GATE 2025 PH – Question 15
The Joule-Thomson expansion of a gas is
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Correct answer: (B) Isenthalpic
Explanation
In the **Joule-Thomson (throttling) process** a gas flows steadily through a porous plug or valve from high pressure to low pressure, with no heat exchange and no work, and with negligible kinetic and potential energy changes.
The first law for steady flow then gives
$$h_1=h_2 ,$$
that is, the **enthalpy is constant**. The process is **isenthalpic** (option B).
It is not isentropic, because throttling is irreversible and the entropy increases. The pressure falls, so it is not isobaric either.