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Re: Impedance &c.



The chapter on impedance matching is only one of 18 in this book. The
"further reading" suggestions for that chapter are all from electrical
engineering, except for two:

Firestone F.A. 1956 'Twixt earth and sky with rod and tube; the mobility
and classical impedance analogies J. Acoust. Soc. Am. 28 (6) 1117-53
[article appears also in American Physical Society Handbook 1957 (New York:
McGraw-Hill) pp3-140]

MacInnes I 1972 The lever as an impedance matching device Physics Education
7 509-10

"Similarities in Physics" is indeed a fascinating book, but does not take
the impedance matching discussion a whole lot further than this thread has
done so far. The Firestone reference looks intriguing.

Mark

At 12:30 11/01/04 -0800, Leigh Palmer wrote:
Wolfgang Rueckner writes:

> There is a chapter on this very question in a wonderful little book
> entitled "Similarities in Physics" by John Shive and Robert Weber.
> (That's the same Shive that gave us the Shive Wave Machine, which has
> become a staple in lecture demonstrations.) In general, if you
> regard impedance as the ratio of cause to effect [and let's NOT get
> into cause/effect arguments here], then in electrical circuits the
> ratio would be the applied voltage to resulting current and the power
> is their product.

There is a small religious problem here. Part of the catechism we
physics teachers are raised with is the proposition that force does not
cause velocity. Any percipient bicycle rider knows that this is untrue,
but still we do try to get students to believe it. To introduce force
and velocity as a cause-effect pair would pose more than philosophical
problems; it would be a heresy. Of course they are a cause-effect pair
in steady state motion of, say, an object being propelled in a viscous
medium.

I want to relate the idea of impedance to other tasks: the throwing of
a ball, for example. Why is it easier (as Fred Bucheit points out) to
impart kinetic energy to a thrown baseball than to a thrown BB or a
bowling ball? How can this problem, matching human throwing ability to
either of the latter tasks, be approached using a transformer? The
problem was solved in prehistory, of course. We have the atlatl, the
archery bow, and the trebuchet as examples of impedance matching
devices of increasing sophistication. How do we describe the
improvements to the archery bow in terms of impedance matching? I want
a systematic treatment centered on a concept, impedance, that my
intuition tells me is there. The concept is well developed in
electricity and less well developed in mechanics.

> Shive and Weber give examples from many areas, whether they be wave
> physics or mechanics, etc. and talk about impedance matching devices
> (transformers) in all of these situations for maximum transfer of
> power. The book is a lot of fun.

Wolfgang has given me one valuable reference of the sort I seek,
"Similarities in Physics". Unfortunately our library does not shelve
this book, and Amazon is enigmatic (check it out). Blackwell's shows it
at ninety pounds (!) and out of print.

Leigh

Mark Sylvester
UWCAd
Duino Trieste Italy