Category: Thermodynamics Objective Questions

Thermodynamics Multiple Choice Questions on “Types of Equilibrium”.

1. For an isolated system,
a) dS<0
b) dS>0
c) dS=0
d) none of the mentioned
Answer: b
Clarification: We have dU+pdV-TdS<0 and for isolated system, U and V are constant.

2. The entropy of an isolated system always ____ and reaches ____ when equilibrium is reached.
a) remains constant, maximum
b) decreases, minimum
c) increases, maximum
d) none of the mentioned
Answer: c
Clarification: This is because dS>0 for an isolated system.

3. Which constraints must be imposed on system to make the Helmholtz function decrease?
a) constant T and p
b) constant U and T
c) constant U and V
d) constant T and V
Answer: d
Clarification: In these constraints, the Helmholtz function decreases and becomes minimum at final equilibrium state.

4. If the constraints are constant p and T, then the Gibbs function of a system decreases.
a) true
b) false
Answer: a
Clarification: The Gibbs function becomes minimum at the final equilibrium state.

5. Which of the following statement is true?
a) a system is in equilibrium state if, when it is disturbed, it comes back to its original state
b) if there is a spontaneous change in the state, the system is not in equilibrium
c) during a spontaneous change, the entropy of system increases
d) all of the mentioned
Answer: d
Clarification: During a spontaneous change, the entropy of system increases and reaches a maximum when equilibrium is reached.

6. A system is said to be in a state of unstable equilibrium when
a) dG<0, dF<0, dS<0
b) dG<0, dF<0, dS>0
c) dG>0, dF>0, dS>0
d) dG>0, dF>0, dS<0
Answer: b
Clarification: These conditions refer to a spontaneous change which makes unstable equilibrium.

7. What is the criterion of stability?
a) dG<0, dF<0, dS<0
b) dG<0, dF<0, dS>0
c) dG>0, dF>0, dS>0
d) dG>0, dF>0, dS<0
Answer: d
Clarification: For these conditions, a system is said to be in a state of stable equilibrium.

8. For a system to be in a state of neutral equilibrium,
a) dS=dG=dF=0
b) dS=0, dG=dF<0
c) dS=0, dG=dF>0
d) none of the mentioned
Answer: a
Clarification: The thermodynamic criterion of equilibrium thus remains at constant value for all possible variations.

9. If a system is stable to small but not large disturbances, it is said to be in metastable equilibrium.
a) true
b) false
Answer: a
Clarification: This is the definition of metastable equilibrium and an example can be mixture of oxygen and hygrogen.

10. Which of the following is true for a system at equilibrium?
a) S=Smax and ∂S=0
b) F=Fmin and ∂F=0
c) G=Gmin and ∂G=0
d) all of the mentioned
Answer: d
Clarification: These are necessary but not the sufficient conditions for equilibrium.

11. For thermal stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) both of the mentioned
d) none of the mentioned
Answer: a
Clarification: Since T>0K , Cv>0 for thermal stability.

12. For mechanical stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) (∂p/∂V)<0, at constant temperature
d) all of the mentioned
Answer: c
Clarification: This is the condition for mechanical stability.

13. For total stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) (∂p/∂V)<0, at constant temperature
d) all of the mentioned
Answer: d
Clarification: All these three conditions are required for stability.

14. The third law is a fundamental law of nature and cannot be proved.
a) true
b) false
Answer: a
Clarification: The third law is not derivable from second law and cannot be proved as is the case with zeroth, first and second laws.

15. For magnetic cooling, ____ salt is used.
a) diamagnetic
b) paramagnetic
c) both of the mentioned
d) none of the mentioned
Answer: b
Clarification: A paramagnetic salt like gadolinium sulphate is used for magnetic cooling.

Thermodynamics Multiple Choice Questions on “Aircraft Propulsion”.

1. Gas turbines are used in aircraft propulsion because
a) they are light
b) they are compact
c) they have high power-to-weight ratio
d) all of the mentioned
Answer: d
Clarification: These are the reasons why aircraft propulsion uses gas turbines.

2. The type of aircraft gas turbines include
a) turbojet
b) turbofan
c) turboprop
d) all of the mentioned
Answer: d
Clarification: These are the types of aircraft gas turbines.

3. In the ideal case, turbine work is greater than the compressor work.
a) true
b) false
Answer: b
Clarification: The turbine work is equal to the compressor work in ideal case.

