Category: Thermodynamics Objective Questions

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.

Thermodynamics Multiple Choice Questions on “Temperature-Entropy Plot”.

1. For a reversible heat transfer and process being adiabatic, which of the following is true?
a) dQ=0
b) dS=0
c) S=constant
d) all of the mentioned
Answer: d
Clarification: dQ=0 since process is reversible and adiabatic and dS=dQ/T.

2. A reversible adiabatic process is an isentropic process.
a) true
b) false
Answer: a
Clarification: dQ=0 and dS=0 and hence S=constant.

3. The area under the curve ∫TdS is equal to the
a) work done
b) heat transferred
c) internal energy change
d) none of the mentioned
Answer: b
Clarification: Q(reversible)=∫TdS.

4. Which of the following statement is true?
a) for reversible isothermal heat transfer, Q=t(Sf-Si)
b) for reversible adiabatic process, S=constant
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: For reversible isothermal heat transfer, T=constant and for reversible adiabatic process, dS=0.

5. A Carnot cycle has following processes.
a) 4 reversible isotherms
b) 4 reversible adiabatics
c) 2 reversible isotherms and 2 reversible adiabatics
d) none of the mentioned
Answer: c
Clarification: Two reversible isotherms and two reversible adiabatics constitute a Carnot cycle.

6. Net work in a Carnot cycle is given by (T1=temperature of heat addition and T2=temperature of heat rejection)
a) (T2-T1)(S1-S4)
b) (T1-T2)(S1-S4)
c) (T1-T2)(S4-S1)
d) none of the mentioned
Answer: b
Clarification: Net work=Q1-Q2=(T1-T2)(S1-S4).

7. According to the principle of Caratheodory, the first law in differential form is written as dQ=Adx+Bdy+Cdz.
a) true
b) false
Answer: a
Clarification: Here, x,y,z are the three thermodynamic coordinates and A,B,C are the functions of x,y,z.

8. For adiabatic, reversible transition,
a) Adx+Bdy+Cdz=-1
b) Adx+Bdy+Cdz=1
c) Adx+Bdy+Cdz=0
d) none of the mentioned
Answer: c
Clarification: dQ=Adx+Bdy+Cdz=0 for adiabatic and reversible process.

9. For quasi-static, adiabatic path
a) Adx+Bdy+Cdz=TdS
b) Adx+Bdy+Cdz=1
c) Adx+Bdy+Cdz=0
d) none of the mentioned
Answer: a
Clarification: This comes from Caratheodory’s theorem.

10. The infinitesimal change in entropy dS due to reversible heat transfer dQ at temperature T is dS=dQ/T.
a) true
b) false
Answer: a
Clarification: For a reversible process, dS=dQ/T .

Thermodynamics Multiple Choice Questions on “Availability-1”.

1. When we obtain useful work during a process in which a finite system undergoes a change of state, when should that process terminate?
a) when the pressure of system equals the pressure of surroundings
b) when the temperature of system equals the temperature of surroundings
c) when the system has reached the dead state
d) all of the mentioned
Answer: d
Clarification: The process goes on until the system reaches the dead state.

2. The availability(A) of a given system is defined as the ____ work that is obtainable in a process in which system comes to equilibrium with its surroundings.
a) useful work
b) maximum useful work
c) minimum useful work
d) none of the mentioned
Answer: b
Clarification: Maximum useful work is given by total work minus pdV work.

3. Availability is a composite property.
a) true
b) false
Answer: a
Clarification: This is because it depends on the state of both the system and surroundings.

4. Availability function for a steady flow system is given by
a) H+TS+(m*V*V/2)+(m*g*z)
b) H-TS+(m*V*V/2)+(m*g*z)
c) H-TS-(m*V*V/2)-(m*g*z)
d) H-TS-(m*V*V/2)+(m*g*z)
Answer: b
Clarification: This term comes very frequent and is considered as availability function for a steady flow system.

5. 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 for a closed system.

6. Which of the following is true for an internal combustion engine?
a) the reactants are in pressure and temperature equilibrium with the surroundings
b) the products are in pressure and temperature equilibrium with the surroundings
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: This is true if the products are cooled to atmospheric temperature To before being discharged from the engine.

7. The Helmholtz function F is given by
a) U-TS
b) U+TS
c) -U-TS
d) -U+TS
Answer: a
Clarification: Helmholtz function F is a property which is defined by the relation F=U-TS.

8. Which of the following statement is true?
a) maximum work is done when process is reversible
b) if the process is irreversible, work is less than the maximum
c) W<=(F1-F2)
d) all of the mentioned
Answer: d
Clarification: W<=(F1-F2) when initial and final temperature of system is same as that of surroundings.

9. Gibbs function G is given by
a) G=H-TS
b) G=U+pV-TS
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: G=H-TS and H=U+pV.

10. If the process is irreversible, the useful work is less than the maximum.
a) true
b) false
Answer: a
Clarification: Useful work is maximum for a reversible process.

