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It says that the energy of the EM wave decays rapidly if it travels through a good conductive material. Therefore, much of the wave is travelling on the surface of the copper wire. (since it decays rapidly if it travels downward and toward (z-direction in the figure) the core of the copper wire).

But this doesn't make sense. Even if the EM wave is travelling on the surface of the copper wire, it is still on the copper medium. This means that it couldn't travel a distance longer than a few "skin-depth" distances even at the surface (x-direction in the figure).

What?

enter image description here

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  • \$\begingroup\$ First of all, EM doesn't travel in the conductor, only the electric current flows trough the conductor. \$\endgroup\$ – Marko Buršič Mar 14 '16 at 21:24
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The EM wave travels in the midspace and arround two conductors. EM wave is a pure energy transfer, while the current in the wires is a pure loss. Actualy you can imagine two conductors like train bins that guide that EM wave. If the frequency is higher you can observe a phenomena skin effect due to eddy currents in the conductor, so the majority of current flows on the surface. EDIT: enter image description here

EM wave is copmosed by electric(E) and magnetic(H) field, in the picture you can see the pattern of E,H field arround two conductors that deliver power to the load - there is a voltage in between them and the same current flows in both directions (forward and return). The energy of the EM wave is a Poynting vector S = E x H (cross product). You may imagine that energy density is larger in the space near conductors, while it soon drops because the H vectors originated from two wires subtracts each other in outer space, but they sum in the midspace.

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  • \$\begingroup\$ the textbook mention that the conductor serves as wave-guide for the EM. What does it mean? \$\endgroup\$ – randy Mar 14 '16 at 21:49
  • \$\begingroup\$ without the conductors, the EM wave would spread out spherically. With the conductor pair, the EM wave is localised round them, and travels along them. Waves, guided. \$\endgroup\$ – Neil_UK Mar 14 '16 at 22:24
  • \$\begingroup\$ The electricity travels inside the wires, but the energy does not. The energy travels in the space adjacent to the wires. Perhaps you were taught that electricity is a form of energy? Nope, wrong. That's a common mistake made in non-tech books, especially in kids science books. Electricity flows in a complete circle, while electrical energy does not. In AC, electricity wiggles while energy flows forward. Amps measure electricity flow, watts measure energy flow. Quantity of electricity comes in coulombs, quantity of energy is in joules. They're two entirely different things. \$\endgroup\$ – wbeaty Mar 15 '16 at 6:29

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