Keith Dysart wrote:
So some times
Ptot = Pf - Pr

Waves traveling in opposite directions have no effect
on each other. The result is standing waves.

while at other times
Ptot = P1 + P2 + Pfudge

Actually, Ptot = P1 + P2 +/- Pinterference

Coherent waves traveling in the same direction have an
effect on each other. As quoted from "Optics", the last
term in the irradiance (power) equation is known as the
"interference term".

Neither of the equations is the "algebraic sum".

Of course not, power does not follow the superposition
principle. The irradiance (power) equation is the correct
way to add the power scalars that exist in superposed waves.

I'm posting an energy brain teaser problem under a new
thread by that name. Try solving the problem by conventional
methods and then try an energy analysis.
--
73, Cecil http://www.w5dxp.com