In an isothermal process for an ideal gas, which thermodynamic quantity remains constant?
APressure
BVolume
CTemperature
DInternal energy and temperature
Answer & Solution
Correct answer: D. Internal energy and temperature
Isothermal means at constant temperature. For an ideal gas, internal energy depends only on temperature ($U$ is a function of $T$ alone), so if $T$ is constant, $U$ is also constant. Hence both temperature and internal energy stay fixed. Pressure and volume change reciprocally to keep $PV = nRT$ constant.
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