On Dec 24, 2:50*pm, Cecil Moore wrote:
Keith Dysart wrote:
When does P(x,t) not equal V(x,t) * I(x,t)?

Any time the angle between V(x,t) and I(x,t) is
not 0 or 180 degrees. The correct equation is:

P(x,t) = V(x,t) * I(x,t) * cos(A)


This is non-sensical. V(x,t) and I(x,t) are functions
representing the instanteous values of the voltage
and current with respect to place and time.

The expression you really mean is

Pavg = Vrms * Irms * cos(A)

You have to keep the time domain equations separate
from the phasor equations.

If you write the above equation as:

P(x,t) = V(x,t) * I(x,t)

then you have implied the *dot product* of those
terms.

No. I mean multiply the instantaneous value by
the instantaneous value, or more correctly,
the function representing the instantaneous
value of voltate by the function representing
the instantaneous value of current.

No vectors or phasors; just time domain functions.

...Keith