Here is a list of MCQs for Unit 7: Thermodynamics, with answers and explanations:
1. The first law of thermodynamics is a statement of the principle of?
A) Conservation of momentum
B) Conservation of energy
C) Conservation of mass
D) Conservation of heat
Answer: B) Conservation of energy.
Explanation: The first law of thermodynamics is essentially the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or converted from one form to another.
2. The work done in an isothermal process is?
A) Zero
B) Positive
C) Negative
D) Equal to the change in internal energy
Answer: B) Positive.
Explanation: In an isothermal process (constant temperature), the work done by the system is equal to the heat added to the system, as there is no change in the internal energy (since temperature is constant).
3. The heat capacity of a body is defined as?
A) The amount of heat required to raise the temperature by 1°C
B) The amount of work done on the body
C) The amount of heat required to increase the pressure by 1 atm
D) The heat required to change the volume of the body by 1 unit
Answer: A) The amount of heat required to raise the temperature by 1°C.
Explanation: Heat capacity is the amount of heat required to raise the temperature of a body by 1°C or 1 K. It is a measure of a body’s ability to absorb heat.
4. Which of the following statements about the second law of thermodynamics is true?
A) Energy cannot be created or destroyed.
B) Heat cannot spontaneously flow from a colder body to a hotter body.
C) The total energy of an isolated system is constant.
D) The entropy of the universe is always constant.
Answer: B) Heat cannot spontaneously flow from a colder body to a hotter body.
Explanation: The second law of thermodynamics states that heat naturally flows from a hotter object to a colder one, and it cannot spontaneously flow in the opposite direction without external work being done.
5. The term ‘enthalpy’ is used to refer to the sum of?
A) Internal energy and pressure
B) Internal energy and volume
C) Internal energy and temperature
D) Internal energy and pressure-volume work
Answer: D) Internal energy and pressure-volume work.
Explanation: Enthalpy is defined as H=U+PVH = U + PVH=U+PV, where UUU is the internal energy, PPP is the pressure, and VVV is the volume of the system.
6. The efficiency of a heat engine depends on?
A) The temperature of the hot reservoir only
B) The temperature of the cold reservoir only
C) The temperatures of both the hot and cold reservoirs
D) The size of the engine
Answer: C) The temperatures of both the hot and cold reservoirs.
Explanation: The efficiency of a heat engine is given by η=1−TcTh\eta = 1 – \frac{T_c}{T_h}η=1−ThTc, where TcT_cTc is the temperature of the cold reservoir and ThT_hTh is the temperature of the hot reservoir. Efficiency depends on both temperatures.
7. The change in the internal energy of a system is equal to?
A) The work done on the system only
B) The heat added to the system only
C) The difference between heat added and work done
D) The sum of heat added and work done
Answer: C) The difference between heat added and work done.
Explanation: According to the first law of thermodynamics, the change in internal energy is the difference between the heat added to the system and the work done by the system: ΔU=Q−W\Delta U = Q – WΔU=Q−W.
8. In a cyclic process, the net change in the internal energy of the system is?
A) Zero
B) Equal to the work done by the system
C) Equal to the heat added to the system
D) Equal to the pressure of the system
Answer: A) Zero.
Explanation: In a cyclic process, the system returns to its original state, so the net change in internal energy is zero. This means that the heat absorbed is equal to the work done by the system.
9. An ideal gas expands against a constant external pressure. The work done by the gas is?
A) Zero
B) Negative
C) Positive
D) Equal to the change in internal energy
Answer: C) Positive.
Explanation: When an ideal gas expands against a constant external pressure, the work done is positive because the gas does work on the surroundings by expanding, i.e., W=PΔVW = P \Delta VW=PΔV, where ΔV\Delta VΔV is positive.
10. The second law of thermodynamics introduces the concept of?
A) Enthalpy
B) Work
C) Entropy
D) Internal energy
Answer: C) Entropy.
Explanation: The second law of thermodynamics involves the concept of entropy, which is a measure of the disorder or randomness of a system. Entropy tends to increase in an isolated system.
