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Re: brightness vrs. power



I want to partly disagree with Leigh and partly agree.

First, the disagreement. I don't want to pass myself off as too big of an
authority, but, unless things have changed since I worked in the Electronics
Division of Los Alamos, a tungsten light bulb is ohmic. When working with
semiconductors, and making contacts, etc. you have to have a pretty good
understanding of ohmic and nonohmic behavior. The group I worked in dealt
with this on a weekly or daily basis.

In order for something to be non-ohmic (not follow Ohm's Law) it has to have
a nonlinear current-versus-voltage curve for some reason other than
temperature dependence. A material that has varying resistance simply
because of temperature dependence is still an ohmic material. If you view
Ohm's law as I = V/R and R must be constant, period, then you might not want
to say that a light bulb obeys Ohm's Law, but nothing else will obey Ohm's
Law either unless it is temperature regulated. The identification of R as
constant, period, is not Ohm's Law.

One of my regular labs for students is to show that standard resistors do
not give linear I-versus-V plots unless you put heatsinks on them (or unless
you run them way below their rated power). A 1/4-watt carbon resistor
running near its rated power (at the high end of the I-versus-V plot) can be
warm (depends on design) and has an observable non-linear character. A
wire-wound ceramic-package resistor definitely runs hot at its design power,
and shows nonlinear plots. My students actually determine whether the
resistor is carbon or nichrome by seeing if the temperature coefficient is
positive (nichrome) or negative (carbon). The nonlinearity for standard
resistors is nothing as great as with a tungsten light bulb, but this is
only because the temperature change is smaller for the resistors and larger
for the light bulb. It has little to do with the temperature coefficient of
the material, i.e. the temperature coefficient for the resistivity of
tungsten is not unusually high.

If the behavior of a device is I = V/R where R is R(0)*[1 + a*delta-T], then
this device is following Ohm's Law. R(0) is the resistance at some
reference temperature (usually 20 Celsius) and a is the temperature
coefficient, and delta-T is the difference between the current operating
temperature and the reference temperature.

Nonohmic is typically used to describe a rectifying contact, or a situation
where the overall current has some dependence on varying charge-carrier
population, where the charge-carrier population change is caused by
something other than temperature, e.g. avalanche effects.

Second, the agreement. I agree that trying to demonstrate simple circuit
behavior with light bulbs can confuse students because of the large change
in temperature as the light bulbs vary in brightness. I understand the
desire to use devices that the students can identify with. I understand
that using standard resistors (of sufficient size that they don't get hot)
is boring... they just sit there. But this is what we do. In our general
education physics labs we have students do simple experiments with I = V/R
using standard resistors.

On the other hand, as mentioned, our labs for science majors include quite a
bit of analysis about what might cause apparent deviations from established
laws. Temperature effects with Ohm's Law has already been mentioned. The
voltage drop across the ammeter is an example where circuits won't appear to
follow the loop rule if you don't take the ammeter into account. This
sometimes gets tough because we have students analyze a circuit with one
voltmeter and one ammeter. That means they have to move the ammeter around.
That means the ammeter voltage drop keeps moving around the circuit as they
measure the current at different points.

The overall point here is this: when you want to do experiments in the lab
to confirm Ohm's Law, or the Loop Rule, or the Junction Rule how picky do
you want to be? If you want 1% or better agreement, you have to worry about
temperature and about ammeter voltage drops, and possibly voltmeter current.
If you want 5-10% results then you don't have to worry about those things as
much. But light bulbs burning at variable brightness are even outside of
this range.

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


I don't know who Dr. Bob Beichner is, but he is wrong headed to approach
Ohm's law in this way. He must feel that light bulbs are to Ohm's law as
the orbit of Mercury is to Newton's law: corrections can be made later,
as they are needed, and as the student grows more sophisticated. Well, I
couldn't disagree with him more. The use of light bulbs to illustrate
Ohm's law is the sort of thing that gives "conceptual physics" a bad
reputation. No student will ever believe the lie that two bulbs in series
produce half the light of a single bulb connected to the same source. If
she does, she is not yet ready for physics. If she doesn't, she will look
elsewhere for enlightenment.

Leigh
he's just WRONG.