11. Find the availability when 100 kW is delivered at 500 K when the ambient temperature is 300 K.
a) 20 kW
b) 30 kW
c) 40 kW
d) 50 kW
Answer: c
Clarification: Φ = W(revHE) = [1-(To/T)]Q = [1-(300/500)](100) = 40 kW.

12. A control mass gives out 10 kJ of energy in the form of heat transfer at 500°C. Find the change in availability of the control mass.
a) −4.14 kJ
b) −5.14 kJ
c) −6.14 kJ
d) −7.14 kJ
Answer: b
Clarification: ∆Φ = -[1-(To/Th)]Q = -[1-(298.15/773.15)](10) = −6.14 kJ.

contest

Thermodynamics Aptitude Test on “Ideal Gas-3”.

1. The ratio of cp/cv is designated by the symbol ɣ.
a) true
b) false
Answer: a
Clarification: This ratio is of importance in ideal gas computations.

2. Which of the following relation is correct?
a) cv = R/(ɣ-1)
b) cp = ɣR/(ɣ-1)
c) ɣ = cp/cv
d) all of the mentioned
Answer: d
Clarification: Since ɣ=cp/cv and cp-cv=R.

3. Which of the following values of n are correct?
a) for isobaric process, n=0
b) for isothermal process, n=1
c) for isentropic process, n=ɣ
d) all of the mentioned
Answer: d
Clarification: For isobaric process, pressure=constant, for isothermal process, temperature=constant and for isentropic process, entropy=constant.

4. For a isometric or isochoric process, n=infinity.
a) true
b) false
Answer: a

5. A spherical helium balloon of 10m diameter is at 15°C and 100 kPa. How much helium does it contain?
a) 57.5 kg
b) 67.5 kg
c) 77.5 kg
d) 87.5 kg
Answer: a
Clarification: V = (4π/3)r^3 = (π/6)D^3 = 523.6 m^3
m(He) = ρV = PV/RT
=(100 × 523.6)/(2.0771 × 288) = 87.5 kg.

6. A rigid tank of 1 m^3 contains nitrogen gas at 600 kPa and 400 K. If 0.5 kg of gas flows out then what is the final pressure given the final temperature is 375 K?
a) 501.9 kPa
b) 503.9 kPa
c) 506.9 kPa
d) none of the mentioned
Answer: c
Clarification: m =(PV/RT) = (600 × 1)/(0.2968 × 400) = 5.054 kg
therefore m2 = m – 0.5 = 5.054 – 0.5 = 4.554 kg
P2 = (m2RT2)/V = (4.554 × 0.2968 × 375)/1 = 506.9 kPa.

7. A cylindrical gas tank 1 m long having inside diameter of 20 cm, is evacuated and filled with carbon dioxide gas at 25°C. What pressure should be the pressure if there is to be 1.2 kg of carbon dioxide?
a) 2052 kPa
b) 2152 kPa
c) 2252 kPa
d) 2352 kPa
Answer: b
Clarification: V = A × L = (π/4)[(0.2)^2] × 1 = 0.031416 m^3
P V = mRT
P = [1.2×0.1889×(273.15 + 25) K)]/(0.031416 m^3)
= 2152 kPa.

8. A hollow metal sphere having an inside diameter of 150-mm is weighed first when evacuated and then after being filled to 875 kPa with an unknown gas. If the difference in mass is 0.0025 kg, and the temperature is 25°C, find the gas.
a) helium
b) argon
c) hydrogen
d) nitrogen
Answer: a
Clarification: V = (π/6)(0.15)^3 = 0.001767 m^3
M = (mRT/PV) = (0.0025 × 8.3145 × 298.2)/(875 × 0.001767) = 4.009
this is the mass of helium gas.

9. An auto-mobile tire has air at −10°C and 190 kPa. After sometime, the temperature gets up to 10°C. Find the new pressure.
a) 210.4 kPa
b) 224.4 kPa
c) 200.4 kPa
d) 204.4 kPa
Answer: d
Clarification: Assume constant volume and that air is an ideal gas
P2 = P1 × T2/T1
= 190 ×(283.15/283.15)
= 204.4 kPa.

10. A piston cylinder contains air at 600 kPa, 290 K and a volume of 0.01m^3. A constant pressure process gives 54 kJ of work out. Find the final temperature of the air.
a) 2700 K
b) 2800 K
c) 2900 K
d) 3000 K
Answer: c
Clarification: W = ∫ P dV = PΔV
ΔV = W/P = 54/600 = 0.09 m^3
V2 = V1 + ΔV = 0.01 + 0.09 = 0.1 m^3
Assuming ideal gas, PV = mRT, then we have
T2 = P2V2/(mR) = (P2V2)*T1/(P1V1)= (V2/V1)*T1
= (0.1/0.01)*290 = 2900 K.

Thermodynamics for aptitude tests,