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February 16th 06, 12:08 AM posted to rec.radio.amateur.antenna

On Wed, 15 Feb 2006 17:13:45 -0600, (Richard
Harrison) wrote:

Roy Lewallen, W7EL wrote:
"The average power is therefore relatively small, much smaller than the
product of RMS volts times RMS amps."

RMS is short for root-mean-square. RMS is synonymous with the "effective
value" of a sinusoidal waveform.

Therefore, the average power for the time period of one complete cycle
or any number of complete cycles is the product of the effective volts
times the effective amperes.

Leaving aside your new confusing term "effective value", if you
multiply Vrms by Irms in an AC circuit you get Apparent Power (units
are Volt Amps or VA).

Apparent Power is the vector sum of two quadrature components Real
Power (Watts) and Reactive Power (VAR).

Real Power is the thing you describe when you talk about average
power. It is Real Power that is the rate of flow of energy averaged
over a complete AC cycle.

See page 19 of "Alternating Current Fundamentals" for derivations of the
proof.

Average power is exactly the product of rms volts times rms amps in
usual circumstances.

Depends on what you mean by "usual circumstances". Your rule does not
apply if there is a phase difference between V and I, which is
commonly the case in power distribution, and is commonly the case in
RF where loads circuit impedances may have a reactive component.

To an electrician, (Real) Power = Vrms * Irms * PF where PF (the power
factor) is the cosine of the phase angle between V and I. This isn't
engineering stuff, sparkies know and apply it every day.

Owen
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