Re: R = V/I ?

[Mark Sylvester <msylvest@SPIN.IT>]

At 16.28 08/05/00 -0400, Michael Edmiston wrote:
> That's what I've been trying to say all along.  I would probably reword his
> statement say "We have to determine why the device has a potential
> difference across it."  A battery has a potential difference across it
> because of an electrochemical reaction, not because of I*R.  A capacitor has
> a potential difference across it because V = Q/C, not because if I*R.  A
> reverse-biased zener diode has a potential difference across it because the
> potential difference is required to get breakdown and obtain charge carriers
> at the junction, not because of I*R.  Whether you use Robert's words or
> mine, the conclusion is the same... don't calculate a resistance for these
> devices using R = V/I... it is inappropriate to assign a resistance to these
> devices.

I have a problem with saying that a p.d. exists "because of I*R" in *any*
circumstances. A p.d. exists fundamentally because energy gets transformed
when there is a movement of charge. The volts are the joules per coulomb,
or the watts per ampere. I try not to budge from this when teaching. There
is "resistance" when the energy transformation is immediately dissipative,
i.e. it simply heats the device.

In a zener diode, does I*V quantify the heating effect of the current? Or
is there energy going into freeing charge carriers? If the latter, then the
dissipation-resistance link is strengthened in my view.

Mark



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