# RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric

From: Sweetman, Eric (Eric) ([email protected])
Date: Mon Feb 19 2001 - 07:31:09 PST

I once had an E&M Physics professor who, not facetiously, stated that the
energy of an EM signal travels through the dielectric medium (or free
space); the conductors only provide the boundary conditions.

Eric Sweetman
Lucent Technologies Bell Labs
Princeton, NJ
[email protected]

> ----------
>
> Date: Sun, 18 Feb 2001 10:24:26 +0200 (IST)
> From: Offer Kaye <[email protected]>
> Subject: RE: [SI-LIST] : Simple question about propagation speed vs. diele
> ctric
>
> Pat, that's the same equation as mentioned in the previous replies, it's
> simply that the equation: V=C/sqrt(Er) assumes that mu=1 (that's realtive
> mu), which is a pretty good assumption since most materials we (EE's)
> use are dielectric, not magnetic, in nature. In Optics, for example, that
> assumption is usually false.
>
> One question to everyone who answered Keith's question- why are you
> assuming Keith meant the propogation speed of Electromagnetic signals?
> Because while the above equation is true for EM signals, and as noted in
> another answer, the sqrt(Er) can simply be called the "refractive index
> (nr)" of the material, how do you know Keith didn't mean the propogation
> speed of electrical signals in a PCB conductor (or other conductor, for
> that matter)?
> In that case, the speed is inversly proportional to sqrt(Er), so if Er
> increases V(sig) decreases. However, the exact formula changes according
> to:
> 1. Geometry (stripline, microstrip, etc.)
> 2. Er used should be *effective* Er, which can be hard to calculate.
> factors to the equation.
>
> Last but not least, remember the basic difference between EM wave
> propagation and electrical signals propagation speed: EM waves depend on
> the Er of the medium through which they are travelling, electrical signals
> depend on the Er of the surrounding dielectric.
>
> Hope this helps... :)
>
> Offer Kaye
> [email protected]
>
>
> On Thu, 15 Feb 2001 [email protected] wrote:
>
> > There's also that old equation:
> > v = c/(sqrt(mu * epsilon))
> >
> > So as epsilon, or Er, gets bigger, v gets smaller.
> >
> > --Pat
> >
> > -----Original Message-----
> > From: Kowal, Keith [mailto:[email protected]]
> > Sent: Thursday, February 15, 2001 2:19 PM
> > To: '[email protected]'
> > Subject: [SI-LIST] : Simple question about propagation speed vs.
> dielectric
> >
> >
> >
> > Folks,
> > IN general , is it true or false, signals travel faster the lower the
> Er
> > ,, or not //// why or whynot?
> >
> > keith
> > Digital Networks, LLC
> >
> >
>
>
>
>
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