1
$\begingroup$

I'm confused by the difference between the phase velocity and transmission line propagation velocity

From https://en.wikipedia.org/wiki/Speed_of_electricity, in a good conductor, phase velocity (speed of an electromagnetic wave) is given by $\sqrt{(2*angular\, frequency) / (permeability * conductivity)}$. From this we can calculated that 60Hz wave travels along a wire at about 3.2m/s (and group velocity is about twice this speed). This does not gel with my experience of turning on a light switch!

On the other hand, the Telegraphers equations (https://en.wikipedia.org/wiki/Telegrapher%27s_equations) say that the speed of propogation in a lossless tranmission line is 1/sqrt(inductance * capacitance), which tends to give velocities up around the speed of light.

There is disconnect here that I can't quite put into words. Best I can do is - what does the phase velocity of 3.2m/Sec mean and how does it relate to the result derived from the telegraphers equation (if it relates at all)?

Cheers

$\endgroup$
1
$\begingroup$

The speed of light (or any EM wave) slows in a material because the EM wave makes charges, such as electrons, move. The movement of the charges then re-radiates the EM wave. The net effect is that the EM wave slows down. In an insulator (which is what we hams might call a dielectric) the electrons don't like to move, and therefore the EM wave doesn't slow down very much. In a conductor, the electrons are easy to move, and therefore the EM wave slows down a lot.

I think your confusion is really about energy flow. When you flip on the light switch, the light comes on pretty much instantaneously. That's because the wire delivers energy to the light bulb right away. But... the energy does not flow through the conductor! Instead, it flows through the space around the conductor. Mathematically, the energy flow is given by the Poynting vector $S = E \times H$, where $E$ is the time-varying electric field and $H$ is the magnetic field. Here (from Wikipedia) is what happens inside a coaxial cable:

Poynting vector in a coaxial cable

Small circles with an "x" in the middle are vectors pointing into the page, and those with a dot in the middle are vectors pointing out of the page. You can see that the energy flow takes place within the dielectric, not the conductor. As you point out, the speed of an EM wave in a dielectric is close to that in a vacuum, so the energy moves quickly around the wire to the light bulb.

$\endgroup$
  • $\begingroup$ nah, for low-frequency current, energy is actually transported through the cable. Power cables aren't coax! Anyway, the point is that the electrical field gradient that makes electrons move actually travels at close to speed of light. $\endgroup$ – Marcus Müller Jul 30 '17 at 18:07
  • $\begingroup$ Take the limit of a perfect conductor. No electric field can exist within the conductor, yet current flows nevertheless. I think you would agree that when current flows, a magnetic field exists around the conductor. The equation for the Poynting vector, $S = E \times H$, tells you that if there is an energy flow, there must be both a magnetic field and an electric field. If either $E$ or $H$ are zero, there is no energy flow, since then $S=0$. The electric field is outside the conductor, since by definition $E = 0$ inside the conductor. Energy flow occurs outside it, not inside. $\endgroup$ – Rodney Price Jul 30 '17 at 18:46
  • $\begingroup$ ok, fair and true, that is basically the skin effect on a larger scale. I was mistaken. Still might be a good idea to point out that in the process of "switching on", we do have limited speed of change through Maxwell! $\endgroup$ – Marcus Müller Jul 30 '17 at 19:11
0
$\begingroup$

The first equation describes a wave propagating in a conductor. The second equation describes a wave propagating in a dielectric.

Electromagnetic waves don't propagate well in good conductors. If the conductor is good, that means a small electric field in the conductor can result in a large current. With the large current comes large ohmic losses, and thus the energy is rapidly dissipated.

If the conductor is ideal, with infinite conductivity, an electric field can't exist in it at all.

However, a conductor can exist at a different electric potential than the things around it, provided it's separated some kind of thing that can support an electric field. Like air, PTFE, or indeed most everyday materials that aren't conductors. These things are all dielectrics. An electric field can exist in this dielectric, and it's this field which is responsible for energy transfer in everyday experience.

$\endgroup$

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.