Re: [Phys-l] Frequency dependence of resistance

[Brian Whatcott <betwys1@sbcglobal.net>]

At 02:38 AM 9/18/2008, Antti, you wrote:
> Hi,
>
> 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.
>
> Regards,
>
> Antti Savinainen
> Kuopio Lyseo HS
> Finland



It seems to me that there is no compelling reason to suppose that resistance
remains the same at various frequencies. On this basis, it would be well not
 to explain what ain't necessarily so.     A review of physical 
properties shows that
a parameter like viscosity (for example) varies widely with 
temperature in various
fluids;  oil, water, air....    If one offers a fluid metaphor for 
electrical current, one
could I suppose mention that the charge carriers in fluids like water are
 dissipating energy, and local heating may well modify their 
transport properties
with increasing frequency. A more plausible mechanism is a changing 
cross-section
of the conductive channel with frequency: skin effect is a member of 
this class,
 I suppose, but the sample URL below from UMichigan,AnnArbor notes a
changing channel resistance in a particular field effect transistor 
with frequency,
and the Hall effect also  comes to mind.

  <http://www.ee.nthu.edu.tw/shhsu/conference%20papers/isdrs99_frequency%20dependent%20output%20resistance.pdf>

..or for the page-break impaired, a tiny url:
http://tinyurl.com/3zanrw


Brian Whatcott    Altus OK    Eureka!