[Phys-L] Re: light bulb transient

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 01:59 PM 11/13/2005,  Tim F., you wrote:

> I will certainly agree that the _current_ will be extra large as the
> bulb is turned on, but that will only last until the bulb warms.  When
>  the bulb is still cool, the power is greatest.  As the filiment warms
> the power in (in the form of IV) decreases while the power out
> increases (primarily in the form of sigma*T^4).  I just don't see any
> source of "thermal inertia" that will cause the temp to keep rising and
>  overshoot the equilibrium value before settling back.

//

> Tim F

Here, the reasoning Tim uses, supposes that the temperature
of the entire filament will increase equally at all points, at turn on.
  There are several problems with this particular model.

1) evaporation of the tungsten wire and so its diameter varies
along the length of the filament. This leads to bright hotspots.
2) If the filament diameter were perfectly constant, the thermal
resistance varies with distance from the support wire, and position
  in the coiled coil structure which impacts thermal reabsorption and
convective transfer

There are other plausible reasons why filament lamps fail
at turn on more often than they should.
In the 120 + 120 volt supply commonly used in the US, the distribution
  transformer carries the center neutral to ground - or should.
  A heavy current on one leg can unbalance the voltage division,
as can a light load with a high resistance ground path.
A lamp at turn on can see a descending input voltage in this circumstance.


Brian Whatcott    Altus OK    Eureka!
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l