The spectral, hemispherical emissivity of tungsten may be approximated by the distribution depicted below. Consider a cylindrical tungsten filament that is of diameter D = 0.8 mm and length L = 20 mm. The filament is enclosed in an evacuated bulb and is heated by an electrical current to a steady-state temperature of 2900 K.

(a) What is the total hemispherical emissivity when the filament temperature is 2900 K?

(b) Assuming the surroundings are at 300 K. what is the initial rate of cooling of the filament when the current is switched off?

(c) Generate a plot of the emissivity as a function of filament temperature for 1300 (d) Estimate the time required for the filament to cool from 2900 to 1300 K.

Transcribed Image Text:

0.6 = 0.45 * 0.4 0.2 &=0.10 2. (um)