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836 posts

Ultimate Geek

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  #143006 4-Jul-2008 17:28
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michaeln:
Fraktul:
Copper is not opaque so the speed of light is copper is not relevent - you are reffering to the speed of electromagnetic wave propogation outside a copper wire.

No, I'm referring to the speed of electromagnetic radiation propagation (c, familiarly the 'speed of light') through copper. I.e. 'light' in this case meaning photons, for which copper is anything but opaque, given the appropriate frequency of photon. Likewise, 'copper' in this context could encompass UTP, STP or various waveguides with a diverse range of dialectrics. The formula is no less relevant and for the copper based transmission media 'about 75%' is a perfectly fine approximation.


Should have read copper is opaque. Copper obviously does not transmit light very well, it is mostly reflected or absorbed. I'm not saying the formula is not relevent, I'm saying that reffering to the speed of light in copper is pretty nonsensical when you are in fact meaning the electromagnetic wave propogation.

3539 posts

Uber Geek

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  #143018 4-Jul-2008 18:25
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Fraktul: ...I'm saying that reffering to the speed of light in copper is pretty nonsensical when you are in fact meaning the electromagnetic wave propogation.

Actually Fraktul **cough** putting on my Electrical Engineer's hat here, it isn't Electromagnetic Wave Propagation we are dealing with either.  That only happens in Free Space.

The mechanism involved is Electron Drift Velocity and this little snippet sums it up quite well:

At the given drift speed it would take about 2000 seconds for an electron to travel 20cm distance.  It is about half an hour.  But discharge happens instantaneously.  This is because the wire is already full with electrons!


If you think of a pipeline full of electrons it makes it easier to visualise.  Push an extra electron into the pipe and another one more-or-less instantaneously pops out the other end.  The speed of this mechanism is controlled by the electric field applied from one end of the conductor to the other.

According to some info. here:

http://www.physics.wayne.edu/~apetrov/PHY2140/Lecture8.ppt

The velocity of the electric field is about 10^8 metres per second i.e. 1/3 of the speed of light.

 
 
 
 


836 posts

Ultimate Geek

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  #143024 4-Jul-2008 18:48
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grant_k:
Fraktul: ...I'm saying that reffering to the speed of light in copper is pretty nonsensical when you are in fact meaning the electromagnetic wave propogation.

Actually Fraktul **cough** putting on my Electrical Engineer's hat here, it isn't Electromagnetic Wave Propagation we are dealing with either.  That only happens in Free Space.

The mechanism involved is Electron Drift Velocity and this little snippet sums it up quite well:

At the given drift speed it would take about 2000 seconds for an electron to travel 20cm distance.  It is about half an hour.  But discharge happens instantaneously.  This is because the wire is already full with electrons!


If you think of a pipeline full of electrons it makes it easier to visualise.  Push an extra electron into the pipe and another one more-or-less instantaneously pops out the other end.  The speed of this mechanism is controlled by the electric field applied from one end of the conductor to the other.

According to some info. here:

http://www.physics.wayne.edu/~apetrov/PHY2140/Lecture8.ppt

The velocity of the electric field is about 10^8 metres per second i.e. 1/3 of the speed of light.


I am well aware the actual speed of electrons in a DC tramsmission is very low (in AC it is obviously effectively 0 as a vector). "Discharge" cannot happen instantionously - think about what that would imply, instantaneous communications. I stand corrected on wave vs field.

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