A tungsten filament inside an incandescent light bulb can be approximated as a…
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Modern Physics · Modern Physics
Problem
A tungsten filament inside an incandescent light bulb can be approximated as a uniform cylinder 4.0cm long with a diameter of 0.020cm. When the bulb is operating at its nominal voltage, the filament’s steady-state temperature is 2800K and its surface emissivity is 0.32. The electrical power delivered to the filament is 60W. Ignore any heat conduction through the filament’s leads and any convection within the bulb; assume radiation is the only way the filament loses energy.
(a) Calculate the total external surface area of the filament. (Neglect the area of the filament’s circular ends.)
(b) Determine the rate at which the filament radiates energy at 2800K.
(c) Compute the ratio of the radiated power found in part (b) to the electrical power supplied to the filament, and interpret this ratio in terms of the filament’s efficiency as a light source.
(d) Briefly discuss one physical reason, other than radiation, that could cause the actual filament temperature to differ from the temperature calculated using an ideal radiation-only model.