Re: delta V = I R

[Michael Edmiston <edmiston@BLUFFTON.EDU>]

* * * *
Richard Tarara comments about taking data in a manner that treats
potential difference as the independent variable and current as the
dependent variable.  This would most commonly prompt one to plot I
versus delta-V, and the slope would be 1/R.

I do not have a problem with this, and it is the way I have students do
it.  The fact that R is the reciprocal of the slope rather than the
slope does not cause any distress for me.  I am not aware of it causing
distress for the students.  I usually talk about conductance as well as
resistance since chemists and biologist take my course as a major
requirement.  I just don't see any problem here.

* * * *
Bob Sciamanda asks what we call (delta-V)/I for a non-ohmic device.  I
don't think we call it anything.  Although some people would still call
it "resistance," I don't think a non-ohmic device has a resistance.
 "Having" a resistance implies a definite value.  If (delta-V)/I yields
a different result for each value of delta-V, then this ratio has no
set value for that device/material.  What would be the point of giving
a name to it?

* * * *
Van Neie's comment that we might not want to call delta-V = IR as Ohm's
law is something I have advocated for some time.  My understanding is
that we should not view Ohm's law simply as an algebraic equation.  We
need to view it as saying that the current is a linear function of
potential difference for devices that are ohmic.

But I would shy away from saying that delta-V = IR is a general
equation that relates current and voltage and resistance because  of
the statement I made in response to Bob Sciamanda.  I don't think
non-ohmic devices have "a resistance."

So therein lies the problem.  Delta-V = IR is an equation that relates
the current, potential difference, and resistance for devices that are
ohmic.  Hence, in some respects it seems to be Ohm's law.   I would
rather say that delta-V = IR is an equation that applies to ohmic
devices, i.e. devices that follow Ohms' law.

We do this type of thing all the time.  PV = nRT is the ideal gas law.
 It applies to any gas which is behaving ideally.  If the gas is not
behaving ideally, then PV = nRT does not apply.

Likewise, delta-V = IR applies to devices that are behaving in an ohmic
manner.  If the device is not ohmic, then this equation does not apply.
 Is that too complicated?  If so, is PV = nRT too complicated?

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



-----Original Message-----
From:   Richard Tarara [SMTP:rtarara@SAINTMARYS.EDU]
Sent:   Monday, November 08, 1999 9:23 AM
To:     PHYS-L@lists.nau.edu
Subject:        Re: delta V = I R

----- Original Message -----
From: Michael Edmiston <edmiston@BLUFFTON.EDU>

> Not only should we think of Ohm's law as delta-V = IR, we should also
> remember that the real "meat and potatoes" of Ohm's law is that R
> should be constant.  Or, better stated, that current through a device
> is a linear function of the potential difference across the device.

The only problem with this is when we go into the laboratory with intro
students.  Yes we would like to take data, have them plot it, have it
be
linear, and have the slope end up the value of the resistance.  The
problem
is, what is normally done is to vary the potential difference (V) and
measure the current (I).  The standard plot should then yield I =
(1/R)V.
The slope is 1/R (or as our Chemistry friends would say--the
conductance).
We can always have them plot the data as V vrs I, but even many of the
intro
students know that's not 'correct' in terms of the experiment.  **I
have the
same problem with a N2 experiment where we vary the applied force and
measure the acceleration but we really want to plot F vrs a so that the
slope ends up the mass of the accelerating system.**

Rick

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Richard W. Tarara
Department of Chemistry & Physics
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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