> Hi
>
> What would be a reasonable figure for the speed of an electrical signal?
>
> Danny
If you are refering to the speed at which the electromagnetic wave
propagates through a conductor, it is given by a slightly modified
version of the formula, derived from Maxwell's equations, that gives the
speed for free space wave propagation:
c = sqrt(1/(u0*e0))
where:
c is the speed of light, or "phase velocity" of an electromagnetic wave
in vacuum, in [m/s].
u0 is the magnetic permeability of free space = 1.2566371E-6 N/A^2
e0 is the electric permittivity of free space = 8.8541878E-12 F/m
If you work out the formula for yourself, you will find that
c = 2.9979246E8 m/s which we have all memorized.
The dielectric material between conductors, however, usually has a
greater value for the electric permittivity than e0, which causes the
speed of "light" or really, the EM wave propagating through a conductor
(actually, a transmission line), to be less than c.
The formula is modified to account for this:
v_p = sqrt(1/(u0*e0*er))
where now v_p is the velocity of propagation
er is the dielectric constant, or relative electric permittivity of the
medium.
Specifically, typical dielectric constants might be 4.7, for instance,
for the average circuit board material.
Plugging that in to the formula gives:
v_p = 1.38E8 m/s (actually a little faster, since er is really
effectively a little lower than the 4.7 of the material since some of
the E field is in air, not the material).
Note that this is about 46% of the speed of light in vacuum.
Sound like fun? Take yourself a formal course in Electromagnetic Fields
theory, and you will know all about this stuff. And quickly forget most
of it a few weeks after finishing it.
Good day!
--
_____________________
Christopher R. Carlen
crobc@earthlink.net
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