# What limits the speed of information through a copper wire?

tech99
Gold Member
I cannot imagine any model based on that would produce a propagation speed of more than a small fraction of c. More like the speed of sound, I should have thought. Do you have any reference to that idea or is it one of your own?
I am afraid it is my own idea, based on plasmons. I have been using this concept to explain surface waves and radiation.
I cannot imagine any model based on that would produce a propagation speed of more than a small fraction of c. More like the speed of sound, I should have thought. Do you have any reference to that idea or is it one of your own?
I thought that as the guided wave travels along, surrounding the wire, each electron in turn will give a back and forth kick as the wave passes. When viewed as a whole, this would constitute a longitudinal wave travelling in the wire at almost c.

davenn
Gold Member
am afraid it is my own idea, based on plasmons. I have been using this concept to explain surface waves and radiation.
Then you may want to review the PF rules about expounding one's own theories ...
briefly ... its a no-no

regards
Dave

No one has yet addressed the fiber optic part to the question....

Two points spring to mind , we need to know the speed of light in the medium the cable is made of , perhaps around 60% c ...and the light does not take a direct rout, but to some extent bounces around inside the fiber.

What limits the speed of information through a copper wire?
The simple answer is in two parts.

First, the speed of propagation of an electromagnetic wave is reduced when it travels through material. All electrical signals are waves, but normally the wave behavior of electricity can be ignored. In copper, the speed of an electromagnetic wave is reduced from 299,792,458 meters per second to 285,102,627. So the speed of the wave is reduced in the first place by the fact that it is travelling through a medium, but only by about 5%.

Second, the copper wire has non-zero impedance. That is to say that the copper wire has some resistance over its length, but more importantly due to self-inductance it also has reactance, the imaginary part of impedance (resistance is the real part) that can be thought of as something like a frequency-dependent resistance. The impedance increases as the frequency of the signal increases, and this means that for higher frequencies the amplitude is reduced and the phase of the signal is shifted, so it becomes harder to protect the signal from being degraded by background noise.

So it's not really the velocity of the propagating wave, ie its "speed", that's important, but rather the way that electrical conductors behave when they're carrying high-frequency signals. Limited frequency means that it's harder to send information as quickly.

These two problems are the basis of transmission line theory.

What is the speed of signal in optic fibres? Is it faster and why?
Optical fibers are made of what is essentially glass, so the speed of the propagating wave is reduced to about 200,000,000 meters per second. This may seem significantly lower than with the copper fiber, but remember that it's the maximum frequency the line can carry that matters much more than the speed of propagation of the signal through the wire in terms of carrying information.

Optical fiber can support a vastly higher frequency than copper wire. Glass is not an electrical conductor, so self-inductance does not occur. Furthermore, in a copper line the current is going to want to induce currents in nearby conductors, forcing it to do work, and thereby expending the energy of the signal. It's also going to be influenced by currents induced by background noise. None of this is a problem in optical fiber, so it can support a higher frequency due to not needing to spend energy protecting the signal from corruption by noise.

sophiecentaur