EIT Thermodynamics 3 — Questions and Answers
Question 1: In a steady-flow system, the energy equation per unit mass is: h1 + V1²/2 + gz1 + q = h2 + V2²/2 + gz2 + w. For an adiabatic nozzle with negligible elevation change and no work, which simplification applies?
- h1 = h2
- h1 + V1²/2 = h2 + V2²/2 (Correct answer)
- V1 = V2
- q = w
Correct answer: h1 + V1²/2 = h2 + V2²/2
With q = 0, w = 0, and Δz ≈ 0, the energy equation reduces to h1 + V1²/2 = h2 + V2²/2.
Question 2: Which of the following is a valid statement of the second law of thermodynamics?
- Heat can spontaneously flow from cold to hot
- The entropy of an isolated system can decrease
- No heat engine can have 100% thermal efficiency (Correct answer)
- Work can be completely converted to heat in a cycle
Correct answer: No heat engine can have 100% thermal efficiency
The Kelvin-Planck statement of the second law prohibits any cyclic device from converting heat entirely to work.
Question 3: For a van der Waals gas, the equation of state is (P + a/v²)(v - b) = RT. The term a/v² accounts for:
- Molecular volume
- Intermolecular attractive forces (Correct answer)
- Kinetic energy of molecules
- Pressure losses due to viscosity
Correct answer: Intermolecular attractive forces
The term a/v² corrects for intermolecular attractive forces that reduce the effective pressure.
Question 4: In a Rankine cycle, what component increases the pressure of the working fluid?
- Boiler
- Turbine
- Condenser
- Pump (Correct answer)
Correct answer: Pump
The pump increases the pressure of the liquid condensate from condenser pressure to boiler pressure.
Question 5: For an ideal gas, the internal energy is a function of temperature only. This means that (∂u/∂v)_T equals:
- Cv
- P
- 0 (Correct answer)
- R
Correct answer: 0
For an ideal gas, internal energy depends only on temperature, so (∂u/∂v)_T = 0.
Question 6: The polytropic process PV^n = constant becomes isothermal when n equals:
- 0
- 1 (Correct answer)
- k (Cp/Cv)
- Infinity
Correct answer: 1
When n = 1, PV = constant, which for an ideal gas gives T = constant (isothermal process).
Question 7: A heat pump has a COP of 4 and delivers 8 kW of heat to a building. How much power does it consume?
- 32 kW
- 0.5 kW
- 2 kW (Correct answer)
- 6 kW
Correct answer: 2 kW
COP_HP = Q_H / W_net → W_net = Q_H / COP = 8 / 4 = 2 kW.
EIT Thermodynamics 3
In a steady-flow system, the energy equation per unit mass is: h1 + V1²/2 + gz1 + q = h2 + V2²/2 + gz2 + w.
For an adiabatic nozzle with negligible elevation change and no work, which simplification applies?