Robert Cohen asks why we cannot use V/I as the definition of resistance. I
think this is a sufficiently different topic that I will boldly change the
subject.
I believe many people, and many textbooks, do use this definition. The
textbook I am using (Tipler) says this.
The problem is: this sometimes leads people to assign a resistance to
something that does not really have a resistance. Of course we assume a
current-carrying device has a current through it and potential difference
across it. But that does not mean it has a resistance. Semiconductor
devices are mostly what I am thinking of here. We don't typically refer to
the collector-emitter "resistance" of a transistor.
Here is a better example. In the old days (analog meters) one had to
consider the resistance of the ammeter to understand how it perturbed the
circuit it was inserted into. It indeed behaved as if we added an Ohm's-Law
resistor into the circuit. Today, with digital multimeters, the digital
ammeter does not have a resistance. It has a "voltage burden." It does not
act like a resistor because it in no way acts like an Ohm's-Law device. As
the current through it varies, the potential difference across it stays
fairly constant (assuming the scale switch is not changed). Look in the
manual (or on the meter back) for an analog meter and you'll probably find a
resistance value. Look in the manual for a digital ammeter and you'll find
a "voltage burden."
Hence, I am not in complete agreement with defining resistance as V/I
because this definition doesn't make any sense when applied to some types of
devices.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817