Internal Energy Of An Ideal Gas
An insulated chamber lets a sample of ideal gas expand freely into an adjoining vacuum with no piston to push and no heat exchange. What happens to the temperature of the gas?
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About This Question
- Subject
- physics
- Chapter
- thermodynamics
- Topic
- internal energy of an ideal gas
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
It stays the same
Free expansion of an ideal gas into a vacuum is a classic adiabatic and zero-work process: the chamber is insulated so Q=0, and the gas expands against nothing, so W=0 because there is no opposing pressure to do work against. The First Law Δcup=Q−W then gives Δcup=0. For an ideal gas internal energy depends only on temperature, so constant internal energy means constant temperature. The option that temperature rises is wrong because no work is done on the gas to add energy. The option that it falls confuses this with controlled adiabatic expansion against a piston, where the gas does real work and cools. The option that it depends on final volume is incorrect because ideal-gas internal energy is volume-independent. A consistency check: even though volume and pressure both change, the product PV=nRT keeps T fixed because U and hence T are unchanged, confirming the temperature is preserved.
This easy difficulty physics question is from the chapter thermodynamics, covering the topic of internal energy of an ideal gas. It appeared in the 2025 exam.
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