# Engineering Thermodynamics MCQ (Multiple Choice Questions)

1. What is thermodynamics?
a) study of the relationship between heat and other forms of energy
b) study of the conversion of chemical energy to other forms of energy
c) study of the relationship between mechanical energy to other forms of energy
d) study of the conversion of mechanical energy to other forms of energy

Explanation: The branch of physical science is concerned with the relationship between heat, and other forms of energy like mechanical, electrical, chemical energy, etc.

2. Which of the following is a branch of thermodynamics?
a) Equilibrium thermodynamics
b) Classical thermodynamics
c) Chemical thermodynamics
d) All of the mentioned

Explanation: The branches of thermodynamics include:
1) Equilibrium thermodynamics
2) Classical thermodynamics
3) Chemical thermodynamics
4) Statistical mechanics or Statistical thermodynamics

3. Which of the following is a thermodynamics law?
a) Zeroth law of thermodynamics
c) Ideal Gas Law of thermodynamics
d) Boyle’s Law of thermodynamics

Explanation: Thermodynamics is primarily based on a set of four rules that are universally applicable when applied to systems that fall within their respective limitations. They are as follows:

• Zeroth law of thermodynamics
• First law of thermodynamics
• Second law of thermodynamics
• Third law of thermodynamics

4. Heat does not spontaneously flow from a colder body to a hotter one. Which of the following thermodynamics law states this?
a) Zeroth law of thermodynamics
b) First law of thermodynamics
c) Second law of thermodynamics
d) Third law of thermodynamics

Explanation: The concept of entropy as a physical attribute of a thermodynamic system is established by the second law of thermodynamics. Despite following the necessity of energy conservation as specified in the first law of thermodynamics, entropy predicts the direction of spontaneous processes and determines whether they are irreversible or impossible.

5. Which of the following is an application of thermodynamics?
a) Refrigerators
b) Gas compressors
c) Power plants
d) All of the mentioned

Explanation: The second law of thermodynamics applies to all refrigerators, deep freezers, industrial refrigeration systems, all forms of air-conditioning systems, heat pumps, and so on. Thermodynamic cycles govern the operation of all forms of air and gas compressors, blowers, and fans. The study of the feasibility of employing various forms of renewable energy sources for household and commercial purposes is an important topic area of thermodynamics.

6. Which of the following is a type of thermodynamic system?
a) Open system
b) Closed system
c) Thermally isolated system
d) All of the mentioned

Explanation: The types of thermodynamic systems are:
i) Open system
ii) Closed system
iii) Thermally isolated system
iv) Mechanically isolated system
v) Isolated system

7. Which of the following occurs without a change in the internal energy?
a) Isochoric process
b) Isenthalpic process
d) Isenthalpic process

Explanation: The following are some of the most commonly studied thermodynamic processes:
i) Heat does not lose or acquire energy in an adiabatic process.
ii) At a constant enthalpy, the isenthalpic process occurs.
iii) The isobaric process happens when the pressure remains constant.
iv) The isochoric process (also known as isometric/isovolumetric) happens at a constant volume.
v) At a constant temperature, an isothermal process occurs.
vi) Internal energy does not change in a steady-state operation.
vii) Isentropic process: adiabatic reversible process with constant entropy.

8. Which of the following thermodynamic law gives the concept of enthalpy?
a) First law of thermodynamics
b) Second law of thermodynamics
c) Third law of thermodynamics
d) Fourth law of thermodynamics

Explanation: The first law of thermodynamics connects a system’s many kinds of kinetic and potential energy to the work it can do and the heat it can transfer. This law, which also includes an extra state variable, enthalpy, is occasionally used to define internal energy.

9. What is the value of the absolute thermodynamic temperature scale?
a) 3K
b) 0K
c) 1K
d) 4K

Explanation: The temperature of an object on a scale with 0 as absolute zero is known as absolute temperature, also known as thermodynamic temperature. Kelvin and Rankine are absolute temperature scales.

10. Which of the following follows the Carnot theorem?
a) Heat engines
b) Gas turbine engines
c) Gas compressors
d) All of the mentioned

Explanation: Carnot’s theorem states that all heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs.

11. If a piston/cylinder with a cross-sectional size of 0.01 m2 is resting on the stops, what should the water pressure be to lift the piston with an outside pressure of 100 kPa?
a) 218kPa
b) 168kPa
c) 198kPa
d) 318kPa

Explanation: Pw = Po + mg/A = 100000 + (100*9.8/0.01) = 198kPa.