4. The processes in compressor, turbine, diffuser and nozzle are
a) reversible
b) adiabatic
c) reversible and adiabatic
d) none of the mentioned
Answer: c
Clarification: This is an assumption for the aircraft gas turbine.

5. The thrust developed in turbojet engine is the
a) unbalanced force
b) balanced force
c) both of the mentioned
d) none of the mentioned
Answer: a
Clarification: It is caused by the difference in momentum of air entering the engine and exhaust gases leaving the engine.

6. Which of the following statement is true?
a) mass flow rates of gases at engine inlet and exit are same
b) the pressure at inlet and exit of engine are ambient pressures
c) both of the mentioned
d) none of the mentioned
Answer: b
Clarification: The mass flow rates at engine exit and inlet are different because of different combustion rate of fuel.

7. The air-fuel ratio used in jet engine is very small.
a) true
b) false
Answer: b
Clarification: The air-fuel ratios used in jet propulsion is usually very high.

8. When flying at high altitudes,
a) air is of less density
b) air exerts less drag force on aircraft
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: This is the reason why commercial airplanes fly at high altitudes to save fuel.

9. The propulsive efficiency is given by
a) work done by engine / propulsive power
b) propulsive power / work done by engine
c) energy input rate / propulsive power
d) propulsive power / energy input rate
Answer: d
Clarification: This tells us how efficiently the energy which is released during combustion is getting converted to propulsive power.

10. We cannot use regenerators and intercoolers on aircraft engines.
a) true
b) false
Answer: a
Clarification: The reason being space and weight limitations.

11. In aircraft propulsion the most widely used engine is
a) turbojet
b) turbofan
c) turboprop
d) all of the mentioned
Answer: b
Clarification: In turbofan engine, turbine drives a large fan which forces air through a duct surrounding the engine.

12. The bypass ratio is the ratio of
a) mass flow rates of two streams
b) pressure ratio of inlet and exit
c) volume flow rate of inlet and exit
d) none of the mentioned
Answer: a
Clarification: This is the definition of bypass ratio.

13. Increasing the bypass ratio of turbofan engine ____ thrust.
a) does not affect
b) decreases
c) increases
d) none of the mentioned
Answer: c
Clarification: As the bypass ratio increases, the thrust also increases.

14. Removing the cowl from the fan gives us
a) turbojet
b) turbofan
c) turboprop
d) all of the mentioned
Answer: c
Clarification: This is how we get a turboprop engine.

15. In turbofans, the bypass ratio is 5-6 and in turboprop it is as high as 100.
a) true
b) false
Answer: a
Clarification: This is the main difference in turbofan and turboprop engine.

Thermodynamics Multiple Choice Questions on “Energy”.

1. Energy is a
a) point function
b) property of the system
c) extensive property
d) all of the mentioned
Answer: d
Clarification: Energy has a definite value for every state of the system.

2. The specific energy, e=E/m is an extensive property.
a) true
b) false
Answer: b
Clarification: The specific energy is an intensive property.

3. (m*V*V)/2 gives the
a) macroscopic kinetic energy
b) microscopic kinetic energy
c) macroscopic potential energy
d) microscopic potential energy
Answer: a
Clarification: The formula gives the macroscopic kinetic energy of the fluid element by virtue of its motion.

4. (m*g*z) gives the
a) macroscopic kinetic energy
b) microscopic kinetic energy
c) macroscopic potential energy
d) microscopic potential energy
Answer: c
Clarification: The above formula gives the macroscopic potential energy of the fluid element by virtue of its position.

5. Which of the following types of energy can be present in molecules?
a) translational and rotational kinetic energy
b) electronic energy and vibrational energy
c) chemical energy and nuclear energy
d) all of the mentioned
Answer: d
Clarification: The molecules may be subjected to rotation as well as vibration due to a collision.

6. The total internal energy of the system is given by
a) U=N/ɛ
b) U=Nɛ
c) U=ɛ/N
d) none of the mentioned
Answer: b
Clarification: U=Nɛ where N is the total number of molecules in the system and ɛ represents the energy of one molecule.

7. In an ideal gas there are no intermolecular forces of attraction and repulsion, and the internal energy is a function of temperature only.
a) true
b) false
Answer: a
Clarification: For an ideal gas U depends only on T.

8. Which of the following is true in regard to the energy of an isolated system?
a) dQ≠0
b) dW≠0
c) E=constant
d) all of the mentioned
Answer: c
Clarification: For an isolated system, dQ=dW=0 and hence, dE=0 by first law.

