On Wed, 6 Oct 2004 14:44:17 -0700, "El Conjeturar"
wrote:

How about:
average power =

where V and I are understood to be the effective or rms values of the
voltage and current.

No, no, no! RMS and average are two totally different things!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Read the URL
http://hyperphysics.phy-astr.gsu.edu...c/powerac.html

it did not say rms equals average

The page was written by a Carl R. (Rod) Nave Department of Physics and
Astronomy Georgia State University

a.. Associate Professor, Georgia State University
a.. Faculty Advisor, Undergraduate Program
a.. Author, HyperPhysics
..



"Paul Burridge" wrote in message
...
On Wed, 6 Oct 2004 14:44:17 -0700, "El Conjeturar"
wrote:

How about:
average power =

where V and I are understood to be the effective or rms values of the
voltage and current.

No, no, no! RMS and average are two totally different things!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Only quantities which have been arithmetically derived by the equation -

P = Sqrt( Sqr(A) + Sqr(B) + Sqr(C) + . . . )

or its equivalents, can properly be referred to as RMS.

Occasionally, when there is something specially significant in the way a
value been calculated, it may be descriptively convenient to precede its
name with RMS.

But use of the term in a non-arithmetical context s meaningless.

The term 'RMS value' is also used in conjunction with other than electrical
quantities, eg., as in Statistics.
----
Reg.

On Wed, 06 Oct 2004 13:38:19 -0700, Bill Turner
wrote:

On Wed, 06 Oct 2004 18:15:11 GMT, Gary Schafer
wrote:

How about "average power" the correct term.

_________________________________________________ ________

This will be the third time I've asked for an official source for this
"correct" term. If there is no reply, I shan't be asking again.


References:

Same one I gave before in an earlier post.
2000 ARRL handbook 6.6 chapter 6, RMS VOLTAGES AND CURRENTS. Read on
to the pep power paragraph too.

Here is another:
Here are quotes directly from: Electronics Pocket Handbook by Daniel
L. Metzger. Page 13.
This is a nice little book if you ever run across one pick it up.
About 280 pages.

Peak, Average and rms.

1. Use peak voltage or current to calculate maximum instantaneous
power only.

2. Use average current to calculate average power when the voltage is
fixed dc. Use average voltage to calculate average power when the
current is unvarying dc.

3. Use rms voltage and /or rms current to calculate average power when
the load is a linear device (resistor) and both V and I are ac in
phase and of the same waveshape. Use IV, I^2 R, V^2/R.

4. Rms measure is assumed in any ac voltage or current notation unless
peak, peak to peak, or average is specified.

5. The factor .707 for converting peak to rms applies to sine waves
only.

73
Gary K4FMX

Bill, here's the story.

RMS and average are basic mathematical functions whose definitions you
can find in numerous references(*). I'll state them here.

The average value of any periodic function is the time integral over a
cycle of the instantaneous value of the function, divided by the period.

The RMS value of any periodic function is the square root of the average
(mean value) of the square of the function, where the average is defined
as above.

First, let's look at these two values for a sine wave with peak
amplitude of V. The instantaneous value (value at any time t) is V *
sin(wt) where w is omega = 2 * pi * frequency. The integral over a cycle
is zero (since the wave spends equal amounts of time at equal amplitudes
above and below zero), so the average value is zero. Some careless
references will give a non-zero value for the "average" of a sine wave,
but this is really the average of the absolute value (that is, the
full-wave rectified value) of the sine wave. The actual average value of
a sine wave with no DC offset is zero. (If it has a DC offset, the
average value is simply the value of the offset.)

The RMS value of the sine wave is the square root of the average of the
square of the original sine function, which is V^2 * sin^2(wt). If you
graph this, you see that it looks like a rectified sine wave -- it never
goes negative. If you go through with the math to get the average of
this squared function, you get the nice value of V^2 / 2 for the
average, hence V / sqrt(2) ~ 0.707 * V for the RMS.

Now let's apply that sine wave to a resistor and look at the power.

The *instantaneous* power, that is the power at any instant, dissipated
by the resistor is v * i = v^2 / R where v is the instantaneous value of
the voltage: v = V * sin(wt). So v^2 / R = V^2 * sin^2(wt) / R. Look
familiar? So what's the average power? Using the definition of average,
the average power is the integral over a period of the instantaneous
power, divided by the period. In other words, it's average value of V^2
* sin^2(wt) / R. Looking at what we did to get the RMS voltage above,
you can see that the average power is simply the square of the RMS
voltage, divided by R.

That's why the *average* power is the square of the *RMS* voltage
divided by R. It's important to realize that this holds true for any
periodic voltage function -- square wave, triangle wave, what have you.

You can use the basic definition of RMS to calculate an RMS value of
power from the instantaneous power, but it's not useful for anything. A
resistor dissipating 10 watts of average power gets exactly as hot if
that average power is supplied by DC, a sine wave, or any other
waveform. That's not true of the RMS power -- different waveforms
producing the same average power and causing the same amount of heat
will produce different RMS powers. So average power is a very useful
value, while RMS power is not.

