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Re: Ohm's Law



I think John Mallinckrodt summed it up fairly well (see below). But I have
a couple amens and a couple quibbles.

(1) Amen to this...this is the way I prefer to say it. Implicit in this
would be a constant of proportionality, (perhaps some emphasis on the word
constant) and that constant could be the conductance or the inverse of
resistance. But neither the word resistance nor conductance needs to be in
the statement of Ohm's law, and the presence of one or the other probably
just confuses things. The way John said it is best.

(2) & (3) Mostly amen. But... when current doesn't stay linear with voltage
because of something like a temperature change, I don't think that makes the
stuff non-ohmic. We wouldn't place that constraint on other things. For
example, we wouldn't call a first-order chemical reaction not a first order
reaction if it heated up as a result of the reaction and then "appeared" to
have its rate constant "change." Implicit in determining rate constants is
that you run your reactions at constant temperature. I would expect the
same thing for testing if something is ohmic or not... it's implicit that
you have to keep the temperature constant.

(4) This is the one I quibble with the most. If a device is non-ohmic, then
I would not even say it has a resistance. I don't think the typical
electrical engineer would speak of an operating diode or transistor as
having a resistance even though it has a potential difference across it and
a current through it.

(5) This seems redundant. It's like saying that ohmic devices have a
"constant" of proportionality (between current and potential difference)
that is... well... constant.

I would combine (4) and (5) this way... "The constant of proportionality in
Ohm's law is the conductance of the device, which can also be expressed as
the reciprocal of the resistance of the device."

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



John M. said...

1. Ohm's law says that the current through a device is directly
proportional to the applied potential difference.

2. Ohm's law is not true in general.

3. Devices that *do* obey Ohm's law are called "ohmic" or
"linear."

4. The resistance of a device is defined by R = delta_V/I

5. Therefore, Ohmic devices have a resistance that is independent
of delta_V or I. (You could, for instance, label them permanently
with a value that represents the ratio of delta_V to I or simply
band them with stripes that code that value.)