Category: Thermodynamics Objective Questions

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).

Thermodynamics Multiple Choice Questions on “Simple Steam Power Cycle and Rankine Cycle”.

1. A power cycle continuously converts ____ into ____
a) heat, heat
b) work, heat
c) heat, work
d) work, work
Answer: c
Clarification: Here heat is the energy released by burning of fuel and work is done as shaft work.

2. In the vapour power cycle, working fluid undergoes a change of phase.
a) true
b) false
Answer: a
Clarification: Here working fluid is water.

3. The path followed in a vapour power cycle is
a) boiler-condenser-turbine-pump
b) boiler-turbine-condenser-pump
c) boiler-turbine-pump-condenser
d) boiler-pump-turbine-condenser
Answer: b
Clarification: In the boiler, water takes heat then expands in turbine going into condenser where it condenses into water and then it is pumped back into boiler.

4. For a fluid undergoing cycle process,
a) there is no net change in its internal energy
b) energy transfer as heat is equal to the energy transfer as work
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: When a fluid undergoes a cycle process, this changes take place.

5. For a vapour power cycle,
a) net heat input is converted into net work output
b) Q1-Q2 = Wt-Wp
c) efficiency = 1 – (Q2/Q1)
d) all of the mentioned
Answer: d
Clarification: Here Q1 is the heat transferred to the fluid and Q2 is the heat rejected, Wt is work transferred from fluid and Wp is work transferred into fluid.

6. In a Rankine cycle, all the processes are ideal.
a) true
b) false
Answer: a
Clarification: The Rankine cycle is an ideal cycle and also a reversible cycle.

7. For a Rankine cycle, which of the following is true?
a) a reversible constant pressure heating process happens in steam boiler
b) reversible adiabatic expansion of steam in turbine
c) reversible constant pressure heat rejection in condenser
d) all of the mentioned
Answer: d
Clarification: All the processes are ideal in Rankine cycle.

8. The liquid water handled by pump is
a) incompressible
b) with increase in pressure, there is a little change in density or specific volume
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: In a pump, reversible adiabatic compression of liquid takes place.

9. The work ratio is defined as the ratio of
a) positive work output to net work output
b) net work output to positive work output
c) heat input to work output
d) none of the mentioned
Answer: b
Clarification: The work ratio = Wnet / Wt.

10. Steam rate is the rate of steam flow required to produce unit shaft output.
a) true
b) false
Answer: a
Clarification: It is the capacity of a steam plant and steam rate = 1/(Wt-Wp).

11. Heat rate is given by (in kJ/kWh)
a) cycle efficiency
b) 3600 / cycle efficiency
c) cycle efficiency / 3600
d) cycle efficiency * 3600
Answer: b
Clarification: Heat rate is the rate input required to produce unit work output.

12. Which of the following statement is true?
a) during compression, specific volume of the fluid should be kept small
b) during expansion, specific volume of the fluid should be kept large
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: The larger the specific volume, more is the work produced or consumed by the steady-flow device.

13. Work output of turbine is ____ the work input to the pump.
a) much larger
b) much smaller
c) equal to
d) none of the mentioned
Answer: a
Clarification: This is the reason why steam power plants is so popular.

Thermodynamics Multiple Choice Questions on “Reversed Heat Engine”.

1. Refrigeration is the cooling of any system below its surroundings temperature.
a) true
b) false
Answer: a
Clarification: This is what refrigeration means.

2. Dry ice is suitable for ____ temperature refrigeration.
a) high
b) low
c) all range of
d) none of the mentioned
Answer: b
Clarification: Dry ice when exposed to atmosphere sublimates by absorbing latent heat of sublimation.

3. A reversed heat engine
a) receives heat from a low temperature region
b) gives heat to a high temperature region
c) receives a net inflow of work
d) all of the mentioned
Answer: d
Clarification: This is the basic working of reversed heat engine.

4. Which of the following is a reversed heat engine cycle?
a) heat pump cycle
b) refrigeration cycle
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These are the reversed heat engine cycles.

5. For a heat pump, coefficient of performance is given by (Q1 is the heat taken from low temperature region and Q2 is the heat given to high temperature region)
a) Q1/(Q1-Q2)
b) Q2/(Q1-Q2)
c) 1- (Q1/Q2)
d) 1- (Q2/Q1)
Answer: a
Clarification: For a heat pump, COP = Q1/W = Q1/(Q1-Q2).

