chuck wrote:

Energy passing through an imaginary surface (or point or plane) would
not actually do any work in passing through, and in fact would retain
its full potential to do work after having passed through.

What then is power density?

The full name is power *flux* density, implying the rate at which energy
*flows through* unit area of a defined reference plane. SI units are
watts per square metre.

Is it the amount of work that the energy passing through a unit area
of the surface "could have done" had it been actually and fully
"captured" at that surface?

Yes, that is the implication - except that it's the *rate* of energy
capture, ie the amount that could be captured from unit area in unit
time.

This is only a concept, because it isn't physically possible to
intercept the power flux through unit area of an EM wavefront - your
wave-catcher would disturb the wavefront around its edges, and the
shadow behind it would be filled in by diffraction. However, very
similar concepts apply to power flux in a transmission line - and in
that case you really *could* capture the exact steady-state power flux
at any point, by cutting the line and substituting a dummy load of the
correct impedance.


There is no real power at that surface, is there?

That rather depends on your personal definitions of the words "real",
"at" and possibly "is" :-) I think you'd caught it correctly in the
previous paragraph... but if you squeeze too hard, the waves will slip
through your fingers.




--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek