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Re: brightness vrs. power



Leigh Palmer wrote:

... 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.

Bob Beichner is the coauthor (with R. Serway) of the 5th edition of
"Physics for Scientists and Engineers". The "over-idealization" can
be seen in question 23 of chapter 28, page 894. Also in problem 63,
page 901.

Whether or not a constant R approximation is valid depends on the
applied DOP. It is a reasonable approximation when 12 V is applied
to a bulb designed to operate at 120 V, but not reasonable at normal
operating conditions. The question 23 does not provide any DOP.
But the problem 63 does refer to 120 V and asks students to "assume
that the resistance of each bulb conforms to Ohm's law (even though
in reality the resistance increases markedly with current)".

After assigning this problem we had a classroom demonstration in
which the dependence of R on I was demonstrated. This provided
an opportunity for talking about idealizations.
Ludwik Kowalski