Context: 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.

Question: Why does inert gas reduce efficiency?
Answer: conducts heat from the filament, thereby cooling the filament

Question: What affects the thermal conductivity of a gas?
Answer: molecular weight of the gas and the cross sectional area of the gas molecules

Question: How does the molecular weight of a gas relate to thermal conductivity?
Answer: Higher molecular weight gasses have lower thermal conductivity

Question: Why is xenon gas not more widely used?
Answer: more expensive, so its use is limited to smaller lamps

Question: What is the advantage of using inert gas in a light bulb?
Answer: reduces filament evaporation

Question: What does not reduce filament evaporation?
Answer: Unanswerable

Question: What conducts heat and heats the filament?
Answer: Unanswerable

Question: What does not affect the thermal conductivity of a gas?
Answer: Unanswerable

Question: Why is xenon gas widely used?
Answer: Unanswerable

Question: What is a disadvantage of using inert gas in a light bulb?
Answer: Unanswerable