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Re: brightness vrs. power

Here's my take on this:

The only really 'physical' representation of Ohm's Law is

I = V/R

That is, the amount of current depends linearly on the potential difference
and inversely on the resistance. This is a 'causal' relationship, not a
definition. The other forms are computationally convenient in certain
circumstances, but don't define anything.

Of course the complication, as is being discussed in this thread, is that R
is dependent on I through the temperature dependence of R.

R is best defined as (rho x L)/A) where rho = rho_0(1 + alpha x delta T)
where the delta-T will be effected by the energy dissipated by the resistor
through ohmic heating.

I don't want to get between Leigh and Michael other that to say that I do
believe that there is a pedagogical defense for considering light bulbs as
resistors for certain classes. I'm thinking of non-science majors for whom
Ohm's Law is going to be the 'end game' in their electrical studies. For
example, in the class I teach, we first do a 'discovery' lab with bulbs and
batteries where students first light a single bulb with a given voltage.
They are then asked to light three bulbs. Almost all start with the series
circuit and find that the bulbs light, but dimly. Eventually they stumble
across the parallel circuit where all bulbs light as brightly as the one
originally did. {They also have to come up with the 'stairwell' wiring of
one bulb controlled independently by two switches.} The next lab is a more
formal Ohm's Law lab where they measure current (in mA) as a function of
voltage (0-10V) through some (literally) black boxes (standard lab
adjustable resistors with the values obscured). They make measurements on
two such boxes, then the boxes in series and the boxes in parallel. We have
introduced Ohm's Law prior to this lab, but after the light bulb lab.

Now the connection: I have them go back to the light bulb lab and try to
explain the 'gross' behavior of the light bulbs in terms of Ohm's Law. I
tell them to consider the bulbs as resistors AND that the brightness depends
on the amount of current flowing through the bulbs. {I don't say that the
brightness is DIRECTLY proportional to the current, and BELIEVE ME, with
this group, no one assumes such a thing--for 90% such a mathematical
relationship would never occur to them.} Of course what I am after IS that
if you assume constant resistance then the series circuit draws 1/3 the
current of the single bulb and the parallel circuit draws 3 times the
current but splits it equally. This relates to Joe Bellina's earlier
comment about the pedagogical goal of showing that the battery is NOT a
constant current device. While only some of the students will fully
recognize the relationships above, NONE are harmed by the 'unrealistic'
assumption that the bulbs act as simple resistors. The point to all of this
is that depending on the audience, the pedagogical goals can justify using
some less than precise physics. We do it all the time and the 'light bulb
as resistor' is a case where I firmly believe that Leigh's strong
condemnation of the practice is not universally warranted.

Rick

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Richard W. Tarara
Associate Professor of Physics
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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----- Original Message -----
From: "Robert A Cohen" <bbq@ESU.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, May 04, 2000 7:58 AM
Subject: Re: brightness vrs. power


I've always assumed that ohm's law was stated as R is independent of I.
Now I see that Ohm's law is really that R is independent of I at constant
T.

Now I realize that I've always assumed R was defined as V/I. Is this
correct?

----------------------------------------------------------
| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
----------------------------------------------------------

On Thu, 4 May 2000, Brian McInnes wrote:

Among other points he makes in his e-mail describing the results of
his experimental observations with bulbs and illuminance, Leigh writes


There's plenty to talk about in this demonstration. My most
important point is that Ohm's law is not an important one, and
that light bulbs are an unsatisfactory example of the application
of Ohm's law.



In Chabay and Sherwood's excellent text, "Ohm's Law" does not appear
in the index! In the text they note that "in some material s
conductivity is nearly a constant, independent of the amount of
current flowing through the resistor. We call ,such
constant-conductivity materials ohmic." That is certainly a good
put-down of the glorified Ohm's law.

Further, in their Instructor's Manual, they write: "In our experience,
students who have previously studied electric circuits have almost
always over-generalized Ohm's law, confusing it with the Kirchhoff
loop rule. They tend to think that V = IR is a fundamental physical
principle rather than merely describing the approximate behavior of
some materials (if the temperature doesn't change much)."

Brian McInnes