Thermodynamics Flashcards

Which Maxwell relation is derived from the Gibbs free energy G = H - TS?

Answer: (∂s/∂P)_T = -(∂v/∂T)_P

From dG = -sdT + vdP, the Maxwell relation (∂s/∂P)_T = -(∂v/∂T)_P follows from equality of mixed partials.

An ideal Otto cycle operates with a compression ratio of 8 and k = 1.4. What is the thermal efficiency?

Answer: 56.5%

η_Otto = 1 - 1/r^(k-1) = 1 - 1/8^0.4 = 1 - 1/2.297 ≈ 0.565 = 56.5%.

For a psychrometric process, the dew point temperature is the temperature at which:

Answer: Moisture begins to condense from air

The dew point is the temperature at which water vapor in moist air begins to condense when cooled at constant pressure.

The Joule-Thomson coefficient μ_JT = (∂T/∂P)_h is zero for:

Answer: Ideal gases

For an ideal gas, enthalpy depends only on temperature, so throttling (constant-h) produces no temperature change, giving μ_JT = 0.

In an air-standard Brayton cycle, increasing the pressure ratio (while maintaining the same temperature limits) will:

Answer: Increase thermal efficiency up to a point determined by the temperature limits

Brayton cycle efficiency increases with pressure ratio, but the optimal pressure ratio for maximum work is determined by the temperature ratio.

The third law of thermodynamics states that the entropy of a perfect crystalline substance at absolute zero temperature is:

Answer: Zero

The third law (Nernst theorem) establishes that the entropy of a perfect crystal at 0 K is exactly zero.

Steam quality (x) of 0.75 in a two-phase mixture means:

Answer: 75% of the mixture by mass is saturated vapor

Quality x = m_vapor / m_total, so x = 0.75 means 75% of the mixture mass is saturated vapor.