Re: impedance of free space

["James S. Marsh" <jsmarsh@uwf.edu>]

The way I like to remember and use the impedence of free space is that the
time averaged irradiance I, or Poynting vector S, for a plane
electromagnetic wave can be written in terms of the electric field
amplitude of the wave, Eo, as I or  S=(1/2)Eo^2/Z, where Z is the impedence
of free space.  This looks like the time average power dissipated in a
resistor with an AC voltage with amplitude Vo applied, P=(1/2)Vo^2/R, where
R is the resistance.  This also looks like the time averaged power
transferred by an endless transmission line with an AC voltage with
amplitude Vo applied, P=(1/2)Vo^2/Z, where Z is the impedence of the
transmission line.  The notion of impedence unites these three examples,
and expresses them in the same way.  JSM

> Sorry if this is a stupid question, but I'm rusty on this and none of the
> H&R or Serway level books give me a good answer.  My students asked, "If
> free space is totally empty, why does it have impedance [of 377 ohms]?"  I
> was about to answer that free space isn't really empty and launch into
> something about a sea of virtual particles, but then I thought I'd better
> ask the list first.
>
> Thanks in advance,
> Larry