Thermodynamics MCQs

This comprehensive set of Thermodynamics MCQs is designed to cover all essential topics required for success in understanding the principles and applications of thermodynamics in physics and engineering. Focused on key subjects such as Laws of Thermodynamics, Thermodynamic Processes, Heat Transfer, and Entropy, these MCQs are crafted to help students build a strong foundation in thermodynamic concepts.

Who should practice Thermodynamics MCQs?

Students pursuing degrees in physics, mechanical engineering, or chemical engineering with a focus on thermodynamics.
Individuals preparing for competitive exams and certifications in the field of thermodynamics and heat transfer.
Students targeting high-yield topics like thermodynamic cycles, phase changes, and energy conservation principles.
Anyone aiming to strengthen their foundational understanding of how thermodynamic principles apply to real-world systems.
Candidates focused on developing critical thinking and application-based problem-solving skills specific to thermodynamics requirements.
Suitable for all students preparing for assessments related to thermodynamics, including those seeking to improve problem-solving speed and build exam confidence.

1. What does the first law of thermodynamics state?

A) Energy cannot be created or destroyed, only transformed
B) Entropy always increases
C) Energy is always lost in a system
D) Heat flows from colder to hotter bodies

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2. In which of the following processes is the heat exchange zero?

A) Isobaric process
B) Isochoric process
C) Adiabatic process
D) Isothermal process

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3. What is the unit of entropy in the SI system?

A) Joules
B) Kelvin
C) Joules per Kelvin
D) Watts

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4. Which thermodynamic cycle is most commonly used in power plants?

A) Carnot cycle
B) Rankine cycle
C) Brayton cycle
D) Otto cycle

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5. What does the second law of thermodynamics state?

A) Energy is conserved in a closed system
B) Heat cannot spontaneously flow from a colder body to a hotter body
C) The total energy in the universe increases
D) Work can be extracted from a system without losses

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6. What is the efficiency of a Carnot engine dependent on?

A) The speed of the engine
B) The heat input
C) The temperatures of the heat source and sink
D) The type of fuel used

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7. In an isochoric process, what remains constant?

A) Temperature
B) Volume
C) Pressure
D) Entropy

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8. What is the thermodynamic property used to measure disorder in a system?

A) Work
B) Enthalpy
C) Entropy
D) Internal energy

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9. Which of the following is a state function in thermodynamics?

A) Work
B) Heat
C) Enthalpy
D) Force

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10. In the context of thermodynamics, what is a reversible process?

A) A process that can be reversed without leaving any change in the surroundings
B) A process that creates maximum entropy
C) A process that requires external work
D) A process that occurs at constant pressure

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11. What is the significance of the zeroth law of thermodynamics?

A) It defines the concept of heat
B) It introduces the concept of temperature equilibrium
C) It explains the conservation of energy
D) It defines entropy

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12. In a closed system, how does the internal energy change if only work is done on the system?

A) It increases
B) It decreases
C) It remains constant
D) It depends on the type of work

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13. Which thermodynamic potential is minimized at constant temperature and pressure?

A) Enthalpy
B) Helmholtz free energy
C) Internal energy
D) Gibbs free energy

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14. What is the Clausius statement of the second law of thermodynamics?

A) No process is possible in which the sole result is the transfer of heat from a cooler to a hotter body
B) The total entropy of an isolated system always decreases
C) Energy cannot be created or destroyed
D) Heat can flow spontaneously from cold to hot

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15. What is the term for the amount of heat required to change the temperature of one kilogram of a substance by one degree Celsius?

A) Heat capacity
B) Latent heat
C) Specific heat
D) Thermal conductivity

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16. Which of the following is true for an ideal gas in an isothermal process?

A) Internal energy remains constant
B) Work done is zero
C) Heat added increases the temperature
D) Pressure remains constant

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17. What is the relation between heat (Q), work (W), and internal energy (ΔU) in the first law of thermodynamics?

A) ΔU = Q + W
B) ΔU = Q – W
C) Q = W
D) ΔU = W/Q

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18. Which thermodynamic process takes place at a constant temperature?

A) Adiabatic process
B) Isobaric process
C) Isochoric process
D) Isothermal process

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19. What happens to the pressure in an adiabatic expansion of an ideal gas?

A) Pressure increases
B) Pressure decreases
C) Pressure remains constant
D) Pressure fluctuates

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20. In a Carnot cycle, where does the heat rejection take place?

A) At the heat source
B) At the heat sink
C) At the turbine
D) At the compressor

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21. What is the principle behind a heat engine?

A) It converts heat into work
B) It transfers heat from a cooler to a hotter body
C) It minimizes energy loss
D) It creates energy from nothing

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22. Which of the following thermodynamic processes occurs at constant pressure?

