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Re: Ohm's Law



On Fri, 7 Mar 2003, Stuart Leinoff wrote:

The filament of the incandescent bulb (again, as I understand it) dep=
ends on its dimensions (length, cross-sectional area), type of materi=
al (tungsten?) and the temperature.

Since the filament gets significantly hotter as current flows through=
it, its resistance changes significantly and thus "I" will not vary =
directly with "V". =20

But does this make it "non-ohmic"?

If we could somehow couple the filament to a heat sink so that the te=
mperature of the filament does not change with the increasing voltage=
would "I" then vary directly with "V"? (...and would it then be "Oh=
mic"?)
The tungsten filament has a sublimation rate which depends on the
temperature. For example, if the initial operating temperature of the
tungsten filament was 3100K then the sublimation rate would be
2.6e-5 kg/m^2s. This sublimation would cause the cross sectional area to
change which will change the resistivity of the filament and typically the
temperature. The temperature change increases the sublimation rate which
increases the rate at which the filament loses mass. Eventually the
filament temperature approaches the melting point of tungsten and failure
occurs. Holding the temprature constant via a heat sink would not prevent
the sublimation from occuring and the resistivity of the sample will
change oveer time.

That being said the tungsten filament is IMHO still ohmic. My
understanding is that nonohmic is typically used in solid state
environments where a small change in voltage results in a non linear
response in current.

Bruce Esser
Physics Teacher Something witty
Marian High School Should go here
http://marian.creighton.edu