alt.sci.physics

On Oct 4, 6:02 am, "N:dlzc D:aol T:com \(dlzc\)"
wrote:
> Dear eric h:"eric h" wrote in message
>
> news: @ ...
>
> > Hi,
>
> > Trying to find this out for a project (not a school one).
>
> > I've been looking all over the place for this information
> > and it just doesn't seem to exist.
>
> > We all know that incandescent lightbulbs get very hot.
>
> The filament is close to the temperature of the surface
> of the Sun.
>
> > Most of us know that the Easy-bake oven used to
> > use lightbulbs to cook food.
>
> > But how hot does a 100 Watt lightbulb actually get,
> > if you assume perfect insulation around it?
>
> > I've read that the filament "burns" at 1722 F, but that
> > doesn't sound right.
>
> Probably is right, when the filament is immersed in a
> normal ambient temperature.
>
> > Any ideas?
>
> Infinitely hot, with a perfect insulator, and neglecting
> any conduction of heat from the space containing
> the lamp by the current carrying wires. And allowing
> infinite time.
>
[link now broken]
>
> The rate of temperature increase in the space will
> be a function of how much mass is being heated,
> and what its "specific heat capacity" is.

Additions necessary via private email...
It would be better to respond in group. I could be lying to you, but
being in a public place, it would be less likely.

With *perfect* insulation, the temperature would climb until the
*real* filament vaporized, at some temperature above "1722
degF" (which I have not verified). In order for this to occur, the
lamp exterior temperature will be somewhat lower (say a little less
than 1000 degF).

A perfect filament and a perfect insulator will go to infinite
temperature, as I said. Energy dissipated in the filament, raises the
filament temperature *above* whatever ambient temperature is, such
that the necessary power can leave the filament. Energy in = energy
out + energy storage.

If you are going for heating, I would recommend "appliance lamps",
even though they are lower wattage. The reason is that they are made
so that the lamps will not shatter at high temps, and the filament is
thicker so they survive the higher ambient a while longer.

[added, not in email]
The darkening of the *inside* of the glass is vaporized filament that
has condensed against the relatively cooler glass. The thicker glass
also helps prevent rapid diffusion of oxygen (or gasses in general)
into the interior vacuum. Oxygen will destroy the hot filament post
haste.

David A. Smith