9. A perpetual motion machine of first kind
a) is a fictitious machine
b) can supply mechanical work without dissipating energy
c) violates first law
d) all of the mentioned
Answer: d
Clarification: There cannot be any machine which would continuously supply mechanical energy without other form of energy being dissipated.

10. The limitation of the first law is
a) does not indicate the possibility of a spontaneous process proceeding in a definite direction
b) it assigns a quality to different forms of energy
c) indicates the direction of any spontaneous process
d) none of the mentioned
Answer: a
Clarification: This is the main limitation of first law and the second law overcomes it.

Thermodynamics Multiple Choice Questions on “The Property of Entropy”.

1. Integral of dQ/T is independent of reversible path connecting between two points.
a) true
b) false
Answer: a
Clarification: For two reversible paths, dQ/T doesn’t depend on the path taken.

2. Integral of dQ/T of a reversible path is given by
a) Si-Sf
b) Sf-Si
c) Si+Sf
d) -Si-Sf
Answer: b
Clarification: Integral of dQ/T is = Sf-Si where i=initial equilibrium state and f=final equilibrium state.

3. Entropy is a
a) path function, intensive property
b) path function, extensive property
c) point function, intensive property
d) point function, extensive property
Answer: d
Clarification: Fact about entropy and unit of entropy is J/K.

4. Specific entropy is given by( where m is the mass)
a) Sm
b) m/S
c) S/m
d) none of the mentioned
Answer: c
Clarification: s=S/m with unit J/kg K.

5. For any process which is undergone by a system
a) dQ/T>=ds
b) dQ/T<=ds
c) dQ/T≠ds
d) none of the mentioned
Answer: b
Clarification: For any process dQ/T<=ds and this comes from Clausius theorem.

6. For a reversible process,
a) dS=dQ/T
b) dS>dQ/T
c) dSd) none of the mentioned
Answer: a
Clarification: For a reversible process, dQ/T is equal to the net change in entropy.

7. For an irreversible process,
a) dS=dQ/T
b) dS>dQ/T
c) dSd) none of the mentioned
Answer: b
Clarification: For a irreversible process, change in entropy is greater than dQ/T.

8. For two different paths between same two points, entropy change is
a) depends on path taken
b) different
c) same
d) none of the mentioned
Answer: c
Clarification: This is because entropy is a property.

9. For the general case, we can write
a) S2-S1<=dQ/T for a path
b) S2-S1>=dQ/T for a path
c) S2-S1≠dQ/T for a path
d) none of the mentioned
Answer: b
Clarification: The equality sign holds good for a reversible process and the inequality sign for an irreversible process.

10. When two equilibrium states are infinitesimally near,
a) dQ/T=dS
b) dQ/T>dS
c) dQ/Td) none of the mentioned
Answer: a
Clarification: dS is an exact differential because S is a point function and a property.

Thermodynamics Multiple Choice Questions on “Useful Work”.

1. Useful work is given by
a) actual work + p(V2-V1)
b) actual work – p(V2-V1)
c) actual work + p(V2+V1)
d) none of the mentioned
Answer: b
Clarification: Useful work = actual work – work performed on the atmosphere.

2. In a steady flow system, which of the following does not change?
a) mass
b) volume
c) both of the mentioned
d) none of the mentioned
Answer: b
Clarification: The volume of the system remains constant for a steady flow system.

3. Which of the following is true?
a) in a steady flow system, no work is done on the atmosphere
b) in case of unsteady flow system, the volume of the system changes
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: In a steady flow system, the volume of the system does not change but it changes for unsteady flow system.

4. Availability function for a closed system is given by
a) U-pV+TS
b) U+pV+TS
c) U-pV-TS
d) U+pV-TS
Answer: d
Clarification: This term comes very frequent and is considered as availability function.

5. When a system exchanges heat with a thermal energy reservoir in addition to the atmosphere, the maximum useful work
a) increases
b) decreases
c) remains constant
d) none of the mentioned
Answer: a
Clarification: The maximum useful work increases in this case.

6. When the system is in equilibrium with the surroundings, this state of the system is known as dead state.
a) true
b) false
Answer: a
Clarification: In this state, the system has pressure and temperature as that of surroundings, there is no chemical reaction mass transfer and has minimum potential energy.

