A closed system is cooled reversibly from 100°C to 50°C. If no work is done on the system________________?
Correct answer: B. U and S both decreases
- A. its internal energy (U) decreases and its entropy (S) increases
- B. U and S both decreases
- C. U decreases but S is constant
- D. U is constant but S decreases
Explanation
For reversible cooling, heat leaves the system, so entropy decreases because dS = δQrev/T is negative. With no work, the energy balance gives ΔU = Q, so internal energy also decreases.
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