Which of the following is a reversible process in the thermodynamic sense?
AQuick compression of a gas with a piston
BFree expansion of a gas into vacuum
CHeat flow from a hot body to a cold body
DAn infinitely slow isothermal compression
Answer & Solution
Correct answer: D. An infinitely slow isothermal compression
1. A reversible process passes through equilibrium states, requiring it to be infinitely slow (quasi-static).
2. Free expansion is irreversible: the gas can't be put back without external work.
3. Heat flow across a finite temperature difference is irreversible by the second law.
4. Sudden compression involves dissipation; not reversible.
5. Only the infinitely slow isothermal compression qualifies, in idealisation.
_Source: NCERT Class 11 Physics Ch 11, §11.9 Reversible and Irreversible Processes_
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