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.

No, I'm sorry, that isn't true. The average power isn't the product of
the product of the RMS voltage times the RMS current, except in the
single circumstance of their being in phase.

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

I don't have this book, but I know that in the past you've quoted from
books without having fully understood the context of the quote. I'm sure
that's the case here. Any electrician or technician should know that for
sinusoidal waveforms, Pavg = Vrms * Irms * cos(theta) where theta is the
phase angle between V and I. And hopefully you can see with a few
moments and a calculator that if theta = 90 degrees, Pavg = zero
regardless of V and I.

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

Perhaps your "usual circumstances" are that the load is purely
resistive. But that's not "usual circumstances" for a host of applications.

Roy Lewallen, W7EL

How about P = Square(V) / R watts

or P = Square(I) * R watts

for no phase angles.

Reg Edwards wrote:
How about P = Square(V) / R watts

or P = Square(I) * R watts

for no phase angles.

Those are fine by me.

Just for the record, though:

V is the voltage across R, I is the current through R. This is important
when there are other components, hence possibly other values of V and I,
in the circuit. P can be average and V and I RMS; or P, V, and I can all
be functions of time -- the formulas are ok in either case. And finally,
just for the fuss-budgets, we're assuming R isn't varying with time.

Picky as this might seem, it's awfully important to make perfectly clear
just what a formula applies to and what it doesn't. Otherwise it's sure
to be misapplied in situations where it isn't valid. (Clarification
reduces the chance of misapplication from certain to only highly likely.)

Roy Lewallen, W7EL

Roy Lewallen, W7EL wrote:
"V is the voltage across R, I is the current through R."

Yes. The voltage drop across a resistor is always in-phase with the
current through the resistor as there is no energy storage in a
resistor.

Best regards, Richard Harrison, KB5WZI

On Wed, 15 Feb 2006 17:13:45 -0600 in rec.radio.amateur.antenna,
(Richard Harrison) wrote,
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.

That is true for the Special Case where your load is a pure
resistance. In general, no.

Reg Edwards wrote:
How about P = Square(V) / R watts

or P = Square(I) * R watts

for no phase angles.

Actually, the "R" implies an impedance of R+j0,
i.e. a phase angle of zero.
--
73, Cecil http://www.qsl.net/w5dxp

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.'

Best regards, Richard Harrison, KB5WZI

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
--

Owen Duffy wrote:
"Work this example through with your textbook."

Don`t need the textbook. I`ve been working these for over 60 years.

Pythagoras gave us the solution in ancient times when electricity was
produced by rubbing an amber rod with an animal pelt. The impedance is
close enough to 120 ohms.

I=E/Z
E= 1 ampere

Power=Isquared x R
Power = 85 watts

Best regards, Richard Harrison, KB5WZI

On Thu, 16 Feb 2006 17:03:29 -0600, (Richard
Harrison) wrote:

Owen Duffy wrote:
"Work this example through with your textbook."

Don`t need the textbook. I`ve been working these for over 60 years.

Pythagoras gave us the solution in ancient times when electricity was
produced by rubbing an amber rod with an animal pelt. The impedance is
close enough to 120 ohms.

I=E/Z
E= 1 ampere

Power=Isquared x R
Power = 85 watts

Nothing like Vrms * Irms though, is it?


Best regards, Richard Harrison, KB5WZI
--