12. At 100°C, a sealed rigid vessel with a volume of 1 m3 and 2 kg of water has a volume of 1 m3. The vessel has now been warmed up. What pressure should a safety pressure valve be adjusted to achieve a maximum temperature of 200°C if one is installed?
a) 431.3 kPa
b) 231.3 kPa
c) 831.3 kPa
d) 131.3 kPa

Explanation: Initial specific volume (v1) = 1 m3/2 kg = 0.5 m3/kg
Interpolating, pressure for the same specific volume at 200°C
= 400 + {(0.53422-0.5)/(0.53422-0.42492)}*(500-400) = 431.3 kPa.

13. When the diver is 8 meters below the surface, the pressure gauge on his air tank reads 60 kPa. The gauge pressure will be 0 at what depth?
a) 44.118 m
b) 24.118 m
c) 34.118 m
d) None of the mentioned

Explanation: Pressure at 10 m depth = Patm + ρgh = 101.325 + 1000*9.80665*8/1000 = 179.778 kPa
Absolute pressure of the air in the tank = 179.778 + 60 = 239.778 kPa
Depth at which gauge pressure is zero (H) = (239.778 – 101.325)*1000/(1000*9.80665) = 14.118 m.

14. Which of the following is the standard fixed point of thermometry?
a) The steam point
b) The triple point of water
c) The ice point
d) All of the mentioned

Explanation: After 1954, only one fixed point has been used.

15. Which of the following is chosen as the standard thermometric substance?
a) Liquid
b) Solid
c) Gas
d) None of the mentioned

Explanation: Smallest variation is observed among different gas thermometers.

16. What is the magnitude of mechanical work?
a) product of the force and distance travelled perpendicular to the force
b) product of the force and distance travelled parallel to the force
c) sum of the force and distance travelled perpendicular to the force
d) sum of the force and distance travelled parallel to the force

Explanation: The work is done by a force acting on a body moving in the force’s direction.

17. Which of the following gives the Mechanical efficiency of the engine?
a) IP/BP
b) 1/(BP*IP)
c) (BP*IP)
d) BP/IP

Explanation: Brake power is less than indicated power and mechanical efficiency is given by BP/IP.

18. Which of the following type of motion does Shaft uses to do work?
a) vertical motion
b) horizontal motion
c) rotational motion
d) none of the mentioned

Explanation: When a shaft is rotated by a motor, there is work transfer into the system.

19. A refrigerator with a 2 kW motor for powering the compressor gives 6000 kJ of cooling to the refrigerated space during 30 minutes of operation in a thermally insulated kitchen. Calculate the change in internal energy of the kitchen if the condenser coil behind the refrigerator rejects 8000 kJ of heat to the kitchen over the same time period.
a) 3600 kJ
b) 2400 kJ
c) 4800 kJ
d) none of the mentioned

Explanation: QKitchen = 0 (Insulated!),
W(Electrical) = – P*∆τ = – 2 kW*30*60 sec = – 3600 kJ
(It is negative because work is done on to the system)
Change in internal energy of the kitchen (∆UKitchen) = QKitchen – WElectrical
= 0 – (–3600) = 3600 kJ.

20. At 15°C, a steel pot with a 5 mm thick bottom is filled with liquid water (conductivity 50 W/m K). The pot has a 10 cm radius and is now placed on a stove with a heat transmission of 250 W. Calculate the temperature on the bottom of the outer pot assuming the inner surface is 15°C.
a) 15.8°C
b) 16.8°C
c) 18.8°C
d) 19.8°C

Explanation: Steady conduction, Q = k A ∆T/∆x ⇒ ∆Τ = Q ∆x / kΑ
∆T = 250 × 0.005/(50 × π/4 × 0.22) = 0.796
T = 15 + 0.796 = 15.8°C.

21. Heat flow into a system is ____, and heat flow out of the system is ________
a) positive, positive
b) negative, negative
c) negative, positive
d) positive, negative

Explanation: The direction of heat transfer is taken from the high-temperature system to the low-temperature system.

22. Joule was the first to prove that heat is a type of energy, laying the groundwork for the fundamental law of thermodynamics.
a) False
b) True

Explanation: Prior to Joule, the heat was considered to be an invisible fluid flowing from a body of higher calories to a body of lower calories.

23. What does (m*g*z) give?
a) macroscopic kinetic energy
b) microscopic kinetic energy
c) macroscopic potential energy
d) microscopic potential energy

Explanation: The above formula gives the macroscopic potential energy of the fluid element by virtue of its position.

24. The enthalpy and internal energy are the function of temperature for
a) all gases
b) steam
c) water
d) ideal gas

Explanation: The enthalpy of an ideal gas depends only on the temperature because the internal energy of an ideal gas depends only on the temperature.

