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For Roy Lewallen et al: Re Older Post On My db Question
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February 16th 06, 10:00 PM posted to rec.radio.amateur.antenna
Owen Duffy
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For Roy Lewallen et al: Re Older Post On My db Question
On Thu, 16 Feb 2006 09:20:34 -0600,
(Richard
Harrison) wrote:
Owen Duffy wrote:
"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."
Exactly, except the term "effective value is as old as a-c power
calculations. The value of the a-c volt was chosen to produce the same
effect, lamps as bright, heat as warm, as d-c does.
My electronic dictionary says:
"rms amplitude - Root-mean-square amplitude, also called effective
amplitude. The value assigned to an alternating current or voltage that
results in the same power dissipation in a given resistance as dc
current or voltage of the same numerical value.'
Richard, your discussion here is limited to DC circuits.
Your proposition in another post that Vrms * Irms gives the Real Power
(ie indicates net energy flow over time) does apply to DC circuits,
but it does not apply generally.
You have cited a text book to support your position, however it is
likely that you have misinterpreted the text book.
Work this example through with your textbook:
We have 120Vrms AC 60Hz (sinusoidal) impressed across a load of 85
ohms of resistance and an ideal inductor of 85 ohms reactance in
series. The load impedance is 85+j85. The Circuit current is
120/(|85+j85|) or 1Arms.
The power dissipated in the resistance is 85W, and since it is the
only resistance dissipating power, the Real Power for the entire
circuit is 85W. The circuit Apparent Power is 120 * 1 or 120VA, the
Reactive Power is 85VAR.
The circuit Vrms * Irms does not give the Real Power for this circuit.
Owen
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