While inert gas reduces filament evaporation, it also conducts heat from the filament, thereby cooling the filament and reducing efficiency. At constant pressure and temperature, the thermal conductivity of a gas depends upon the molecular weight of the gas and the cross sectional area of the gas molecules. Higher molecular weight gasses have lower thermal conductivity, because both the molecular weight is higher and also the cross sectional area is higher. Xenon gas improves efficiency because of its high molecular weight, but is also more expensive, so its use is limited to smaller lamps. COUNT 5
QSN1 Why does inert gas reduce efficiency? ANS1 conducts heat from the filament, thereby cooling the filament QSN2 What affects the thermal conductivity of a gas? ANS2 molecular weight of the gas and the cross sectional area of the gas molecules QSN3 How does the molecular weight of a gas relate to thermal conductivity? ANS3 Higher molecular weight gasses have lower thermal conductivity QSN4 Why is xenon gas not more widely used? ANS4 more expensive, so its use is limited to smaller lamps QSN5 What is the advantage of using inert gas in a light bulb? ANS5 reduces filament evaporation