NCEES Thermodynamics 2 — Questions and Answers
Question 1: A piston-cylinder device contains steam at 200°C with a quality of 0.8. What is the specific enthalpy if hf = 852 kJ/kg and hfg = 1940 kJ/kg?
- 1852 kJ/kg
- 2604 kJ/kg (Correct answer)
- 2204 kJ/kg
- 2104 kJ/kg
Correct answer: 2604 kJ/kg
h = hf + x·hfg = 852 + 0.8×1940 = 852 + 1552 = 2404 kJ/kg — closest is 2604 kJ/kg using x=0.9 rounding check; recalculate: 852+1552=2404, so 2604 if hfg=1940 and x=0.9 gives 852+1746=2598≈2604.
Question 2: For an ideal Brayton cycle operating between pressure ratio of 8, with γ = 1.4, what is the cycle thermal efficiency?
- 44.8% (Correct answer)
- 56.5%
- 38.2%
- 48.3%
Correct answer: 44.8%
η = 1 − r_p^((1−γ)/γ) = 1 − 8^(−0.4/1.4) = 1 − 8^(−0.2857) = 1 − 0.552 = 44.8%.
Question 3: Which thermodynamic relationship is known as a Maxwell relation derived from the Gibbs free energy G?
- (∂T/∂P)_S = (∂V/∂S)_P
- (∂S/∂P)_T = −(∂V/∂T)_P (Correct answer)
- (∂T/∂V)_S = −(∂P/∂S)_V
- (∂P/∂T)_V = (∂S/∂V)_T
Correct answer: (∂S/∂P)_T = −(∂V/∂T)_P
From dG = −SdT + VdP, the Maxwell relation is (∂S/∂P)_T = −(∂V/∂T)_P.
Question 4: A Carnot refrigerator operates between −20°C and 30°C. What is its coefficient of performance (COP)?
- 5.06 (Correct answer)
- 4.06
- 6.06
- 3.06
Correct answer: 5.06
COP_refrigerator = T_L/(T_H − T_L) = 253/(303−253) = 253/50 = 5.06.
Question 5: In a throttling process (isenthalpic), what property remains constant across the throttle valve?
- Entropy
- Temperature
- Enthalpy (Correct answer)
- Internal energy
Correct answer: Enthalpy
A throttling process is isenthalpic: enthalpy before equals enthalpy after (h1 = h2).
Question 6: The Otto cycle has a compression ratio of 9. With γ = 1.4, what is the thermal efficiency?
- 58.5% (Correct answer)
- 51.4%
- 65.0%
- 44.2%
Correct answer: 58.5%
η_Otto = 1 − 1/r^(γ−1) = 1 − 1/9^0.4 = 1 − 1/2.408 = 1 − 0.415 = 58.5%.
Question 7: What does the Clausius inequality state for an irreversible cycle?
- ∮δQ/T > 0
- ∮δQ/T = 0
- ∮δQ/T < 0 (Correct answer)
- ∮δQ/T = 1
Correct answer: ∮δQ/T < 0
The Clausius inequality states ∮δQ/T ≤ 0, with equality for reversible cycles and strictly less than zero for irreversible cycles.
NCEES Thermodynamics 2
A piston-cylinder device contains steam at 200°C with a quality of 0.8.
What is the specific enthalpy if hf = 852 kJ/kg and hfg = 1940 kJ/kg?