Thanks for the suggestions, Steve. I've updated the file to incorporate
them. The new file is at http://eznec.com/Amateur/RMS_Power.pdf.

During recent trips to Powell's Technical Bookstore and the library, I
looked through a number of books about electrical circuits which are
oriented toward hobbyists and technicians (that is, ones lacking the
math of a college level circuits text). I'm glad to say I didn't find
any which were plainly wrong about average and RMS power (like the Joe
Carr book quoted earlier here). But what nearly all of them do is to
introduce RMS voltage and current pretty early on in the text, and
explain that the RMS values of voltage and current are important because
they represent equivalent heating values (which is correct). From then
on, they simply use E and I with the assumption that they represent RMS
values of voltage and current. At some point, they introduce the
equation P = E * I or, in the more advanced ones, E * I * cos(phase
angle), and maybe at that point mention that P is the equivalent heating
power (which is also correct). What I didn't see in any of them was the
fact that the product of the RMS values of E and I is the *average*, and
*not* the RMS value of P. It's easy to understand, then, why a lot of
people, like a number of the folks who posted comments and questions
here, naturally (and incorrectly) assume that the product of Erms and
Irms is RMS power. The books simply don't contain the information you'd
need in order to discover that Erms * Irms = Pavg. Hopefully the paper
posted by Rosenbaum and the one I did will help fill the void.

Roy Lewallen, W7EL

Steve Nosko wrote:
Both the Rosenbaum and Lewallen papers look very good and well written.

I would, however, Roy, make a more explicit statement that while RMS power
can be calculated, is has no practical value for the normal considerations
of power...and add a note indicating something like that:

"The phrase "RMS Power" has been used in some circles not as an exact use of
the term RMS, but rather as an informal "standard" that actually means
"average power under specific text conditions". (I refer to an earlier post
telling of the single channel, steady state audio power amp measurement)
This use of the term "RMS" was originally initiated to call attention to the
specific test. Unfortunately, this use has caused some confusion in the use
of this terminology and is some of the motivation for the paper."

73, Steve, K9DCI

On Thu, 18 Nov 2004 15:52:41 -0800, Roy Lewallen
wrote:

Thanks for the suggestions, Steve. I've updated the file to incorporate
them. The new file is at http://eznec.com/Amateur/RMS_Power.pdf.

During recent trips to Powell's Technical Bookstore and the library, I
looked through a number of books about electrical circuits which are
oriented toward hobbyists and technicians (that is, ones lacking the
math of a college level circuits text). I'm glad to say I didn't find
any which were plainly wrong about average and RMS power (like the Joe
Carr book quoted earlier here). But what nearly all of them do is to
introduce RMS voltage and current pretty early on in the text, and
explain that the RMS values of voltage and current are important because
they represent equivalent heating values (which is correct). From then
on, they simply use E and I with the assumption that they represent RMS
values of voltage and current. At some point, they introduce the
equation P = E * I or, in the more advanced ones, E * I * cos(phase
angle), and maybe at that point mention that P is the equivalent heating
power (which is also correct). What I didn't see in any of them was the
fact that the product of the RMS values of E and I is the *average*, and
*not* the RMS value of P. It's easy to understand, then, why a lot of
people, like a number of the folks who posted comments and questions
here, naturally (and incorrectly) assume that the product of Erms and
Irms is RMS power. The books simply don't contain the information you'd
need in order to discover that Erms * Irms = Pavg. Hopefully the paper
posted by Rosenbaum and the one I did will help fill the void.

Hi Roy,

there does seem to be an unsettling amount of misinformation, errors
and poor explanations in the majority of text books I've encountered,
I'm sorry to say. No wonder there's such a huge amount of confusion
surrounding these aspects of our hobby. :-(
--

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