I am still concerned about the utility and the correctness of ideas written
by those of you who are write something like: "Since we can measure a
potential difference across a device, and we can measure the current through
the device, what's wrong with calling this the resistance of that device?"
When I began this thread I mentioned that we don't' say digital ammeters
have a resistance; rather, we say they have a "voltage burden." Apparently
that example, and my comments about semiconductor devices haven't sunk in
yet. Let me try another one.
Let make a circuit consisting of two batteries, a resistor, and an ammeter
all hooked in series. The batteries are arranged opposite to each other so
the higher-voltage battery can act to recharge the lower voltage battery.
Let's assume the resistor is large enough that we do not need to consider
the internal resistance of the batteries (although considering the internal
resistance would not change my argument). Let's assume the ammeter's
resistance or voltage burden is negligible (although taking it into
consideration would not change my argument).
Let's say battery-one is 12 volts, battery-two is 8 volts, and the resistor
is 10 ohms. The current will be 0.4 amperes. Put a voltmeter across each
device, and put the red probe at the end where positive current enters each
device. Across the resistor we read 4 volts, across B2 we read 8 volts,
across B1 we read negative 12 volts.
Using V/I we conclude the resistance of R is (4 volts) / (0.4 amps) = 10
ohms.
Using V/I we conclude the resistance of B2 is (8 volts) / (0.4 amps) = 20
ohms.
Using V/I we conclude the resistance of B1 is (-12 volts) / (0.4 amps) =
negative 30 ohms.
I hope this strikes most of you as pretty crazy... assigning a resistance to
the batteries in this manner. But that is what can happen when you define
resistance as V/I. At this point you might be saying "But I would never do
that with a battery or other active device." Good, I would say the same
thing. But in my way of seeing things, I would say you are equally crazy if
you assign a resistance to a transistor, or zener diode, or digital ammeter,
etc.
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