11. The temperature at which the entropy of a perfect crystal becomes zero is?
A) 0°C
B) 0 K
C) 273 K
D) 100 K
Answer: B) 0 K.
Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal at absolute zero (0 K) is exactly zero.
12. In an adiabatic process, the system?
A) Absorbs heat from the surroundings
B) Loses heat to the surroundings
C) Does not exchange heat with the surroundings
D) Neither absorbs nor loses heat
Answer: C) Does not exchange heat with the surroundings.
Explanation: In an adiabatic process, there is no heat exchange between the system and its surroundings. All the energy change in the system is due to work done by or on the system.
13. What is the value of the work done in an isochoric process (constant volume)?
A) Zero
B) Positive
C) Negative
D) Equal to the change in internal energy
Answer: A) Zero.
Explanation: In an isochoric process, the volume remains constant, so no work is done by the system because work is given by W=PΔVW = P \Delta VW=PΔV. Since ΔV=0\Delta V = 0ΔV=0, W=0W = 0W=0.
14. Which of the following statements is true about a Carnot engine?
A) It is the most efficient engine operating between two temperature reservoirs.
B) It is the least efficient engine.
C) It operates at constant temperature throughout.
D) It violates the first law of thermodynamics.
Answer: A) It is the most efficient engine operating between two temperature reservoirs.
Explanation: The Carnot engine is the ideal heat engine that operates with the maximum possible efficiency, which is determined by the temperatures of the hot and cold reservoirs.
15. The change in the internal energy of a system depends only on?
A) The initial temperature of the system
B) The work done by the system
C) The heat added to the system
D) The initial and final states of the system
Answer: D) The initial and final states of the system.
Explanation: The change in internal energy depends only on the initial and final states of the system, and is independent of the path taken to reach those states. It is a state function.
16. The work done by an ideal gas during an isothermal expansion is?
A) Zero
B) Positive
C) Negative
D) Equal to the internal energy
Answer: B) Positive.
Explanation: In an isothermal expansion (constant temperature), the internal energy of the gas remains unchanged. The work done by the gas is positive as the gas does work on the surroundings during expansion.
17. In a reversible adiabatic process, the temperature of the gas?
A) Increases
B) Decreases
C) Remains the same
D) Depends on the volume only
Answer: B) Decreases.
Explanation: In a reversible adiabatic expansion, the gas does work on the surroundings without heat exchange. As a result, the temperature of the gas decreases.
18. The heat engine efficiency is always less than 100% because of?
A) Friction
B) Heat loss to the environment
C) Entropy increase
D) Irreversible processes
Answer: C) Entropy increase.
Explanation: According to the second law of thermodynamics, some energy is always lost as heat and increases the entropy of the system. This results in less than 100% efficiency for any real heat engine.
19. In a steam engine, the work done is due to?
A) Heat absorbed from the steam
B) Conversion of steam’s thermal energy to mechanical energy
C) Expansion of gases
D) Heat produced by combustion
Answer: B) Conversion of steam’s thermal energy to mechanical energy.
Explanation: In a steam engine, thermal energy from the steam is converted into mechanical energy that performs work.
20. If a system undergoes an isothermal expansion, which of the following remains constant?
A) Temperature
B) Pressure
C) Volume
D) Internal energy
Answer: A) Temperature.
Explanation: In an isothermal process, the temperature remains constant. As the system expands, it does work, and heat is absorbed to maintain the temperature.
21. The specific heat capacity of a substance is the amount of heat required to?
A) Change the temperature of a unit mass by 1°C
B) Raise the temperature of a unit mass by 1 Kelvin
C) Change the pressure of the substance by 1 atm
D) Change the temperature of 1 mole of the substance by 1°C
Answer: A) Change the temperature of a unit mass by 1°C.
Explanation: Specific heat capacity is defined as the amount of heat required to change the temperature of a unit mass of a substance by 1°C (or 1 K).