The only thing that makes RMS voltage or current useful at all or
worthwhile calculating is its relationship to the useful quantity of
average power.

(*)You were asking for references -- you can find the definition of
average on p. 254 and RMS on p. 255 of Pearson and Maler, _Introductory
Circuit Analysis_, and average on p. 423 and RMS on p. 424 of Van
Valkenburg, _Network Analysis_. You'll also find an explanation in both
books similar to the one I just gave. These happen to be the two basic
circuit analysis texts I have on my shelf -- you should be able to find
the same explanation in just about any other circuits text.

Roy Lewallen, W7EL

On Thu, 07 Oct 2004 09:04:08 -0700, Bill Turner
wrote:

On Thu, 07 Oct 2004 01:28:13 -0700, Roy Lewallen wrote:

You can use the basic definition of RMS to calculate an RMS value of
power from the instantaneous power, but it's not useful for anything. A
resistor dissipating 10 watts of average power gets exactly as hot if
that average power is supplied by DC, a sine wave, or any other
waveform. That's not true of the RMS power -- different waveforms
producing the same average power and causing the same amount of heat
will produce different RMS powers. So average power is a very useful
value, while RMS power is not.

_________________________________________________ ________

That goes against everything I've ever read about RMS power, at least
for sine waves. I have always heard that a certain value of RMS power
produces the same heating as the same value of DC power. In your
statement above, you say that's true only for average power, not RMS,
and is true for *any* waveform, including sine waves.

Is there a new world order?

Bill,

If you read carefully in any of the handbooks where they discuss the
resistor heating by AC compared to DC you will see that they say "RMS
VOLTAGE that causes the same amount of heating in a resistor as the
same amount of DC voltage". They do not say the same amount of RMS
power.

Since a constant DC voltage would equate to average power in a
resistor, then if the same amount of AC RMS voltage causes the same
amount of heat it has to also be average power that it produces.

73
Gary K4FMX

Bill Turner wrote:
On Thu, 07 Oct 2004 01:28:13 -0700, Roy Lewallen wrote:


You can use the basic definition of RMS to calculate an RMS value of
power from the instantaneous power, but it's not useful for anything. A
resistor dissipating 10 watts of average power gets exactly as hot if
that average power is supplied by DC, a sine wave, or any other
waveform. That's not true of the RMS power -- different waveforms
producing the same average power and causing the same amount of heat
will produce different RMS powers. So average power is a very useful
value, while RMS power is not.


__________________________________________________ _______

That goes against everything I've ever read about RMS power, at least
for sine waves. I have always heard that a certain value of RMS power
produces the same heating as the same value of DC power.

I'd be interested in where you've read this. It's wrong. I suspect that
if you go back and read carefully, you'll find that it's the RMS value
of the voltage or current that causes the same heating as the same DC
value, not the RMS value of the power.

Incidentally, I should mention that "RMS power" is used by the makers of
audio amplifiers. About four years ago, this topic came up on
rec.radio.amateur.antenna, and Jim Kelley commented about it:

Jim Kelley wrote:

I found out the answer to this usage of RMS power. The audio folks
have co-opted the RMS idea to mean the following: If the signal is a
single sine wave, then RMS power is understood to mean the average
power output due to that sine wave. It is deceptive to the following
extent: An amplifier with a certain "RMS power rating" may go
completely flat on peaks that are only a little greater than the RMS
rating. Anyway, the use of the term is pervasive.

So the term is sometimes used in the consumer audio world, although
incorrectly. One shouldn't look to the audio consumer world for accurate
technical information about anything.

In your
statement above, you say that's true only for average power, not RMS,
and is true for *any* waveform, including sine waves.

Let me demonstrate my statement about the average power being the square
of the RMS voltage divided by R for any waveform.

Call the voltage time waveform v(t). This was V * sin(wt) for my sine
wave example, but let it be any periodic waveform. The RMS value of the
voltage Vrms is, by definition, sqrt(avg(v(t)^2)). Applied to a
resistor, the power time waveform p(t) is v(t)^2/R, so the average power
is avg(p(t)) = avg(v(t)^2/R) = avg(v(t)^2)/R. (The 1/R term can be moved
out of the average since it doesn't vary with time -- the time average
of 1/R = 1/R.) From the definition of RMS voltage, you can see that
avg(v(t)^2)/R is simply Vrms^2/R. This was demonstrated without any
assumption about the nature of v(t) except that it's periodic.

The RMS power caused by that v(t) waveform would be sqrt(avg(p(t)^2)) =
sqrt(avg(v(t)^4/R^2)) = sqrt(avg(v(t)^4))/R. This is different from the
average, with the size of the difference depending on the shape of the
waveform.

Is there a new world order?

