Question

An electric bulb with a tungsten filament was designed for a voltage of 220V and requires a power of 40W. Find the length of the filament of this bulb if the filament diameter is 0.01 mm. The absolute temperature of the filament when the bulb is alight is 2700°K. The resistivity of tungsten at 0° C is 5 X 10^-6Ω cm and increases proportionally to the absolute temperature of the filament.  

Answer 1

Answer: l = 19 cm.

Explanation:

The resistance of the filament of the bulb is

where V is the voltage, N the power, ρ the resistivity, l the length, and S the cross section.

The resitivity of the filament when incandescent is