Re: Ohm's Law

["Van E. Neie" <ven@PHYSICS.PURDUE.EDU>]

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.


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Van E. Neie                       Ph:  765-494-5511
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