7. When a system exchanges heat with a thermal energy reservoir at temperature Tr in addition to the atmosphere, the maximum useful work increases by
a) dQ(1+Tr/To)
b) dQ(1-Tr/To)
c) dQ(1+To/Tr)
d) dQ(1-To/Tr)
Answer: d
Clarification: The maximum useful work will be increased by dQ(1-To/Tr) where dQ is the heat received by the system.

8. When a system changes its state towards that of surroundings, the opportunity to produce more work
a) increases
b) decreases
c) remains constant
d) none of the mentioned
Answer: b
Clarification: More the system goes towards surroundings, lesser is the opportunity to produce more work.

9. When the system is in equilibrium with the surroundings, it must be in
a) pressure equilibrium
b) temperature equilibrium
c) chemical equilibrium
d) all of the mentioned
Answer: d
Clarification: When the system is in equilibrium with the surroundings, its pressure and temperature is Po and To and there should not be any chemical reaction or mass transfer.

10. All spontaneous processes terminate at the dead state.
a) true
b) false
Answer: a
Clarification: At dead state, the system is in equilibrium with the surroundings and hence the opportunity of producing work ceases to exist.

Thermodynamics Multiple Choice Questions on “Ideal Gas-2”.

1. The value of cp and cv depend on
a) temperature of the gas
b) ɣ and R
c) pressure of the gas
d) all of the mentioned
Answer: b
Clarification: The value of cp and cv depends on the number of atoms in a molecule and the molecular weight of the gas.

2. Which of the following statement is true?
a) value of ɣ for monoatomic gases is 5/3
b) value of ɣ for diatomic gases is 7/5
c) for polyatomic gases, the value of ɣ is approximately taken as 4/3
d) all of the mentioned
Answer: d
Clarification: These values of ɣ can be shown by the classical kinetic theory of gases.

3. The maximum and minimum values of ɣ is
a) 1.33, 1
b) 2.00, 1
c) 1.67, 1
d) 1.25, 1
Answer: c
Clarification: ɣ=1 when cp=cv and ɣ=1.67=5/3 for monoatomic gases.

4. Which of the following equation can be used to compute the entropy change between any two states of an ideal gas?
a) s2-s1 = cv*ln(T2/T1) + R*ln(v2/v1)
b) s2-s1 = cp*ln(T2/T1) – R*ln(p2/p1)
c) s2-s1 = cp*ln(v2/v1) + cv*ln(p2/p1)
d) all of the mentioned
Answer: d
Clarification: Any of the given three equations can be used.

5. For a reversible adiabatic change, ds=0.
a) true
b) false
Answer: a
Clarification: For a reversible adiabatic process, change in entropy is zero.

6. For a reversible adiabatic process,
a) p*(v^ɣ) = constant
b) T*(v^(ɣ-1)) = constant
c) T*(p^(1-ɣ)/ɣ) = constant
d) all of the mentioned
Answer: d
Clarification: All these relations come from the pv=RT and p*(v^ɣ) = constant.

7. Which of the following is true for a polytropic process?
a) p(v^n) is used to describe the process
b) it is not adiabatic
c) it can be reversible
d) all of the mentioned
Answer: d
Clarification: These are the properties of an adiabatic process.

8. In the equation p(v^n), n is calculated by
a) (logp1 + logp2) / (logv2 + logv1)
b) (logp1 – logp2) / (logv2 – logv1)
c) (logp1 – logp2) / (logv2 + logv1)
d) none of the mentioned
Answer: b
Clarification: It comes from the p1*(v1^n) = p2*(v2^n).

9. For entropy change in a polytropic process, which of the following statement is true?
a) when n=ɣ, the entropy change becomes zero
b) if p2>p1, for n<=ɣ, the entropy of the gas decreases
c) for n>ɣ, the entropy of the gas increases
d) all of the mentioned
Answer: d
Clarification: This comes from the relation s2-s1 = [(n-ɣ)/{n(ɣ-1)}]*R*ln(p2/p1).

10. Polytropic specific heat is given by cn=cv(ɣ-n)/(1-n) .
a) true
b) false
Answer: a
Clarification: The polytropic specific heat is used in the relation Qr=cn*ΔT.

11. Which of the following is true?
a) for n>ɣ, there will be positive heat transfer and gain in entropy
b) for n<ɣ, there will be negative heat transfer and decrease in entropy
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: This comes from the relation Qr=cn*ΔT and cn=cv(ɣ-n)/(1-n).