"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.