NCEES Thermodynamics 4

Question 1

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

Accepted Answer

(5/2)R

Answer

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

Question 2

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?

Accepted Answer

11,285 kJ

Answer

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

Question 3

Which cycle most accurately models a steam power plant?

Accepted Answer

Rankine cycle

Answer

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

Question 4

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

Accepted Answer

W = mRT ln(V2/V1)

Answer

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

Question 5

The compressibility factor Z for an ideal gas is:

Accepted Answer

1

Answer

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

NCEES Practice Test