Thermodynamics Flashcards

The specific heat at constant volume (cv) for an ideal diatomic gas at moderate temperatures is approximately:

Answer: (5/2)R

A diatomic ideal gas has 5 degrees of freedom (3 translational + 2 rotational) at moderate temperatures, giving cv = (5/2)R.

Water at 100°C and 1 atm has hfg = 2257 kJ/kg. How much heat is required to completely vaporize 5 kg of saturated water?

Answer: 11,285 kJ

Q = m·hfg = 5 kg × 2257 kJ/kg = 11,285 kJ.

Which cycle most accurately models a steam power plant?

Answer: Rankine cycle

The Rankine cycle is the ideal thermodynamic cycle for vapor power plants using steam as the working fluid.

For an ideal gas undergoing an isothermal expansion from V1 to V2, the work done by the gas is:

Answer: W = mRT ln(V2/V1)

For an isothermal process with an ideal gas, W = ∫PdV = mRT ln(V2/V1) since T is constant and PV = mRT.

The compressibility factor Z for an ideal gas is:

Answer: 1

By definition, Z = PV/(nRT) = 1 for an ideal gas, since ideal gases obey PV = nRT exactly.

In a regenerative Rankine cycle, feedwater is preheated using:

Answer: Steam extracted from the turbine

In regeneration, steam is extracted (bled) from intermediate turbine stages to preheat feedwater, increasing cycle efficiency.

A system undergoes a process where ΔU = 200 kJ and Q = 350 kJ. Using the first law, what is the work done BY the system?

Answer: 150 kJ

First law: Q = ΔU + W → W = Q − ΔU = 350 − 200 = 150 kJ (work done by the system).