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
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