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Re: brightness vrs. power



Robert A Cohen wrote:

Back in May, this list had a discussion regarding Ohm's Law and the
resistance of light bulbs. I posted a question on the following series
parallel configuration:
/-----X------X-----\
----- -------
\-----X------X-----/
vs. a single configuration:
---------------X----------------

I wrote that I thought beginning students would guess (assuming constant
resistance) that the total current in each case would be the same and so
each light bulb in the second circuit would be half as bright as the lone
light bulb in the first circuit. ---------------cut



Let me also say a word about the preparation of the students. Although
the pace of the course is pretty quick, I think the course is pretty
normal otherwise. All four students are pretty average (which corresponds
to a C). The students have finished a unit on Electric Forces, Fields and
Potentials, and are currently in the midst of a unit on DC circuits. They
have already covered Ohm's Law. They measured the dependence of
resistance on material, length, area, temperature and potential
difference. (note: they found the relationships before reading from the
book what the "correct" relationships should be). The difficulty in
measuring a value for the resistance of a light bulb was also covered.

They then do a lab in which they investigate the equivalent resistance of
resistors in series and parallel. As part of this they investigate
internal resistance as well. In the last part of the lab, they come up
with a relationship for resistors in series (R = R1+R2+...) and parallel
(1/R = 1/R1 + 1/R2...) and test it by extending their measurements to
three resistors. They then test their relationship by applying it to
lightbulbs. Of course, they find the relationship does not hold. After
much mind probing, I finally reminded them that the resistance of each
light bulb depends upon temperature and that the temperature changes with
current. Note: I did *not* mention the relationship between brightness
and current (neither the naive assumption that illumination is
proportional to current nor the "real" relationship that was discussed in
the list).

It was *right* after this lab that I asked them to complete the "quiz" on
the series-parallel configuration.

I am alone in the quality of students I get? Do people still think I
underestimate them? Am I guilty of poor understanding of the concepts
myself?

No, you have just defined a "C" student. An "A" or "B" student would have "absorbed" the material you gave (rather complete, I think!) and replied correctly. Including the "caveat" constant res.
assumption. I think the answer is one fourth "bright" so (caveat abt. Stefan's, and Wien's law and eyes' response) total "emission" is equal to one alone. Am I a "B" or "C"?

bc

Back in the 70's some one created and published a critique of cartoons called Cartoon Physics. If you got the "C" students in to the 17th Cent. (i.e.) Galilei instead of Aristotle. You did well
enuff.! BTW Who well do (did) your A students do with that ??.



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