[Phys-L] Re: light bulb transient

["Folkerts, Timothy J" <FolkertsT@BARTONCCC.EDU>]

John,

Surely you have your own opinion on this idea, but in my opinion this=
 just seems wrong.

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.  Whe=
n the bulb is still cool, the power is greatest.  As the filiment war=
ms, the power in (in the form of IV) decreases while the power out in=
creases (primarily in the form of sigma*T^4).  I just don't see any s=
ource of "thermal inertia" that will cause the temp to keep rising an=
d overshoot the equilibrium value before settling back.

If anything, as the bulb warms, the filiment will lose less thermal e=
nergy to the surroundings (because the gas in the bulb, the support p=
osts, etc, will be warmer), so the filiment should get slightly warme=
r and brighter.  Anyone have a PASCO or Vernier sensor handy???

Or consider a "slow-motion filiment" i.e. a heating element on an ele=
ctric stove.  Assuming no active temp control, the element will just =
keep slowing heating until an equilibrium is reached.


Tim F

P.S.  The original statement can be found at=20
http://www.physlink.com/Reference/Glossary.cfm


-----Original Message-----
=46rom: Forum for Physics Educators on behalf of John Denker
Sent: Sun 11/13/2005 12:38 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: light bulb transient
=20
I read somewhere:

> Metals are ohmic so long as one holds their temperature constant. B=
ut changing the temperature of
> a metal changes R slightly. Therefore such a device as an electric =
light bulb increases its
> temperature as it warms up, which is why it glows slightly brighter=
 for a very brief time just
> after it is turned on.

Comments, anyone?
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