A) Isochoric
B) Adiabatic
C) Isobaric
D) Isothermal

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23. What is the main consequence of the second law of thermodynamics in terms of entropy?

A) Entropy always remains constant
B) Entropy always decreases in an isolated system
C) Entropy always increases in an isolated system
D) Entropy can be completely converted into work

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24. What is the efficiency of a Carnot engine if the temperature of the heat source is 500 K and the heat sink is at 300 K?

A) 0.40
B) 0.20
C) 0.60
D) 0.33

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25. In which process is no work done on or by the system?

A) Isobaric process
B) Isochoric process
C) Adiabatic process
D) Isothermal process

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26. What is the name of the thermodynamic property that measures the ability of a system to do work?

A) Enthalpy
B) Gibbs free energy
C) Entropy
D) Internal energy

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27. Which of the following statements is true for an ideal gas undergoing an adiabatic process?

A) Heat exchange is maximum
B) The temperature remains constant
C) The temperature changes without heat exchange
D) The volume remains constant

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28. Which of the following describes a heat pump?

A) A device that converts work into heat
B) A device that transfers heat from a cooler body to a hotter one
C) A device that creates heat energy from electrical energy
D) A device that converts heat into mechanical work

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29. In thermodynamics, what is an isolated system?

A) A system that exchanges both matter and energy with its surroundings
B) A system that does not exchange energy with its surroundings but exchanges matter
C) A system that neither exchanges matter nor energy with its surroundings
D) A system that exchanges only energy but not matter with its surroundings

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30. What is the maximum efficiency of a heat engine operating between two temperature reservoirs of 1000 K and 400 K?

A) 60%
B) 40%
C) 50%
D) 20%

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31. In an isothermal expansion of an ideal gas, what happens to the internal energy of the gas?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates

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32. What does a throttling process involve?

A) A constant temperature process
B) An adiabatic process with no work done
C) A process with constant volume
D) A process that increases pressure

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33. In which process does the internal energy of an ideal gas depend only on its temperature?

A) Adiabatic process
B) Isobaric process
C) Isochoric process
D) All of the above

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34. What is the key difference between a refrigerator and a heat engine?

A) A refrigerator works in the opposite direction of a heat engine
B) A refrigerator produces work, while a heat engine consumes work
C) A refrigerator increases temperature, while a heat engine decreases it
D) A refrigerator decreases entropy, while a heat engine increases it

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35. What is the significance of Gibbs free energy in a chemical reaction?

A) It determines the speed of the reaction
B) It measures the heat exchange
C) It predicts the spontaneity of the reaction
D) It calculates the amount of entropy

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36. Which law of thermodynamics defines absolute zero?

A) Zeroth law
B) First law
C) Second law
D) Third law

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37. What is the physical meaning of specific heat capacity?

A) The heat required to raise the temperature of a substance
B) The heat required to raise the temperature of one unit mass of a substance by one degree
C) The heat lost by a substance when cooled
D) The heat required to change the phase of a substance

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38. What is the ideal gas law?

A) PV = nRT
B) P = V/T
C) U = H + PV
D) Q = W + U

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39. Which of the following describes an irreversible process?

A) A process that can be reversed without any energy loss
B) A process where entropy increases
C) A process that occurs without any work done
D) A process that occurs with zero heat exchange

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40. In thermodynamics, what is a “control volume”?

A) A closed system with no matter exchange
B) A fixed region in space where mass and energy can cross the boundary
C) A system that exchanges only work with its surroundings
D) A system that is perfectly insulated

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41. What happens to the temperature of a gas during an adiabatic expansion?

A) It remains constant
B) It decreases
C) It increases
D) It fluctuates

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42. What is the relationship between work and heat in a cyclic process?

A) Work is equal to heat
B) Work done in the cycle is greater than heat
C) Net work done in the cycle is equal to net heat exchanged
D) Heat is zero

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43. What type of thermodynamic process occurs when no heat is transferred to or from the system?

A) Isothermal process
B) Adiabatic process
C) Isobaric process
D) Isochoric process

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44. Which quantity is conserved in a steady-flow system?

A) Work
B) Entropy
C) Mass
D) Heat

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45. What does the term “enthalpy” refer to in thermodynamics?

A) The total kinetic energy of molecules
B) The total energy of a system, including internal energy and the product of pressure and volume
C) The heat lost in an isochoric process
D) The change in entropy during a process

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46. In which type of process is the temperature constant, but the pressure and volume change?

A) Isochoric process
B) Isobaric process
C) Adiabatic process
D) Isothermal process

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47. Which cycle is used in jet engines and gas turbines?

A) Rankine cycle
B) Carnot cycle
C) Otto cycle
D) Brayton cycle

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48. What is the entropy change of an isolated system undergoing a reversible process?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates randomly

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49. What is the efficiency of a Carnot engine operating between a heat source at 800 K and a heat sink at 400 K?