6. For a regenerator, coefficient of performance is given by (Q1 is the heat taken from low temperature region and Q2 is the heat given to high temperature region)
a) Q1/(Q1-Q2)
b) Q2/(Q1-Q2)
c) 1- (Q1/Q2)
d) 1- (Q2/Q1)
Answer: b
Clarification: For a heat pump, COP = Q2/W = Q2/(Q1-Q2).

7. The working fluid in a refrigeration cycle is known as refrigerant.
a) true
b) false
Answer: a
Clarification: A refrigerant is referred particularly to a refrigeration cycle.

8. Which of the following happens in a reversed Carnot cycle?
a) the refrigerant is compressed reversibly and adiabatically
b) it is condensed reversibly
c) it expands reversibly and adiabatically
d) all of the mentioned
Answer: d
Clarification: These processes take place in a reversed Carnot cycle.

9. For a heat pump, coefficient of performance is given by (T1 is the temperature of heat rejection and T2 is the temperature of heat absorption)
a) T1/(T1-T2)
b) T2/(T1-T2)
c) 1- (T1/T2)
d) 1- (T2/T1)
Answer: a
Clarification: For a heat pump, COP = Q1/W = T1/(T1-T2).

10. For a regenerator, coefficient of performance is given by (T1 is the temperature of heat rejection and T2 is the temperature of heat absorption)
a) T1/(T1-T2)
b) T2/(T1-T2)
c) 1- (T1/T2)
d) 1- (T2/T1)
Answer: b
Clarification: For a heat pump, COP = Q2/W = T2/(T1-T2).

11. For the same T1 and T2, COP increases with ____ in temperature difference.
a) increase
b) decrease
c) no change
d) none of the mentioned
Answer: b
Clarification: This means, closer the temperatures T1 and T2, the higher is the COP.

Thermodynamics Multiple Choice Questions on “Enthalpy”.

1. The enthalpy of a substance(denoted by h), is defined as
a) h=u-pv
b) h=u+pv
c) h=-u+pv
d) h=-u-pv
Answer: b
Clarification: This is a basic definition for enthalpy.

2. In a constant volume process, internal energy change is equal to
a) heat transferred
b) work done
c) zero
d) none of the mentioned
Answer: a
Clarification: In a constant volume process, there is no work other than the pdV work.

3. For an ideal gas, enthalpy becomes
a) h=u-RT
b) h=-u-RT
c) h=u+RT
d) h=-u+RT
Answer: c
Clarification: For an ideal gas, pv=RT.

4. Enthalpy is an intensive property of a system.
a) true
b) false
Answer: a
Clarification: Enthalpy is an intensive property measured mostly in kJ/kg.

5. Heat transferred at constant pressure _____ the enthalpy of a system.
a) decreases
b) increases
c) first decreases then increases
d) first increases then decreases
Answer: b
Clarification: At constant pressure, (dQ)=dh where h=u+pv is the specific enthalpy of the system.

6. The enthalpy of an ideal gas depends only on the temperature.
a) true
b) false
Answer: a
Clarification: This is because the internal energy of an ideal gas depends only on the temperature.

7. Total enthalpy of a system H is given by
a) H=h/m
b) H=m/h
c) H=mh
d) none of the mentioned
Answer: c
Clarification: Total enthalpy equals (mass*enthalpy) of substance.

8. The enthalpy and internal energy are the function of temperature for
a) all gases
b) steam
c) water
d) ideal gas
Answer: d
Clarification: The enthalpy of an ideal gas depends only on the temperature because the internal energy of an ideal gas depends only on the temperature.

9. Change in enthalpy of a system is due to heat supplied at
a) constant volume
b) constant pressure
c) both at constant volume and pressure
d) none of the mentioned
Answer: b
Clarification: Change in enthalpy occurs when heat is given to a system at constant pressure.

10. At constant pressure
a) pdv=d(pv)
b) dQ=du+d(pv)
c) dQ=d(u+pv)
d) all of the mentioned
Answer: d
Clarification: For a constant pressure process, dQ=du+pdv.