# Evaporating tungsten wire

Consider an incandescent bulb having a thin filament of tungsten that is heated to high temperature by passing an electric current. The hot filament emits black-body radiation. The filament is observed to break up at random locations after a sufficiently long time of operation due to non-uniform evaporation of tungsten from the filament. The bulb is powered at constant voltage.

Can we solve this situation quanititatively for the temperature gradient or the resistance varitaion with temperature? Before the wire breaks, will it emit light of shorter wavelength? Is there any equation or concept regarding rate of evaporation of solids? Is it possible to predict these variations qualitatively atleast? Also, can tungsten evaporate? It has the highest melting point among all elements after all. And usually, isn't that vapour thing coming off of heated tungsten due to oxidation of tungsten?

• At the operating temperature of the tungsten filament, tungsten has significant vapour pressure, so it evaporates very slowly.
– Gert
Commented May 12, 2019 at 9:35

The non-uniform evaporation probably originates from a small variation in thickness of the filament, or possibly local variations in the degree of perfection of the crystal structure. Now thinner parts of the filament will have a greater than average resistance per unit length, and so will dissipate more power ($$P=I^2R$$). So they will get hotter, as they will tend to the steady state when power in = Power out, that is $$I^2R=\sigma A_{\text{surf}} T^4.$$ [Here we're assuming that the 'hot spot' in the filament radiates as a black body.] Because it's hotter the peak wavelength of emission is lower (Wien's law: $$\lambda_{\text{peak}} \propto T^{-1}$$), which I hope answers one of your questions.