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Re: R = V/I ?



At 10:17 AM -0700 5/8/00, Mark Sylvester wrote:

At 10.52 07/05/00 -0700, Leigh Palmer wrote:

Well, if one wishes to call the ratio V/I the resistance of a light
bulb at a specific operating point one may certainly do so. It is a
matter of some concern to me that this resistance can't be used to
calculate anything else. It does not condense the information in a
way that will simplify a later calculation - operating voltage or
current must still be specified along with this resistance, so how
is that an improvement over specifying two other numbers, say
voltage and power?

We do a lab where students measure I and V for a range of values, using a
12V bulb. They calculate R at each point in order to find the temperature
of the filament (and then go on to plot power vs temperature in pursuit of
the 4th power radiation law). I don't think this is a counter-example to
your comment, but it does show that the concept (of resistance) is useful
even when limited in the sense that you point out.

You make my point; thank you. You have your students plot temperature
vs. power (I assume). If the student has measured I and V, can he not
calculate power without first calculating this questionable resistance
parameter? Of what value, then, is V/I?

On a related issue: do your students use an optical pyrometer to
measure the filament temperature? I think this instrument (which may
be a tad expensive) is a marvelous one to introduce students to, and
it fits well into the course when one is studying Stefan's law as
your students are doing. 42 years ago, while I was working at General
Atomic in La Jolla, a colleague devised a method by which the optical
pyrometer temperature range could be extended downward. He used a
chopper wheel running at about 10 Hz to make the dimly glowing
filaments visible. I think it got written up in RSI or somewhere like
that.

Leigh