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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.
I will bet that this is a wildly inaccurate way to determine temperature.
Why do you expect the resistance to vary linearly with temperature over
a range of nearly a factor of ten when you already know its resistivity
varies by more than factor of ten? You need a thermometer, and the optical
pyrometer is the thermometer of choice for this purpose.
On a related issue, how do your students determine the effective area of
the radiating filament? You should examine a filament with a hand lens
sometime, and you should recognize that its dimension will change
significantly between cold and hot, though not nearly as much as the
resistivity will change.