25. In which of the following systems does mass transfer occur across the system boundary?
a) isolated system
b) closed system
c) open system
d) none of the mentioned

Explanation: Basic definition of an open system.

26. When more than one fluid stream enters or leaves the control volume, which of the following type of balance is taken?
a) mass balance
b) energy balance
c) mass balance and energy balance
d) none of the mentioned

Explanation: Both energy and mass balance are considered here.

27. Fluid flow through which of the following throttles the flow?
a) partially opened valve
b) orifice
c) porous plug
d) all of the mentioned

Explanation: In all of the given cases, there is an appreciable drop in pressure and hence the flow is throttled.

28. Rate of energy increase within the control volume is given by
a) rate of energy inflow * rate of energy outflow
b) rate of energy inflow – rate of energy outflow
c) rate of energy inflow / rate of energy outflow
d) rate of energy inflow + rate of energy outflow

Explanation: The rate of accumulation of energy within the control volume is equal to the net energy flow across the control surface.

29. Which of the following is true for a discharging tank?
a) the process is quasi-static
c) dQ=0
d) all of the mentioned

Explanation: Applying the first law to the control volume and dW=0, dm=0 and KE and PE of the fluid are assumed to be small.

30. A cylinder/piston contains 1kg methane gas at 100 kPa, 20°C. The gas is compressed reversibly to a pressure of 800 kPa. WHat is the work required if the process is isothermal?
a) -116.0 kJ
b) -316.0 kJ
c) -216.0 kJ
d) -416.0 kJ

Explanation: Process: T = constant. For ideal gas then u2 = u1 1W2 = 1Q2 and ∫ dQ/T = 1Q2/T
1W2 = 1Q2 = mT(s2 – s1) = -mRT ln(P2/P1)
= -0.51835× 293.2 ln(800/100) = -316.0 kJ.

31. A piston/cylinder contains carbon dioxide at 300 kPa, 100°C with a volume of 0.2 m^3. Weights are added at such a rate that the gas compresses according to the relation PV^1.2 = constant to a final temperature of 200°C. Find the work done during the process.
a) 60.4 kJ
b) -50.4 kJ
c) 80.4 kJ
d) -80.4 kJ

Explanation: For the Polytropic process PVn = constant
1W2 = ∫PdV = (P2V2 – P1V1)/(1 – n )
Assuming ideal gas, PV = mRT
But mR = P1V1/T1 = 300 × 0.2/373.15 = 0.1608 kJ/K
1W2 = 0.1608(473.15 – 373.15)/(1 – 1.2) = -80.4 kJ.

32. Which of the following statements are true for a mechanical energy reservoir(MER)?
a) all processes within an MER are quasi-static
b) it is a large body enclosed by an adiabatic impermeable wall
c) stores work as KE or PE
d) all of the mentioned

Explanation: These are some important features of an MER.

33. Which of the following is true according to Clausius statement?
a) it is possible to construct a device that can transfer heat from a cooler body to a hotter body without any effect
b) it is impossible to construct a device that can transfer heat from a cooler body to a hotter body without any effect
c) it is impossible to construct a device that can transfer heat from a hotter body to a cooler body without any effect
d) none of the mentioned

Explanation: To transfer heat from a cooler body to a hotter body, some work must be expended.

34. What is the relationship between Kelvin-Planck’s and Clausius’ statements?
a) violation of one doesn’t violate the other
b) not connected to each other
c) virtually two parallel statements of the second law
d) none of the mentioned

Explanation: Kelvin-Planck’s and Clausius’ statements are equivalent in all aspects.

35. Which of the following causes irreversibility?
a) electrical resistance, magnetic hysteresis
b) friction, viscosity
c) inelasticity
d) all of the mentioned

Explanation: These effects are known as dissipative effects.

36. The reversed heat engine takes heat from a ___ temperature body, then discharges it to a ___ temperature body and ___ an inward flow of the network.
a) high, low, gives
b) low, high, gives

Explanation: In a reversed heat engine, the magnitude of energy transfers remains the same and only directions change.

37. If a system undergoes a reversible isothermal process without transfer of heat, the temperature at which this process takes place is called
a) triple point of water
b) boiling point of water
c) absolute zero
d) none of the mentioned

Explanation: The smallest possible value of Q which is the amount of heat supply is zero and the corresponding temperature is zero.

38. Which of the following is known as the inequality of Clausius?
a) cyclic integral of dW/T<=0
b) cyclic integral of dW/T>=0
c) cyclic integral of dQ/T>=0
d) cyclic integral of dQ/T<=0

Explanation: It provides the criterion for the reversibility of a cycle.

