Re: [Phys-l] Frequency dependence of resistance

[Jack Uretsky <jlu@hep.anl.gov>]

"standard" according to whose edict?
				Regards,
					Jack
On Thu, 18 Sep 2008, Dan Crowe wrote:

> Jack,
>
> This is the standard definition of resistance used in electrical engineering.  There are probably hundreds of books that use this definition.  It is a misunderstanding of science, and especially engineering, to think that there are not standard definitions of terms.
>
> Daniel Crowe
> Loudoun County Public Schools
> Academy of Science
> dan.crowe@loudoun.k12.va.us
> > > > Jack Uretsky <jlu@hep.anl.gov> 09/18/08 7:27 PM >>>
> It is a misunderstanding of the nature of science and/or engineering to
> use a phrase like "resistance is defined".  The brutal truth is that you
> are free to define resistance any way you please.  If you want to use
> someone else's definition of ``resistance'', then it is incumbent upon you
> to clue us in as to whose definition of resistance you are referring to.
> If, as I suspect, you have a particular textbook in mind, then you might
> have an issue with the author of that textbook,
> 					Regards,
> 						Jack
>
> On Thu, 18 Sep 2008, Dan Crowe wrote:
>
> > Resistance is defined to be the component of the impedance that is due to the component of the current that is in phase with the potential difference.  The definition of resistance does not imply that resistance is independent of frequency.  At high frequencies, the skin effect increases the resistance of metal wires, for example.
> >
> > Daniel Crowe
> > Loudoun County Public Schools
> > Academy of Science
> > dan.crowe@loudoun.k12.va.us
> >
> > > > > John Denker <jsd@av8n.com> 9/18/2008 11:16 AM >>>
> > On 09/18/2008 12:38 AM, Savinainen Antti wrote:
> >
> > > I was wondering how to explain qualitatively why the resistance of a resistor
> > > stays constant (at least in the first approximation, that is in the HS physics
> > > :-)) when frequency of current increases. The frequency dependence in the case
> > > of reactance of coil or capacitor is quite easy to explain in terms of
> > > self-induction (coil) and charging/discharging (capacitor). These results can
> > > also be derived using simple calculus which is understandable by (good) HS
> > > students.
> > >
> > > One possible explanation might use the idea of storing energy: resistor just
> > > dissipates energy to thermal energy which cannot be returned to the power
> > > supply.  Capacitor and coil can store energy in electric/magnetic fields and
> > > then give it back to the power supply when the cycle proceeds. Can these
> > > considerations lead to a plausible explanation why the resistance of a resistor
> > > stays constant in an AC-circuit, no matter what frequency? Or is so that energy
> > > has nothing to do with this?
> > >
> > > It would also be interesting to hear to what extent resistance is actually
> > > independent of frequency.
> >
> > Answer #1)  I assume the intent was to ask about the _impedance_ of a
> > resistor.  If the impedance is observed to be frequency independent,
> > that's a nontrivial observation.  In contrast, the "resistance" is by
> > definition the frequency-independent part of the impedance.  So asking
> > about a frequency-independent "resistance" would be circular and
> > pointless.
> > _______________________________________________
> > Forum for Physics Educators
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