Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Ohm's Law



I refer to my favo. "bible" Harnwell (2nd ed. 1949).

Chap. V is titled nonohmic circuit elements and alternating currents. The
first paragraph defines ohmic conduction and in contrast nonohmic
conduction. Further high lights:

Two kinds of non linear circuit elements: contingent and intrinsic.
Intrinsic include symmetric (e.g. thyrite) and asymmetric (gas, diodes, (SS
and thermionic), etc.). contingently nonohmic elements discussed in detail
include lamp filaments (carbon and W) and the thermistor.

Harnwell also, in this chapter, devotes some time to the behavior of the
above circuit elements in AC circuits.

So at least in 1949 lamps were clearly nonlinear or nonohmic circuit
elements.

All of you are familiar that gas filled tubes are (were) used as voltage
regulators. Lamps are (were) also used. The most familiar example is the
use of a low wattage house hold lamp to stabilize the early HP audio
oscillators. The + feed back was supplied by a notch filter while the amount
of - feedback was determined by the lamp. Similar tubes using an iron
filament were common before the SS era. They made use of its high thermal
coefficient of resistance and convenience as a current regulator in, inter
alia, series string vacuum tubes.

bc who has some somewhere in his infamous garage

"B. Esser" wrote:

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