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All of us know that the optical fiber cables have more data speed than the traditional copper cables. That's the reason why they're getting replaced in most of the countries. But does even a 200-300 meter piece replacement of copper cable by optical fiber cable make any difference in the speed? It might be possible as we already said for larger cable length the speed increases, but then whats the fundamental reason behind the increase in speed like this?

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  • $\begingroup$ Your speed on your final device will be limited by the slowest device in the chain. $\endgroup$
    – Solar Mike
    Oct 7 '21 at 6:18
  • $\begingroup$ You are focusing on the medium, while ignoring transition from one medium to another. If you read the article you reference, the advantages for distance communications are clearly outlined. These advantages do not extrapolate to short distances. $\endgroup$ Oct 7 '21 at 7:32
  • $\begingroup$ Copper is still used extensively for data transfer including by big smart players like Amazon and Google. Don't assume that fiber is automatically better. $\endgroup$
    – Tiger Guy
    Oct 7 '21 at 14:04
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Fiber cables have higher data transfer speed not because light travels faster in a fiber than electromagnetic waves in a copper wire, but because they can be modulated by signals having far higher frequencies than can be managed in the case of copper wires.

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Niels Nielsen's answer is correct. Just to provide some calculations as to actual propagation speeds (not signal bandwidth):

Typical optical fibers have $n\approxeq 1.5$ , so the propagation speed is $\frac{c}{1.5}$ , or roughly 2E8 m/s . This is the field, or photon, speed.
Using the formula from Wikipedia, the speed of electromagnetic waves in a good conductor is given by:

$$v= \sqrt{\frac{2\omega}{\sigma\mu}} = \sqrt{\frac{4\pi}{\sigma_c\mu_0}} \sqrt{\frac{f}{\sigma_r\mu_r}} \approx \left(0.41 \frac{m}{s} \right)\sqrt{\frac{f/(1\;\text{Hz})}{\sigma_r\mu_r}}$$

And the approximate value reported there of 3.2 m/s at 60 Hz, we can estimate, for, e.g. 100 MHz a propagation speed of the electric field (NOT the electrons) of $3.2 *\sqrt{\frac{1E8}{60}}$, or about 4100 m/s . Of course, we're already in the microwave regime, where a hollow pipe is preferable to a solid conductor.

Note: here's the definition of the variables:

enter image description here

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  • $\begingroup$ for the sake of completeness could you add definitions of the various quantities in the formulas. e.g. n, $\sigma_r$, $\mu_r$ etc $\endgroup$
    – NMech
    Oct 7 '21 at 13:46
  • $\begingroup$ @NMech good point; done. $\endgroup$ Oct 7 '21 at 13:52
  • $\begingroup$ if its not too much of a bother could you also provide some reference/definition for the $n=1.5$. I'd like to read up on that because I've never come across it before. $\endgroup$
    – NMech
    Oct 7 '21 at 13:57
  • $\begingroup$ How do you reconcile your values for propagation in conductors with the statement, in the same wikipedia entry, that the velocity of propagation in circuits is "about 300,000 kilometers per second"? Shouldn't I notice the delay, at 3.2 m/s, when I turn on the lights in a long hallway? $\endgroup$ Oct 7 '21 at 16:46
  • $\begingroup$ @ElliotAlderson I think the part you are quoting refers to the electric field generated in the free space outside the conductor $\endgroup$ Oct 7 '21 at 17:05

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