Entropy & Availability Explained | Unit 3 Thermodynamics | SPPU MECH End Sem Important|2019 and 2024

A) Theory Questions (Very Important)
Available Energy and Unavailable Energy – definition, concept, and physical meaning.
Exergy – definition, expression, and significance.
Dead State – definition and when a system becomes dead.
Clausius Inequality – statement, significance.
Principle of Increase of Entropy – statement + meaning.
Entropy as a Property – show that entropy is a state function.
Applications of Entropy – any 5 points.
Boyle’s Law & Charles’s Law + equation of state for ideal gas.
Helmholtz & Gibbs Functions – definition + difference.
Difference between Available Energy & Unavailable Energy.
Carnot Theorem – statement & significance.
Confusion between reversible, irreversible & impossible heat-engine results – using entropy test.

✅ B) Derivations (Must Prepare 3–5 lines derivations)
Derivation: Clausius Inequality
Derivation: Entropy of ideal gas
Derive: Entropy of reversible heat transfer ​
Derivation: Entropy is a property using cyclic integral.
Derivation: Available Energy (Exergy) lost due to irreversibility:
Derivation: Work done in polytropic process
​✅ C) Numerical Problems (Directly from papers – must solve)
Steam turbine adiabatic expansion – find
 a) Isentropic efficiency
 b) Loss of availability (exergy loss)
Heat engine (600 kJ, 1000 K → 300 K) – check 2nd law using
 a) Clausius inequality
 b) Carnot theorem
Finite-source heat transfer – entropy generated + exergy loss
 (7200 kJ/min from 1000 K to 500 K system, ambient 300 K)
Polytropic process – air at 8 bar, 100°C → 1.8 bar
 Find: ΔS, W, Q
Exergy loss during heat addition
 3 kg gas, cv=0.81, heat = 600 kJ from 1200 K source → surroundings 290 K
Heat engine rejection classification
 300 kJ/s input; rejected =
 i) 215 kJ/s
 ii) 150 kJ/s
 iii) 75 kJ/s
 Identify reversible / irreversible / impossible.