No, but hopefully there's some new knowledge being gained.

Roy Lewallen, W7EL

"Bill Turner" wrote in message
...
On Wed, 6 Oct 2004 16:15:23 -0500, "Steve Nosko"
wrote:

3- I disagree. An adjective or modifier isn't "needed" but it sure can
help
if there may be confusion as to just what the subject is.

__________________________________________________ _______

I think we're having a semantic-fest here. Steve says an adjective
isn't "needed" but can help if there may be confusion.

Well, if it helps avoid confusion, isn't it "needed". Or is confusion
ok?
--
Bill W6WRT

Oh, I suppose, but I maintain this is something you determine within the
discussion, then move on to cover the important aspects of the issue at
hand. After moving on, you won't have to keep saying "Average Power' (or
whatever is decided upon) each time you want to refer to it. You will just
need to say "power", since you have defined the term. This is a
semantic-fest, rather than a discussion of how to determine power. I've
been in many, many discussions, where once the subject is defined, the word
"power" is used repeatedly, never with any adjectives and no one had any
problems.

You guys are too tied up in what is "official". It is what is in common
usage that matters. define your terms and move on.
73
--
Steve N, K,9;d, c. i My email has no u's.

"Bill Turner" wrote in message
...
On Wed, 06 Oct 2004 18:15:11 GMT, Gary Schafer
wrote:

How about "average power" the correct term.

__________________________________________________ _______

This will be the third time I've asked for an official source for this
"correct" term. If there is no reply, I shan't be asking again.

--


What is official to you? It appears you are not sure what the word
'average' means.

Ask Jeeves gave quite a few official looking references for a search of
"average power"
http://ask.com/
See the ones that look like dictionaries of glossaries.

See the "graphics version":
http://www.abdn.ac.uk/physics/bio/fi20www/tsld016.htm



A search via Google and some poking around just gave me:
hyperphysics.phy-astr.gsu.edu/ hbase/electric/powerac.html
http://mitglied.lycos.de/radargrundl...r/pr22-en.html

http://www.twysted-pair.com/dictp.htm


http://www.csgnetwork.com/ohmslaw2.html

Power used by the human body on this one:
http://www.csgnetwork.com/ohmslaw2.html

Look Ma, No adjectives:
http://www.ffldusoe.edu/Faculty/Dene...0&%20%20energy



This should confuse you even more.
http://www.tpub.com/content/neets/14...s/14192_15.htm

Then I found this !!
Looking for average power? eBay has great deals on new and used electronics,
cars, apparel, collectibles, sporting goods and more. If you can t find it
on eBay, it probably doesn t exist.


--
Steve N, K,9;d, c. i My email has no u's.

"Bill Turner" wrote in message
...
On Thu, 07 Oct 2004 01:28:13 -0700, Roy Lewallen wrote:

You can use the basic definition of RMS to calculate an RMS value of
power from the instantaneous power, but it's not useful for anything. A
resistor dissipating 10 watts of average power gets exactly as hot if
that average power is supplied by DC, a sine wave, or any other
waveform. That's not true of the RMS power -- different waveforms
producing the same average power and causing the same amount of heat
will produce different RMS powers. So average power is a very useful
value, while RMS power is not.

__________________________________________________ _______

That goes against everything I've ever read about RMS power, at least
for sine waves. I have always heard that a certain value of RMS power
produces the same heating as the same value of DC power. In your
statement above, you say that's true only for average power, not RMS,
and is true for *any* waveform, including sine waves.

Is there a new world order?

--
Bill W6WRT


Bill,
Please be careful here. You are confusing yourself. The passage you
quoted here is indeed correct, but I believe you are interpreting it
incorrectly because you are thinking of the term "RMS Power" as the "loosely
defined" term in the audio world also spelled "RMS Power".
The above passage is indeed correct IF you understand that it is
referring to an RMS value of the power waveform. (one of the links I posted
previously shows a power waveform in the "graphics version" link) This is a
mathematically defined "Root Mean Square" value of the power waveform. THIS
does indeed have no use. Calculating the Root-Mean-Square of a power
waveform does NOT produce the average power we think of as heating the same
as DC. We don't do it and in the Engineering community we don't use that
term at all. We talk about the 'true' or 'average power' to make things
clear, if needed.
However, the term bandied about in Audio circles which is also spelled
"RMS Power" means something completely different. One term - two meanings.
It has been used (as I gather from earlier posts) to mean what would be
correctly describes as: the average power produced by a channel of an audio
amplifier under sinewave signal conditions. This describes what is
technically called "Average Power", but the audio folks saw a need to have
something to hang their hat on we=hen talking about this measurement and,
unfortunately picled something just to confuse you, Bill.
This is two different uses for the same phrase. The first is a
mathematically defined value (the same math used to get RMS voltages) and
the other is a commonly accepted meaning in a specific field.

Both can be correct IF you understand which deffinition is in use.