NCEES Thermodynamics 4 — Questions and Answers
Question 1: The specific heat at constant volume (cv) for an ideal diatomic gas at moderate temperatures is approximately:
- (3/2)R
- (5/2)R (Correct answer)
- (7/2)R
- R
Correct answer: (5/2)R
A diatomic ideal gas has 5 degrees of freedom (3 translational + 2 rotational) at moderate temperatures, giving cv = (5/2)R.
Question 2: Water at 100°C and 1 atm has hfg = 2257 kJ/kg. How much heat is required to completely vaporize 5 kg of saturated water?
- 11,285 kJ (Correct answer)
- 1,128.5 kJ
- 22,570 kJ
- 5,642.5 kJ
Correct answer: 11,285 kJ
Q = m·hfg = 5 kg × 2257 kJ/kg = 11,285 kJ.
Question 3: Which cycle most accurately models a steam power plant?
- Brayton cycle
- Otto cycle
- Rankine cycle (Correct answer)
- Stirling cycle
Correct answer: Rankine cycle
The Rankine cycle is the ideal thermodynamic cycle for vapor power plants using steam as the working fluid.
Question 4: For an ideal gas undergoing an isothermal expansion from V1 to V2, the work done by the gas is:
- W = P1(V2 − V1)
- W = mRT ln(V2/V1) (Correct answer)
- W = cv(T2 − T1)
- W = 0
Correct answer: W = mRT ln(V2/V1)
For an isothermal process with an ideal gas, W = ∫PdV = mRT ln(V2/V1) since T is constant and PV = mRT.
Question 5: The compressibility factor Z for an ideal gas is:
- 0
- Dependent on temperature
- 1 (Correct answer)
- Equal to Pc·Vc/(R·Tc)
Correct answer: 1
By definition, Z = PV/(nRT) = 1 for an ideal gas, since ideal gases obey PV = nRT exactly.
Question 6: In a regenerative Rankine cycle, feedwater is preheated using:
- External combustion gases
- Steam extracted from the turbine (Correct answer)
- Condenser cooling water
- Boiler flue gases
Correct answer: Steam extracted from the turbine
In regeneration, steam is extracted (bled) from intermediate turbine stages to preheat feedwater, increasing cycle efficiency.
Question 7: A system undergoes a process where ΔU = 200 kJ and Q = 350 kJ. Using the first law, what is the work done BY the system?
- −150 kJ
- 550 kJ
- 150 kJ (Correct answer)
- −550 kJ
Correct answer: 150 kJ
First law: Q = ΔU + W → W = Q − ΔU = 350 − 200 = 150 kJ (work done by the system).
NCEES Thermodynamics 4
The specific heat at constant volume (cv) for an ideal diatomic gas at moderate temperatures is approximately: