Re: [Phys-l] physics of dissipation

[Brian Whatcott <betwys1@sbcglobal.net>]

chuck britton wrote:
> JD writes:
>
>   
> > Nyquist's model can be boiled down to a single sentence:  An
> > ohm is an ohm is an ohm, and a coaxial cable is 50 ohms.  That
> > is, we model the energy being "dissipated" in terms of energy
> > being _radiated_ into an infinitely-long piece of coax.
> >
> > This gives a perfect model of dissipation.
> >     
>
> Yes, I've wondered about '50 ohm' coax and such.
> Isn't this '50 Ohms' somewhat frequency dependent?
>
> Or is it as simple and universal concept as you imply?
> _______________________________________________
>   
John was certainly using  "50 ohms" as shorthand for characteristic 
impedance,
which for coax varies in the  45 - 90 ohms depending on dimensional details.
But your question was this: does the Characteristic Impedance of a coax 
vary with frequency?
I suppose, in answer, I could cite a voltage wave that rises at 0.1 volt 
per minute
into a coax. You will understand that the coax looks quite resistive, at 
a rather high value in this case. And at the other end, if you shine a 
flash light into a coax, it will be reflected and absorbed like a 
(relative) short circuit.

Brian W