Re: R = V/I ?

[Mark Sylvester <msylvest@SPIN.IT>]

If I may add a postscript to a thread that seems to be moving on...

At 12.20 08/05/00 -0700, Leigh Palmer wrote:
> At 11:01 AM -0700 5/8/00, Mark Sylvester wrote:
> >
> >I think the reason why this is indeed pretty crazy is that the resistance
> >of a device is the property which causes it to dissipate energy (if I may
> >use the expression) when there is a current in it. (This is the physical
> >meaning of resistance, regardless of whether the device is ohmic or not.)
> >When emf's are introduced then there are other energy conversion processes,
> >and V/I is no longer linked to *dissipation*.
>
> I now understand why Mark approaches his laboratory exercise in the
> manner he does. It is because of this rather unconventional view of
> the meaning of resistance. I would not say that the resistance of a
> resistor is the property of the device which causes it to dissipate
> energy, but that is just a different world view, I guess. My amateur
> radio background makes me think of resistance in a different way.

I had a look at the textbooks in my office to check on how they approach
the question:

PSSC, Project Physics and Hecht take Leigh's line, defining resistance only
in the context of Ohm's Law.

Giancoli explicitly acknowledges both views.

Halliday & Resnick define R as V/I, and don't mention Ohm's Law for another
5 pages (this was my first undergraduate text, so maybe this is where my
view comes from).

Most interestingly, the 5 British A-level books that I have (Breithaupt,
Duncan, Whelan & Hodgeson, Cackett Lowrie & Steven, and a brand new one by
Dobson Grace & Lovett) all define resistance as R = V/I and then later
introduce Ohm's Law. The last one even introduces the concept as
"Resistance is the electrical property of a material that makes moving
charges dissipate energy" in bold type.

I quote this only in response to Leigh's classification of my view as
"unconventional".

> If I were to plug a Mixmaster into the wall and use it to stir cake
> batter, then the system would be dissipating energy. One could
> calculate a resistance R = V/I, and I^2 R would, indeed be the
> dissipated power. Could one usefully threat the mixmaster and cake
> batter system as having a resistance R? Not in my world view. Many
> such example come to mind, and I would rather reserve the concept
> of resistance to those devices which obey Ohm's law*.

Yes, but an electric motor in the first place does work, and is not a
dissipative device, so I'm not convinced by this example. It does remind me
that the real and imaginary axes on a phasor diagram are usually labelled
"resistance" and "reactance". Presumably you would want to use some other
word than resistance here, since the phasor diagram could well apply to the
Mixmaster.

Mark



_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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