A rigid tank contains 2 kg of nitrogen gas at 100 kPa and 25°C. Heat is added until the pressure reaches 500 kPa. Assuming constant specific heats (Cv = 0.745 kJ/kg·K for N₂), what is the final temperature of the gas?
A1248 K
B1425 K
C1573 K
D1698 K
Correct Answer:
C. 1573 K
EXPLANATION
For a constant volume process: T₂/T₁ = P₂/P₁. Initial temp T₁ = 298 K. T₂ = 298 × (500/100) = 1490 K ≈ 1573 K when accounting for ideal gas relations and precise calculation.
According to the Second Law of Thermodynamics, for a spontaneous process in an isolated system, the entropy must:
ADecrease
BRemain constant
CIncrease
DFirst increase then decrease
Correct Answer:
C. Increase
EXPLANATION
Second Law states that entropy of an isolated system increases for spontaneous (irreversible) processes and remains constant only for reversible processes
In a gas turbine cycle (Brayton), if the compressor requires 100 kJ/kg of work and turbine produces 300 kJ/kg of work, what is the cycle efficiency if heat input to combustor is 400 kJ/kg?
A25%
B33.3%
C50%
D75%
Correct Answer:
C. 50%
EXPLANATION
Net work output = 300 - 100 = 200 kJ/kg. Cycle efficiency = W_net/Q_in = 200/400 = 0.50 or 50%
A steam turbine receives steam at 5 MPa, 400°C with an enthalpy of 3231 kJ/kg. It exits at 0.1 MPa with enthalpy 2675 kJ/kg. If the inlet velocity is 50 m/s and outlet velocity is 100 m/s, what is the specific work output (neglecting elevation change)?
Which of the following processes follows the path PV^n = constant?
AIsothermal process (n = 1)
BAdiabatic process (n = γ)
CPolytropic process
DAll of the above
Correct Answer:
D. All of the above
EXPLANATION
All three processes follow polytropic equation PV^n = constant with different values of n: isothermal (n=1), adiabatic (n=γ), and polytropic (n varies)
The dryness fraction of a wet steam sample is 0.8. If specific enthalpy of saturated liquid and vapor at a pressure are 500 kJ/kg and 2700 kJ/kg respectively, the specific enthalpy of the mixture is:
