I like to think of it this way: If you are talking instantaneous measurements, then watts is indeed always volts * amps. With a resistive load, the signs of volts and amps are always the same, and the product of the two is always non-negative. If you calculate the average of instantaneous watts over time, you get average power.
If you have an inductive load, watts is still volts * amps. But the phase shift between current and voltage means that the instantaneous power is sometimes negative, which means that the load is (at that instant) returning power to the source. But averaging instantaneous watts, both positive and negative values, still gives you average power. The problem comes when we want to calculate watts with devices that only measure voltage, or only measure current. With a resistive load, where the instantaneous power is never negative, you can calculate power by measuring only voltage, calculating the RMS voltage, and knowing the resistance. But that doesn't work for non-resistive loads because the instantaneous current is no longer proportional to the instantaneous voltage. If both are still sinusoidal, knowing the phase shift lets you calculate power. But that doesn't work either for arbitrary waveforms. Dave On Sun, Aug 19, 2012 at 10:32 PM, Chuck Harris <cfhar...@erols.com> wrote: > The long and the short of it is that when AC encounters a reactive > load, it results in a current that is not in phase with the voltage. > Power is equal to volts x amps only when the current and voltage are > in phase.... which can only happen if the load is purely resistive. > > If you hang a perfect capacitor across the power line, or a perfect > inductor, you will draw lots of current, but no power. > > -Chuck Harris > > > Tom Knox wrote: > >> >> Hi Ed; >> I may not have had enough coffee yet, but if Volt X Amps = Watts why >> would there be a difference? >> Best Wishes; >> Thomas Knox >> > > ______________________________**_________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.