A) 0.25
B) 0.50
C) 0.75
D) 0.33

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50. In the context of thermodynamics, what is a “heat reservoir”?

A) A system that can absorb or release infinite amounts of heat without a temperature change
B) A system that stores mechanical work
C) A system that is insulated from its surroundings
D) A system that continuously exchanges heat with its surroundings

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51. What happens to the internal energy of an ideal gas in an isochoric process?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates

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52. Which of the following cycles is used in internal combustion engines?

A) Rankine cycle
B) Brayton cycle
C) Otto cycle
D) Stirling cycle

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53. What is the relationship between pressure and volume in an adiabatic expansion of an ideal gas?

A) Pressure is directly proportional to volume
B) Pressure is inversely proportional to volume
C) Pressure remains constant
D) Volume remains constant

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54. In a throttling process, what is the change in enthalpy?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates

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55. Which of the following laws relates the heat transfer through a substance to its thermal conductivity?

A) Stefan-Boltzmann law
B) Fourier’s law
C) Newton’s law
D) Boyle’s law

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56. What happens to the specific volume of a substance during the phase change from liquid to gas at constant pressure?

A) It decreases
B) It increases
C) It remains constant
D) It fluctuates

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57. In an irreversible process, what happens to the total entropy of the system and its surroundings?

A) It decreases
B) It increases
C) It remains constant
D) It fluctuates

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58. Which thermodynamic process occurs when the pressure remains constant?

A) Isothermal process
B) Isochoric process
C) Isobaric process
D) Adiabatic process

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59. Which statement correctly describes a Carnot cycle?

A) It is an ideal cycle with 100% efficiency
B) It is the most efficient cycle possible for converting heat into work
C) It is used in all real-world engines
D) It involves only isobaric processes

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60. What is the significance of the third law of thermodynamics?

A) It defines the maximum efficiency of heat engines
B) It states that entropy approaches zero as temperature approaches absolute zero
C) It explains the concept of thermal equilibrium
D) It governs the behavior of gases at high temperatures

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61. Which of the following is true for a system in thermodynamic equilibrium?

A) There is no temperature gradient within the system
B) The system continuously does work
C) The pressure varies throughout the system
D) Heat is constantly exchanged with the surroundings

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62. What does the term “adiabatic” imply about a thermodynamic process?

A) No work is done
B) No heat is transferred to or from the system
C) Pressure remains constant
D) The volume remains constant

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63. In the context of entropy, what does the term “reversible process” mean?

A) A process that does not affect the surroundings
B) A process that can occur in reverse without increasing entropy
C) A process that consumes no energy
D) A process that occurs with no work

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64. What is the formula for calculating the work done during an isobaric process?

A) W = P(ΔV)
B) W = ΔU + Q
C) W = Q/P
D) W = ΔV/T

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65. What is the efficiency of a heat engine if the heat input is 600 J and the work output is 300 J?

A) 25%
B) 50%
C) 75%
D) 100%

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66. What is the main consequence of a throttling process on an ideal gas?

A) The temperature decreases
B) The pressure remains constant
C) The enthalpy remains constant
D) The volume decreases

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67. What is the relationship between temperature and volume in an isothermal expansion of an ideal gas?

A) Volume remains constant
B) Volume increases as temperature increases
C) Volume decreases as temperature increases
D) Temperature remains constant while volume increases

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68. Which thermodynamic process describes a constant volume process?

A) Isochoric process
B) Isobaric process
C) Isothermal process
D) Adiabatic process

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69. What happens to the temperature of a gas when it is compressed adiabatically?

A) Temperature increases
B) Temperature decreases
C) Temperature remains constant
D) Temperature fluctuates randomly

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70. What is the result of increasing the temperature of an ideal gas while keeping the pressure constant?

A) The volume decreases
B) The volume increases
C) The entropy decreases
D) The internal energy decreases

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71. What is the efficiency of a reversible heat engine operating between two temperature reservoirs at 900 K and 300 K?

A) 66.7%
B) 33.3%
C) 50%
D) 25%

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72. What is the change in internal energy for an ideal gas in an isothermal process?

A) It increases
B) It decreases
C) It remains constant
D) It depends on the heat added

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73. What is the name of the process where a liquid changes to vapor at constant temperature and pressure?

A) Melting
B) Evaporation
C) Sublimation
D) Boiling

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74. What happens to the total entropy of an isolated system in a natural, spontaneous process?

A) It decreases
B) It remains constant
C) It increases
D) It fluctuates randomly

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75. Which thermodynamic potential is used to determine the work obtainable from a system at constant temperature and pressure?

