Speed of electric potential in copper

[Jdo300]

Hello All,

Is there a way to predict the speed at which an electrical potential travels through a piece of copper wire? I know that the electrons themselves move very slowly (at the drift velocity), but how about the voltage potential itself? Are there any nifty equations out there to determine this? I know it's near the speed of light but so far, I haven't been able to find any actual values.

Thanks,
Jason O

 

[berkeman]

That is governed by the transmission line equations:

http://en.wikipedia.org/wiki/Transmission_line

The propagation velocity is indeed a significant fraction of c. For most cables and transmission lines, it's about half of c or so.

 

[Jdo300]

Hi Berkeman,

Thanks for the tip about the transmission lines. I'm not sure if I can apply these equations directly to what I'm doing though. What I'm working with is coils and I'm sure that the inductance and capacitance will affect the propagation speed. Do you know of any good equations that can help me determine the potential propagation speed through a coil? Also, I'm trying to see how using different materials for the propagation medium effects it. One thing I'm interested in seeing is how fast voltage propagates through iron/steel as well as copper.

- Jason O

 

[berkeman]

I googled transmission line propagation velocity tutorial, and got some good hits. This website appears to have many technical articles overall (maybe it's like wikipedia?), and this is the list of transmission line related pages:

http://www.educypedia.be/electronics/transmissionlines.htm

I clicked on the one link that talked about propagation velocity, and that got me here:

http://www.ycars.org/EFRA/Module%20C/TLChar.htm

The distributed inductance per unit length and distributed capacitance per unit length are how I think you should try to model your system. Even though what you have is not a traditional transmission line, the propagation velocity of the field waves is governed by the same distributed L & C constraints -- that's how the energy moves along.

Hope that helps. Good luck!