache because it is rather difficult to get one's brain wrapped around what is theoretically going on with electromagnetic waves. The truth of the matter is it is not quite as simple as we first imagine.

Here is how I rationalise it.

First and foremost you need to separate the notion that electrons have anything directly to do with electromagnetic waves. They don't. EM waves propagate without the need for any material. In a vacuum they propagate at the speed of light, when the wave encounters a material they are slowed by the materials physical properties.

Having grasped that bit of wisdom, it is easy enough to understand that when you apply a voltage to one end of a bit of wire, it takes time for you to be able to detect that voltage at the other end of the wire. That voltage propagates down the wire at the speed of light for that conductor material. In effect you created, or more accurately, changed the E-Field at one end and the change wave takes time to get to the other end.

Now consider instead modulating the applied voltage, that is, applying a signal to the wire. If we break up that signal in the time domain into infinitely small periods you can see from what we just discussed that there will be that propagation delay for the instantaneous change at one end to reach the other. The E-Field must "carry" those changes to the other end. Again, it is important to remember, this has nothing to do with electrons.

So, to summarize so far, the electromagnetic waves are the carrier for your signal not the signal itself. EM waves happen to be transverse, and that does not change.

The signal, or whatever voltage wave you are transmitting, is effectively a modulation of that carrier, and is usually longitudinal. You are setting up, or transmitting, a difference in the electrical field. It's those local differences that excites the electrons in your conductor and make them move in reaction.

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Conductors have very high absorption, so EM-waves propagating inside a conductor do not reach very far. An ideal conductor has infinite absorption and EM-waves cannot propagate inside an ideal conductor.

EM-waves propagate in waveguides, which may consist of conductors (e.g. coax cable or twisted pair), however not all waveguides need conductors (e.g. fiber optic cable). Also the energy the wave carries is contained in the field which is mainly in between the conductors. The conductors are only needed for confinement of the wave, i.e. that the wave is going where it is supposed to go and not arbitrarily dispersing into free space.

Whether the waves are longitudinal or not (or have some longitudinal component) depends on the waveguide geometry and the type of mode the EM-wave corresponds to. In most cases waves are a mixture and have both longitudinal and transversal components. Note that in free-space no longitudinal EM-waves can exist (but in waveguides this is possible).

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    \$\begingroup\$ I would say that the issue here is that the assumption is made that "wave" is confined to the cable, when in fact it is not. Outside of the conductor we have TEM modes propagating the signal, and inducing currents inside the conductor. Because the currents can easily propagate along the cable, so does the wave. At least that is my first-order understanding of the matter. \$\endgroup\$ – Joren Vaes Sep 8 '17 at 8:36
  • \$\begingroup\$ @JorenVaes: I am not sure if I understand your comment. \$\endgroup\$ – Andreas H. Sep 8 '17 at 12:44
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Electromagnetic waves are Transverse in nature. The E and H field oscillate perpendicular to the direction of propagation of wave. If you consider a transmission line such as a coaxial cable, there are two conductors that carry currents in opposite directions. One current flowing from source to load and other from load to source. It is true that these two conductors carry currents. They are separated by a dielectric layer of foam or polyethylene. Your Electromagnetic wave propagates along the gap between the two conductors. The electromagnetic wave is produced due to the jiggling of electrons. As a result of which a self propagating wave is produced for long distance transmission. In a coaxial cable like RG59, RG6 etc the outer conducor is kept to confine the EM wave inside the cable. This is done to tackle skin effect at high frequencies. Moreover, Ron Schmitt's "Electromagnetics Explained" tells that the electrons flowing thro the inner and outer conductors just serve to guide the EM wave inside the dielectric region. For, dielectrics allow EM waves to pass thro them.

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The presence of a longitudinal wave requires an extension of Maxwell equations to incorporate this extra piece. In addition, the energy conservation equation should include a term that gives momentum and energy to the longitudinal wave besides the transverse wave. In that case one can say that the longitudinal wave exists at a fundamental level.

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