Naked Science Forum

Non Life Sciences => Technology => Topic started by: chris on 27/01/2017 10:13:04

Title: How hot is a glowing tungsten light bulb filament? Why doesn't it melt?
Post by: chris on 27/01/2017 10:13:04

What is the temperature at which the tungsten filament glows in a light bulb to produce light? It must be over 2000 degrees? So how can tungsten tolerate this?

Title: Re: How hot is a glowing tungsten light bulb filament? Why doesn't it melt?
Post by: alancalverd on 27/01/2017 12:02:20

The melting point of tungsten is 3422 deg C, 3695K

"Warm white", i.e. conventional tungsten light, has a color temperature of 2700 - 3000 K. There's plenty of temperature headroom for domestic lighting but photographic "photoflood"  tungsten lamps run a bit hotter and have a much shorter life.   

Title: Re: How hot is a glowing tungsten light bulb filament? Why doesn't it melt?
Post by: evan_au on 27/01/2017 15:23:36

Chemical reactions occur much more quickly as temperature rises (see the Arrhenius equation (https://en.wikipedia.org/wiki/Arrhenius_equation)).
At temperatures approaching 3000K, tungsten would react quickly with oxygen in the air, and the filament would burn out immediately.

However, the glass bulb is filled with an inert gas (eg Argon and Nitrogen) which do not react chemically with the filament. This gas also reduces the rate at which tungsten atoms evaporate from the filament, extending its lifetime.
See: https://en.wikipedia.org/wiki/Incandescent_light_bulb#Gas_fill