Re: Hi-Def video senders (long post, sorry!)

[Jim Noble <yahoo-groups@xxxxxxxxxxxxxxxxxx>]

On 13 May 2011, at 09:37, Paul Gordon wrote:

> Is it even as long as that?... - been a long while, & my memory
isn't so good these days, but if I can remember anything at all from my
school physics lessons...
> Assuming that electric charge travels close to the speed of light... I
know that's a gross simplification, since electrons have mass, and
therefore cannot travel anywhere near the speed of light... but the
electrons themselves are just charge carriers, and don't need to move very
far or very fast for current to flow at near instantaneous speed...

Electron drift velocity is actually pretty slow (quite literally a snail's
pace for a typical low voltage electrical signal in copper), but as you
point out the signal travels much faster.

> Anyway, ISTR that light speed (300,000 Kps) equates to approx 1 metre
per femtosecond (femtosecond = 10 to the -15th power)
> So light would travel 6 metres in 6 femtoseconds...
> If a current flowed through a conductor at only 1% of light speed,
that would still only require 600 femtoseconds to make it through 6 metres
of wire...
> Or am I completely confused?!!?!


You may want to check your maths there... :-)

Electrical wave propagation in copper is a significant fraction of the
speed of light, i.e. in the order of nanoseconds per metre:
Speed of light == ~3x10^8 m/s, so 3.33x10^-9 m/s or 3.33 nanoseconds/metre,
with signal propagation in copper coming in at maybe 5 ns/m or so.

Jim



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