Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: brightness vrs. power

I had written:

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.
[snip]
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?

On Fri, 7 Jul 2000, Bernard G. Cleyet & Nancy Ann Seese wrote:

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"?

Perhaps I am alone on this, but when I talk about the performance of my
students in general terms, I am referring to the average student, not the
"A" and "B" students.

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 ??.

I don't think I have any "A" students this semester. Well, I suppose that
remains to be seen, I suppose.

Upon re-reading the student answers to my question, I have a feeling that
they may have answered "correctly" (one-fourth as bright) by
mis-interpreting my question and applying the following logic. All seemed
to understand that the four-resistor configuration would have a resistance
equal to the one-resistor configuration. I think they may then have
figured that the two configurations must be equivalent in brightness as
well.

With such logic, you might be tempted to give them credit, then. However,
upon questioning one of the "better" students afterward, I found that she
assumed that the current through *each* bulb in the four-bulb
configuration would be the same as that through the single bulb in the
one-bulb configuration. I will try the same "quiz" next time and put in
additional questions to better identify their understanding.

P.S. On Friday, a day after the quiz, I showed them a circuit with a
battery and two light bulbs in series. The two bulbs were different and
the battery emf was such that one bulb was lit and the other was not. I
then asked them to come up with a reason for this and an experiment to
test their explanation. It was clear that they still had significant
misconceptions (and also were confusing current vs. potential difference
vs. energy) that impeded their ability to do the quiz. Next time I will
try changing the order of the activities.

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