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I have a question about quartz halogen bulbs that redeposit the vaporized tungsten on the filament. After the tungsten in vaporized, it would have a tendency to redeposit itself on the coolest part of the filament, which would be the ends of the filament after it has begun to cool. Would this mean that the center of the filament would not recieve tungsten like the rest of the filament?
- Jake Elder (age 17)
Sheldon High School, Eugene, Oregon
That's a really thoughtful question, but our initial answer at least will be somewhat guesswork.
There are a couple competing effects in determining where most of
the tungsten gets redeposited. One is as you say that molecules (of the
tungsten halide gas) are more likely to stick longer to the cooler
spot. However, their decomposition to redeposited metal and halide gas
is driven by heat, so it may go faster at the hotter spots. I'm not
sure where the deposition ends up being fastest.
Since these bulbs do ultimately burn out, effects like these,
which cause the filament to become non-uniform, could be involved. If
the redeposition goes fastest at the hottest spots, that would help
prolong filament life since those will tend to be the thinnest spots in
(republished on 07/24/06)
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