[Phys-L] Re: light bulb transient

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding John Denker's question:

> I read somewhere:
>
> > Metals are ohmic so long as one holds their temperature constant.
> > But 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?

I'm not sure that light bulbs actually *do* glow brighter for a brief
time after they are turned on.  I have not noticed that effect.  But
supposing that that observation really is correct the claimed reason
given above for it makes no sense.  I suspect that the effect (if is
actually real) would be caused by the fact that the filament of a lit
bulb in its normally operating steady state is cooled by a
combination of direct radiation to its cooler surroundings *and* by
sensible heat transfer to the gas filling the bulb which soon
establishes some sort of circulation pattern of convection cells
which carry thermal energy out to the cooler glass bulb from the
central hot filament.

But when the filament is first turned on the surrounding gas is
essentially at the glass bulb temperature and there are no convective
cells of circulating gas cooling the filament.  This means that for
some amount of time as the bulb heats up and before its final steady
state is established, its cooling mechanism is primarily by only
radiation since the convection cooling channel is effectively
non-existent until there is sufficient mass of overheated gas up
close to the filament that begins to be buoyed upward by the cooler
gas around it.  While this state of affairs persists, with the
convection channel turned off and the cooling only taking place
via direct radiation and a small amount of direct conduction to
the gas directly touching the filament (with this gas being a
good insulator rather than a heat conductor since it is still a
stationary rather than a convecting gas present), then it stands to
reason that the filament would glow somewhat brighter at a
temperature higher than its normal operating temperature since it is
not being cooled as efficiently as it is later after the convection
channel is fully established whose operation serves to bring the
filament's steady state temperature down somewhat.

David Bowman
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