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Re: R = V/I with real lightbulbs



Robert A Cohen wrote:

On Thu, 11 May 2000, Bernard G. Cleyet & Nancy Ann Seese wrote:

I (assuming I haven't missed a post) am surprised no one has compared
ohms law to another law. Hook's law! I invite a rancorous debate on how

No spelling flames yet? We're losing our touch!

It seems the first problem being debated is whether equation (1)
necessarily implies equation (2). The second issue being debated is
whether Ohm's law (equation 2) holds for a material for which R depends
upon temperature but is otherwise constant.

I would rather say that resistivity depends on temperature, it is a
closer tie to the physics of what is happening. From my perspective the
light filament has resistance given by {geometrical factor} *
{temperature dependent resistivity}. This resistance changes with the
voltage applied as well as the history of the filament over the normal
range of operation. One cannot even find a unique history independent
I-V plot. It is of little consequence because the sorts of
measurements/calculations one would normally associate with resistance
are more easily done from V&I without finding R. The *resistivity*
however, is a very useful material property - for example Mark is
attempting to use it to determine the actual temperature of the
filament, and with some work may be able to do a good job. BTW, does
anybody know how uniform temperature of the filament is? With a 4th
power dependence this alone could kill off the experiment - whether
using a pyrometer or anything else.

I would contrast this with other nonlinear I-V plots where it doesn't
make sense to talk about a resistance at all. I would rather not think
of the depletion regions in a diode as a resistivity, and even less so
space charge density in a vacuum tube.

()-()-()-()-()-()-()-()-()-()-()-()-()-()-()-()

Doug Craigen
http://www.dctech.com/physics/