On Tue, 14 Feb 2006 14:15:20 -0800, Roy Lewallen
wrote:

Owen Duffy wrote:

I shouldn't use that word ONLY!!!

Only in a DC circuit, or a in an AC circuit (loop) where the current
and voltage measured are in phase. In an AC circuit where the voltage
and current are not in phase you must multiply the product of the RMS
voltage and RMS current by the cosine of the phase difference to get
real power (which is what I think you mean by "average power").


Of course, that only works when the voltage and current are sinusoidal
and of the same frequency.

Yes, implied by the "in phase" condition.

Thinking that through further brings a third case to the "ONLY"
conditions, and that is if the circuit is entirely resistive (eg real
power is the product of Vrms and Irms if the waveform is square and
the circuit contains only resistances).

More generally, the average power is 1/T
times the integral over T of v(t) * i(t) dt, where T is the interval
over which it's being averaged. If the waveforms are periodic, an
interval of one cycle can be used for T.

Roy Lewallen, W7EL
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