EIT Thermodynamics 5 — Questions and Answers
Question 1: The availability (exergy) of a closed system at state (T, P) relative to the dead state (T0, P0) is defined as:
- φ = u - u0 + P0(v - v0) - T0(s - s0) (Correct answer)
- φ = h - h0 - T0(s - s0)
- φ = u + Pv - Ts
- φ = h - T0s
Correct answer: φ = u - u0 + P0(v - v0) - T0(s - s0)
The closed-system exergy (non-flow availability) is φ = (u - u0) + P0(v - v0) - T0(s - s0).
Question 2: A compressor receives air at 100 kPa and 300 K and delivers it at 500 kPa. Assuming isentropic compression with k = 1.4, what is the exit temperature?
- 475 K (Correct answer)
- 565 K
- 650 K
- 390 K
Correct answer: 475 K
T2 = T1 × (P2/P1)^((k-1)/k) = 300 × (5)^(0.4/1.4) = 300 × 5^0.286 ≈ 300 × 1.584 ≈ 475 K.
Question 3: Which statement about entropy generation (S_gen) is always true according to the second law?
- S_gen = 0 for all real processes
- S_gen < 0 for spontaneous processes
- S_gen ≥ 0 for all processes (Correct answer)
- S_gen depends only on heat transfer
Correct answer: S_gen ≥ 0 for all processes
The second law requires S_gen ≥ 0: zero for reversible processes and positive for all irreversible (real) processes.
Question 4: For a liquid-vapor mixture, as heat is added at constant pressure, the temperature remains constant during the phase change. This is because:
- The specific heat of the mixture is infinite
- All energy goes into the latent heat of vaporization (Correct answer)
- The enthalpy of the mixture decreases
- The entropy of the mixture decreases
Correct answer: All energy goes into the latent heat of vaporization
During phase change at constant pressure, added heat provides the latent heat of vaporization without changing temperature.
Question 5: In a throttling device (e.g., expansion valve), which property remains approximately constant?
- Entropy
- Temperature
- Enthalpy (Correct answer)
- Internal energy
Correct answer: Enthalpy
In a throttling process, the energy equation gives h1 ≈ h2 because no work is done and heat transfer is negligible.
Question 6: The specific heat at constant volume (Cv) and at constant pressure (Cp) are related for an ideal gas by:
- Cp = Cv + R
- Cp = Cv - R
- Cp = k × Cv where k > 1
- Both A and C are correct (Correct answer)
Correct answer: Both A and C are correct
For ideal gases, Cp - Cv = R (Mayer's relation) and Cp = k×Cv where k = Cp/Cv > 1, making both A and C correct.
Question 7: An isolated system undergoes an irreversible process. What can be said about its entropy?
- Entropy decreases
- Entropy remains constant
- Entropy increases (Correct answer)
- Entropy change is indeterminate
Correct answer: Entropy increases
For an isolated system (δQ = 0), the second law requires dS ≥ 0, with entropy increasing for irreversible processes.
EIT Thermodynamics 5
The availability (exergy) of a closed system at state (T, P) relative to the dead state (T0, P0) is defined as: