Engineering Questions with Answers - Multiple Choice Questions

# MCQs on Thermodynamic Properties & Compressibility

1 - Question

If there is no exchange of heat between system and surrounding where system comprises of a compressible fluid but the heat is generated due to friction, the process is an adiabatic.
a) True
b) False

b
Explanation: For process to be adiabatic, there is no heat exchange and no heat generation within fluid.

2 - Question

For a compressible fluid, if there is no change in specific volume at constant temperature, what type of process it is?
a) Isothermal process
c) Polytropic process
d) None of the mentioned

a
Explanation: As, specific volume remains constant, density remains constant. Therefore for given temperature there is no change in volume. hence, the process is isothermal.

3 - Question

If the fluid is incompressible, do thermodynamic properties play an important role in its behaviour at varying temperature and pressure?
a) Yes
b) No
c) Depends on the fluid
d) None of the mentioned

b
Explanation: If fluid is incompressible there is not much change in observed properties with variation in temperature and pressure. Hence, no perceivable change.

4 - Question

If for same temperature and pressure change, the value of bulk modulus is compared for isothermal process and adiabatic process, which one would be higher?
a) Isothermal process
c) Value is constant for both the processes
d) None of the mentioned

b
Explanation: For isothermal process
K=p
K=kp
where K=Bulk modulus
k=Polytropic constant
p=Pressure.

5 - Question

The value of gas constant is same for all the gases
a) True
b) False

b
Explanation: The value of gas constant depends on molecular weight. As the molecular weight is different, gas constant will be different.

6 - Question

Calculate the pressure exerted by 9 kg of air at a temperature of 20℃ if the volume is 0.8m3. Assuming ideal gas laws are applicable.
a) 946 kN/m2
b) 1892 kN/m2
c) 1419 kN/m2
d) None of the mentioned

a
Explanation: Ideal gas Law: PV=nRT
n=M/m
P=(9*8314*293)/28.97=946 kN/m2.

7 - Question

A gas weighs 16 N/m3 at 30℃ and at an absolute pressure of 0.35 N/mm2. Determine the gas constant.
a) 708.23
b) 354.11
c) 531.17
d) 1062.34

a
Explanation: R=P/(ρ*T)=3500000*9.81/16*303=708.23.

8 - Question

A cylinder of 0.8 m3 in volume contains superheated steam at 70℃ and .4 N/m2 absolute pressure. The superheated steam is compressed to .3 . Find pressure and temperature.
a) 0.74 N/m2, 422.3℃
b) 1.48 N/m2, 422.3℃
c) 0.74 N/m2, 844.6℃
d) 1.48 N/m2, 844.6℃

a
Explanation: For polytropic process,
P2=(v1/v2)n *P1
=(0.8/0.3)1.3 * 0.4 ……..(for superheated stream n=1.3)
=.74 N/m2
T1=P1v1/nR=422.3℃.

9 - Question

Determine the compressibility of an incompressible fluid, if the pressure of the fluid is changed from 70 N/m2 to 130 N/m2. The volume of the liquid changes by 0.15 percent.
a) 0.0025 m2/N
b) 0.0050 m2/N
c) 0.0070 m2/N
d) 0.0012 m2/N

a
Explanation: Compressibility=1/Bulk Modulus
=1/K
K=(dp*V/dv)
=60/0.15
=400
Compressibility=.0025.

10 - Question

What is the variation of cp, cv and k in case of gases when the temperature increases?
a) cp and cv decreases with temperature, and k increases
b) cp and cv increase with temperature, and k decreases
c) cp and cv increase with temperature, and k increases
d) cp and cv decreases with temperature, and k decreases

View Answer b Explanation: cp is molar heat capacity at constant pressure. As temperature is increased, enthalpy increases, heat capacity increases. Same is for cv, cp is molar heat capacity at constant volume. However cp-cv=R and cp/cv = R Hence, as cp, cv increases R decreases.