39. Which of the following properties describe entropy?
a) point function, intensive property
b) point function, extensive property
c) path function, extensive property
d) path function, intensive property

Explanation: Fact about entropy and unit of entropy is J/K.

40. Which of the following statement is true?
a) for reversible adiabatic process, S=constant
b) for reversible isothermal heat transfer, Q=t(Sf-Si)
c) both of the mentioned
d) none of the mentioned

Explanation: For reversible isothermal heat transfer, T=constant and for reversible adiabatic process, dS=0.

41. The entropy of an isolated system can never ____
a) decrease
b) be zero
c) increase
d) none of the mentioned

Explanation: The entropy of an isolated system always increases and remains constant only when the process is reversible.

42. A car uses power of 25 hp for a one hour in a round trip. Thermal efficiency of 35% can be assumed? Find the change in entropy if we assume ambient at 20°C.
a) 354.1 kJ/K
b) 654.1 kJ/K
c) 254.1 kJ/K
d) 554.1 kJ/K

Explanation: E = ⌠ W dt = 25 hp × 0.7457 (kW/hp) × 3600 s = 67 113 kJ = η Q
Q = E / η = 67 113 / 0.35 = 191 751 kJ
∆S = Q / T = 191 751 / 293.15 = 654.1 kJ/K.

43. If heat Q flows reversibly from the system to the surroundings at To, what will be the effect on entropy?
a) entropy of the system is reduced by Q/To
b) system has lost entropy to the surroundings
c) entropy increase of the surroundings is Q/To
d) all of the mentioned

Explanation: We can say that there is entropy transfer from the system to the surroundings along with heat flow.

44. The entropy of any closed system can increase in which of the following way?
a) dissipative effects or internal irreversibilities
b) by heat interaction in which there is entropy transfer
c) both of the mentioned
d) none of the mentioned

Explanation: These two processes increase the entropy of a closed system.

45. A hot metal piece is cooled rapidly to 25°C, removing 1000 kJ from the metal. Calculate the change of entropy if saturated liquid R-22 at −20°C absorbs the energy so that it becomes saturated vapor.
a) 0.950 kJ/K
b) 7.950 kJ/K
c) 3.950 kJ/K
d) 5.950 kJ/K

Explanation: R-22 boiling at -20°C; m = 1Q2 /h(fg) = 1000/220.327 = 4.539 kg
∆S(R-22) = ms(fg) = 4.539(0.8703) = 3.950 kJ/K.

46. The entropy change of a system between two identifiable equilibrium state is ___ when the intervening process is reversible or change of state is irreversible.
a) depends on the process
b) different
c) same
d) none of the mentioned

Explanation: To determine the change in entropy, a known reversible path is made to connect the two end states and integration is performed on this path.

47. When work is dissipated into internal energy, what is the change in the disorderly motion of molecules.
a) increases
b) remains the same
c) decreases
d) none of the mentioned

Explanation: We know that increase in internal energy causes more random motion.

48. Which of the following is an example of low grade energy?
a) heat from combustion of fossil fuel
b) heat or thermal energy
c) heat from nuclear fission or fusion
d) all of the mentioned

Explanation: These are few examples of low grade energy.

49. The available energy of a system ___ as its temperature or pressure decreases and approaches that of the surroundings.
a) remains constant
b) increases
c) decreases
d) none of the mentioned

Explanation: As temperature decreases, exergy decreases.

50. Which of the following is true?
a) Q for reversible < Q for irreversible and work for reversible < work for irreversible
b) Q for reversible > Q for irreversible and work for reversible < work for irreversible
c) Q for reversible < Q for irreversible and work for reversible > work for irreversible
d) Q for reversible > Q for irreversible and work for reversible > work for irreversible

Explanation: This is because, Q for reversible=(To)*(S2-S1) and Q for irreversible<(To)*(S1-S2).

## Chapterwise Multiple Choice Questions on Engineering Thermodynamics

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## 1. MCQ on Introduction to Engineering Thermodynamics

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## 2. Engineering Thermodynamics Multiple Choice Questions on Work and Heat Transfer

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## 3. MCQ on First Law of Thermodynamics

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## 5. MCQ on Second Law of Thermodynamics

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The section contains Engineering Thermodynamics multiple choice questions and answers on heat engine, vapor compression refrigeration cycle and heat pump system.

## 14. Engineering Thermodynamics Multiple Choice Questions on Psychrometrics

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## 15. Engineering Thermodynamics MCQ on Compressible Fluid Flow

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## 16. Engineering Thermodynamics Multiple Choice Questions on Gas Compressors

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