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Re: Ohm's Law

Consider a diode whose I=f(V) looks like y=x^2 between V=0 and
V=6 volts. You apply 4 V and the current of 2 mA is flowing. You
say R=2000 ohms. This is a dc circuit. Add an ac source in series.
Assume its inner resistance is negligible and the dop amplitude
is 1 volt. The alternating current is superimposed on the dc. Divide
your 1 volt by the amplitude of the current and you have the dynamic
resistance. It is smaller than 2000 ohms. That is how I see it. The ac
and the dc encounter different resistances.

What would be a good mechanical or optical analogy for this?
Ludwik Kowalski

"Van E. Neie" wrote:

Bob Sciamanda wrote:

I always treat it as the definition of resistance. In like manner dV/dI
is the definition of dynamic resistance. I call neither a "law"; to me
they are definitions of useful quantities. The statement that R is a
constant might be called Ohm's law, obeyed - within limits and under
certain constraints - by some materials. But Ohm's law cannot say
anything about resistance until resistance is first defined. To lump both
the definition and the law in a single relation is either redundant or
contradictory, and certainly wasteful of notation, and confusing.

I have a question about dV/dI. If I plot a graph of V vs I, I can find the
resistance by calculating the ratio V/I. That is, I have a value for the
voltage and a value for the current, and according to our basic relationship
V = IR, I can calculate the resistance. This is not, in general, the same
thing as dV/dI. It would be the same only if the curve were linear (Ohm's
Law). So what is dV/dI? You call it dynamic resistance, but how does it
differ from regular resistance, especially when the two numbers can be quite
different?

Thanks.

--
Van E. Neie Ph: 765-494-5511
Purdue University FAX: 765-494-0706
Dept of Physics Home: 157 Ivy Hill Drive
1396 PHYS Bldg W Lafayette, IN 47906-4865
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