A) Enthalpy
B) Gibbs free energy
C) Helmholtz free energy
D) Internal energy

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76. In which type of thermodynamic process is heat transferred in or out of the system while the temperature remains constant?

A) Isobaric process
B) Isochoric process
C) Isothermal process
D) Adiabatic process

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77. What happens to the internal energy of a gas when it expands adiabatically?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates

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78. What does the term “latent heat” refer to?

A) The heat required to raise the temperature of a substance
B) The heat required for a phase change without changing temperature
C) The heat lost during expansion
D) The heat required to maintain a constant volume

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79. What does the third law of thermodynamics imply about the entropy of a pure crystalline substance at absolute zero?

A) It is infinite
B) It is zero
C) It is undefined
D) It fluctuates

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80. What is the condition for a thermodynamic process to be considered reversible?

A) No heat transfer occurs
B) No entropy is generated
C) The process occurs infinitely fast
D) Work done is always zero

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81. What is the relationship between pressure and temperature in Gay-Lussac’s law?

A) Pressure is inversely proportional to temperature
B) Pressure is directly proportional to temperature at constant volume
C) Pressure remains constant while temperature changes
D) Pressure decreases as temperature increases

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82. In which type of system is mass and energy allowed to cross the boundary?

A) Open system
B) Closed system
C) Isolated system
D) Steady system

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83. Which term describes the maximum amount of work that can be extracted from a system at constant temperature and volume?

A) Enthalpy
B) Gibbs free energy
C) Helmholtz free energy
D) Entropy

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84. During a phase change from liquid to vapor at constant pressure, what happens to the temperature of the system?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates

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85. In thermodynamics, what is a “dead state”?

A) A state where no energy transfer can occur
B) A state where the system is in mechanical and thermal equilibrium with its surroundings
C) A state where only entropy increases
D) A state where only heat transfer takes place

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86. What does a PV diagram represent in thermodynamics?

A) The relationship between pressure and work
B) The relationship between pressure and volume for a system
C) The change in temperature and entropy
D) The change in energy and work

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87. What is the efficiency of an ideal refrigerator?

A) It is greater than 1
B) It is equal to 1
C) It is always less than 1
D) It fluctuates

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88. What happens to the temperature of a substance during a phase change?

A) It remains constant
B) It increases
C) It decreases
D) It fluctuates randomly

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89. In thermodynamic processes, what does the term “quasi-static” mean?

A) The process happens infinitely slowly, ensuring equilibrium at every point
B) The process occurs instantly
C) The process happens with constant volume
D) The process occurs with constant pressure

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90. What is the specific heat at constant pressure for an ideal gas?

A) The heat required to raise the temperature of a gas by one unit at constant volume
B) The heat required to raise the temperature of a gas by one unit at constant pressure
C) The energy released by a gas during compression
D) The heat lost during phase change

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91. What is the Carnot efficiency of a heat engine operating between two reservoirs at 1200 K and 300 K?

A) 25%
B) 75%
C) 50%
D) 66.7%

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92. What is meant by a “state function” in thermodynamics?

A) A property that depends only on the current state of the system, not the path taken
B) A property that depends on the process followed
C) A function related to the work done
D) A property that remains constant in all thermodynamic processes

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93. Which cycle represents the ideal cycle for a steam power plant?

A) Otto cycle
B) Carnot cycle
C) Rankine cycle
D) Stirling cycle

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94. In an isobaric process, what is the change in pressure?

A) Pressure increases
B) Pressure decreases
C) Pressure remains constant
D) Pressure fluctuates

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95. What is the relationship between entropy and the reversibility of a process?

A) Reversible processes increase entropy
B) Irreversible processes decrease entropy
C) Reversible processes leave entropy unchanged
D) Entropy is always zero for reversible processes

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96. What happens to the pressure of an ideal gas during an isochoric process?

A) It remains constant
B) It increases as temperature increases
C) It decreases as temperature increases
D) It fluctuates randomly

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97. What does the term “thermal conductivity” refer to?

A) The ability of a substance to conduct heat
B) The capacity of a substance to store heat
C) The ability of a substance to expand upon heating
D) The capacity of a substance to change phase

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98. What is the internal energy change in a cyclic process?

A) It increases
B) It decreases
C) It remains constant
D) It fluctuates randomly

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99. Which of the following describes the behavior of an ideal gas during adiabatic compression?

A) Temperature decreases
B) Temperature increases
C) Temperature remains constant
D) Pressure decreases

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100. In the context of thermodynamics, what is “thermal equilibrium”?

A) The state where no heat transfer occurs between two systems
B) The state where one system has more heat than another
C) The state where the pressure is equal in all systems
D) The state where work is done to equalize temperature

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That concludes 100 unique MCQs on thermodynamics!