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At 11.21 08/05/00 -0700, Leigh Palmer wrote:
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?
It's used to determine the temperature of the filament, from R = Ro(1 +
alpha*theta). For Ro we use the smallest I and V that we can measure to 2
SD, using the best meters we have. We use the tcr of tungsten as found in
the data book.