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Re: brightness vrs. power



I posted the following message earlier, and it never came back. If some of
you already got it, forgive my reposting. Does anyone know why some e-mails
never go through? Where do they go... e-mail heaven/hell?


I think Leigh and I are close, and I can agree to leave the words ohmic and
nonohmic out of the discussion.
I view Ohm's Law as I = V/R, plus the stipulation that R is constant, which
usually implies T must also be constant.

I repeat that nothing is likely to follow Ohm's Law if we demand I = V/R
with R constant but T allowed to vary. (1) If T varies then R varies. (2) T
is almost guaranteed to vary. Any device with a potential difference across
it and a current through it will dissipate energy, usually as thermal
energy. In order to dissipate thermal energy to the surroundings the
temperature of the device must rise. How much it rises depends upon the
power and upon the thermal energy transfer rate (heatsink or not, etc.)

My point is, I don't like the idea of saying a light bulb does not follow
Ohms Law, but saying a standard resistor does follow Ohms Law. Neither one
follows Ohm's Law if a linear plot of I-versus-V is required. It's only a
matter of the size of the deviation from Ohms Law. It's not a matter of
whether there is a deviation with one and not the other.
Of course, it might be possible to keep the temperature of the resistor so
close to constant that our instruments do not have enough
sensitivity/accuracy to notice the deviation from Ohms Law for the resistor;
whereas the light bulb, by nature of its construction, cannot keep a
constant filament temperature.
So I think it is okay to say that neither the light bulb nor the resistor
follow Ohms law (except the resistor comes closer), or I think it is okay to
say they both follow Ohms Law when temperature is taken into account. What
I don't like is saying that one follows Ohms law and one does not. I would
agree to a statement something like "A resistor follows Ohms law more
closely than a light bulb because the temperature change of the resistor as
the power increases is much smaller than the temperature change of the
filament as the power increases."


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