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In almost every physics program, you learn about electricity, electromagnetism, photonics, ...

Within the electricity course, you learn about transporting electric energy as electrons through cables.

Within the electromagnetics course, you learn about transporting electromagnetic energy through cables (transmission lines), as well as through media as electromagnetic waves. The latter being unguided or guided within a waveguide. These waveguides are e.g. hollow rectangular structures with metallic sides.

Within a photonics course, you learn about transporting electromagnetic energy (with frequencies around that of visible light) through an optical guide. But in this case, the guides are suddenly just dielectric media (because this appears to be more efficient considering these frequencies).

Can somebody give me a general framework on solving these problems starting from the Maxwell equations (giving me some intuition by stating boundary conditions for each problem and stating their consequences)? Answering questions like:

  • Why is a metallic waveguide not efficient for optical waves?
  • Why is a dielectric waveguid not efficient for radio waves?
  • Is a regular cable also some kind of wave guide, guiding an electromagnetic wave on the outside of the cable?
  • Is a metallic waveguide also a kind of cable, transporting electrons through its walls when transporting an electromagnetic wave?
  • How fast is energy travelling within each field? How fast are the electrons moving in a regular cable? Are there photons accompanying them on the side of the cable, travelling at the same speed?

I know it appears that I have more than one question, but I am convinced that all the questions boil down to the same confusion considering these "different" fields within physics.

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  • If your physics program teaches that energy is transported by electrons in cables, then it's time to change to a different school, because that is 100% incorrect. The electrons in a cable are merely setting up boundary conditions for the electromagnetic field surrounding the conductors which localize that field around the conductors rather than to allow it to propagate freely. The energy is still being transported in the electromagnetic field itself, which is completely identical to what happens in the case of radio waves in waveguides and light in optics. – CuriousOne Oct 28 '15 at 20:47
  • Isnt this just a matter of how you describe the energy flow? Power P=dE/dt = F.dx/dt = q E(field) dx/dt = I V (with some improper use of differentials), such that the energy is transported through the flow of charge? – BNJMNDDNN Oct 28 '15 at 21:10
  • Nope, it's not. As your own examples show charge is not necessary, at all, to transport energy. There is surely no charge flowing from the sun to a solar panel, is there? There are no charges flowing from a radio transmitter to a receiver, there are no charges flowing from the primary to the secondary winding of a transformer, right? But in each and every case there is energy transport trough the fields. What we should be teaching, but don't teach on the high school level because it's too complicated, is that the relevant quantity for energy transport is a quantity called the Poynting vector. – CuriousOne Oct 28 '15 at 21:53

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