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Re: Ohm's Law

dV/dI is the important quantity for evaluating the effect of small signal
voltages which oscillate about some DC bias value.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Van E. Neie <ven@PHYSICS.PURDUE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, November 08, 1999 10:17 AM
Subject: Re: Ohm's Law


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
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