I know what the Poynting vector is, roughly, but what is its "speed of
propagation"?
Green

Apparently their idea was that a 60 cycle wave train can
be viewed as moving down the transmission line from source to receiver
at a given velocity which they calculated as being roughly the speed
of light. (or infinitiy if they had not rejected the second root of
their equation as physically meaningless.)
Bailey

Question, how does one determine
which way power flows in a wire carrying AC
current at any given voltage?
Ken

John Bailey wrote:

On 2 Aug 2006 16:49:02 -0700, "Edward Green"

I know what the Poynting vector is, roughly, but what is its "speed of
propagation"?

Apparently I learned the term Poynting vector some time after the
sophomore ac course where the propagation of energy down an electrical
transmission line was discussed. Amazingly, I found the remembered
passage in the section of the book Alternating-Current Circuits by
Kerchner and Corcoran, 3rd Edition, 1951!

Sometimes miracles happen. :-)

On pages 420-423,
Transmission Line Calculations, velocity of propagation, they derive
an expression for the apparent movement of the wave down a
transmission line. Recalling that earlier today, I (correctly) made
the identification of the wave as a Poynting vector but Kerchner and
Corcoran make no mention of that term in their discussion.
http://en.wikipedia.org/wiki/Poynting_vector
I find it hard (today, 50+ years later) to recall or infer much
physical meaning to Kerchner and Corcoran's terminology of velocity of
propagation. Apparently their idea was that a 60 cycle wave train can
be viewed as moving down the transmission line from source to receiver
at a given velocity which they calculated as being roughly the speed
of light. (or infinitiy if they had not rejected the second root of
their equation as physically meaningless.)

Hmm... Let's say I know all about phase and group velocity (and wasn't
there a third possibility?), but didn't think of either as particularly
related to energy flux, which is the role of the Poynting vector.

On the other hand, suppose we calculated the average field energy per
unit length of transmission line, and then the power delivered by the
line. That would apparently give us a "velocity of energy". Maybe we
are to integrate the Poynting vector over a surface perpendicular to
the line, average it over a cycle -- which gives us average energy flux
-- and divide this by the average energy density.

I wonder how we get a "meaningless root". I tried to start a thread a
while back about the idea that sometimes "meaningless roots" are dumped
in the trash bin without remorse, and sometimes, found to triumphantly
predict brave new physical phenomenon, which someone had the courage to
maintain rather than discard. But the pond was fished out.