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GATE 2020 ME (ME2) – Question 44

Thermodynamics · Entropy changes, closed and open systems · 2 marks · Multiple choice

One kg of air in a closed system undergoes an irreversible process from 1 bar absolute, 27°C to 3 bar absolute, 127°C. With R = 287 J/(kg·K) and specific-heat ratio γ = 1.4, find specific entropy change (J/(kg·K)).

  1. −26.3
  2. 28.4
  3. 172.0
  4. indeterminate, as the process is irreversible

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Correct answer: (A) −26.3

Explanation

Entropy is a state property, so the actual path need not be reversible. $c_p=\gamma R/(\gamma-1)=1004.5$ J/kg·K. Then Δs=cp ln(400/300)−R ln3=**−26.3 J/kg·K,A**. A system entropy decrease can occur in an irreversible process; total entropy generation remains nonnegative.