Thermodynamics Flashcards

The availability (exergy) of a closed system at state (T, P) relative to the dead state (T0, P0) is defined as:

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).

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?

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.

Which statement about entropy generation (S_gen) is always true according to the second law?

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.

For a liquid-vapor mixture, as heat is added at constant pressure, the temperature remains constant during the phase change. This is because:

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.

In a throttling device (e.g., expansion valve), which property remains approximately constant?

Answer: Enthalpy

In a throttling process, the energy equation gives h1 ≈ h2 because no work is done and heat transfer is negligible.

The specific heat at constant volume (Cv) and at constant pressure (Cp) are related for an ideal gas by:

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.

An isolated system undergoes an irreversible process. What can be said about its entropy?

Answer: Entropy increases

For an isolated system (δQ = 0), the second law requires dS ≥ 0, with entropy increasing for irreversible processes.