Re: Light bulb ohmicity

[Steve Luzader <sluzader@fre.fsu.umd.edu>]

I sent this message on Friday the 13th, but I never got it back.  If 
it arrived at your computer, I apologize for making you look at it 
again.

Steve Luzader
 
Studying the resistance characteristics of light bulb filaments has
been a staple for our senior lab students.  (I have someone doing it
this semester, as a matter of fact.)  Any statement to the effect that
light bulb filaments don't obey Ohm's law must be accompanied with the
the caveat that as the voltage across the filament changes, so does the
temperature of the filament.  It's easy to keep the filament at a
constant temperature by carefully breaking the bulb and immersing the
filament in mineral oil.  Then (I hope to no one's surprise) the
filament _does_ obey Ohm's law!  (This was done by a senior lab student
two years ago.)
   When the current-voltage characteristic is plotted for a normally
operating flashlight bulb, there is an interesting "kink" that appears.
I've been trying to get the students to track down the explanation for
the kink (which shows up as a peak in the "dynamic resistance" dV/dI
that Leigh Palmer recommends studying), but so far no one has gone
beyond simply graphing the data and not really interpreting it.  I hope
this year's student will actually delve into the metallurgy of tungsten
and perhaps come up with the explanation.  Maybe the internet link
given by Leigh will help.
 
Steve Luzader
> James W. Wheeler wrote:
> > But we often do an Ohmic heating lab in which a heating coil of nichrome
> > (or even a resistor) is immersed in water and the temperature rise is
> > limited to 5-10K.
> >
> > On Thu, 12 Mar 1998, Leigh Palmer wrote:
>
> > > David's idea of immersing a tungsten filament in a constant
> > > temperature environment is invincibly problematic. It would be
> > > possible to do such a thing if Joule heating could be ignored,
> > > but it can't. I appreciate the theoretical approach to many
> > > problems, but a dissipationless flow of current in an ohmic
> > > substance is, I'm afraid, out there with the spherical cow.
> > >
> > > Leigh
           
> --
> Stephen Luzader
> Frostburg State University
> Frostburg, MD
> http://antoine.fsu.umd.edu/phys/luzader

-- 
Stephen Luzader
Frostburg State University
Frostburg, MD
http://antoine.fsu.umd.edu/phys/luzader