Re: brightness vrs. power

[Brian McInnes <bmcinnes@PNC.COM.AU>]

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