22. In a reversible adiabatic process, the relationship between pressure and volume is given by?
A) PV=constantP V = \text{constant}PV=constant
B) PVn=constantP V^n = \text{constant}PVn=constant
C) P=constantP = \text{constant}P=constant
D) V=constantV = \text{constant}V=constant
Answer: B) PVn=constantP V^n = \text{constant}PVn=constant.
Explanation: In a reversible adiabatic process, the relationship between pressure and volume is PVn=constantP V^n = \text{constant}PVn=constant, where nnn is the adiabatic index (also called the heat capacity ratio).
23. Which of the following processes is adiabatic?
A) Free expansion of a gas
B) Isobaric heating
C) Isothermal compression
D) Compression of gas in an insulated container
Answer: D) Compression of gas in an insulated container.
Explanation: In an adiabatic process, there is no heat exchange with the surroundings. If a gas is compressed in an insulated container, no heat is exchanged, making it an adiabatic process.
24. The efficiency of a Carnot engine depends on?
A) The temperature of the working substance
B) The temperatures of the hot and cold reservoirs
C) The pressure in the engine
D) The work done by the engine
Answer: B) The temperatures of the hot and cold reservoirs.
Explanation: The efficiency of a Carnot engine is determined by the temperatures of the hot and cold reservoirs. It is given by η=1−TcTh\eta = 1 – \frac{T_c}{T_h}η=1−ThTc, where TcT_cTc is the temperature of the cold reservoir and ThT_hTh is the temperature of the hot reservoir.
25. The second law of thermodynamics implies that?
A) Heat flows spontaneously from a colder object to a hotter one
B) The entropy of the universe tends to decrease
C) It is possible to convert all heat into work
D) Heat flows spontaneously from a hotter object to a colder one
Answer: D) Heat flows spontaneously from a hotter object to a colder one.
Explanation: According to the second law of thermodynamics, heat always flows naturally from a hotter object to a colder object. This is a direction of energy flow that increases the entropy of the universe.
26. If a substance undergoes a phase change at constant temperature, the change in internal energy is equal to?
A) The heat absorbed or released
B) The work done by the system
C) The pressure-volume work
D) The heat capacity of the substance
Answer: A) The heat absorbed or released.
Explanation: During a phase change (such as melting or boiling) at constant temperature, the internal energy of the substance changes due to the heat absorbed or released, while the temperature remains constant.
27. In thermodynamics, a reversible process is one in which?
A) The system does not return to its original state
B) The system can only be done in one direction
C) The system returns to its original state after the process
D) There is a change in the work done during the process
Answer: C) The system returns to its original state after the process.
Explanation: A reversible process is one that can be reversed without leaving any net changes in the system or surroundings. The system can be restored to its original state by reversing the process.
28. What is the work done in an isothermal expansion of an ideal gas?
A) W=PΔVW = P \Delta VW=PΔV
B) W=nRTln(Vf/Vi)W = nRT \ln(V_f/V_i)W=nRTln(Vf/Vi)
C) W=0W = 0W=0
D) W=nRln(Tf/Ti)W = nR \ln(T_f/T_i)W=nRln(Tf/Ti)
Answer: B) W=nRTln(Vf/Vi)W = nRT \ln(V_f/V_i)W=nRTln(Vf/Vi).
Explanation: In an isothermal process, the work done by an ideal gas during expansion is given by W=nRTln(Vf/Vi)W = nRT \ln(V_f/V_i)W=nRTln(Vf/Vi), where VfV_fVf and ViV_iVi are the final and initial volumes, respectively, and nnn is the number of moles of gas.
29. The entropy of a system is a measure of?
A) Energy
B) Temperature
C) Disorder or randomness
D) Work done
Answer: C) Disorder or randomness.
Explanation: Entropy is a measure of the disorder or randomness in a system. The higher the entropy, the greater the disorder or uncertainty of the system’s state.
30. If two bodies are in thermal equilibrium, then?
A) Their temperatures are equal
B) Their masses are equal
C) The total heat in both bodies is the same
D) They are in mechanical equilibrium
Answer: A) Their temperatures are equal.
Explanation: According to the Zeroth law of thermodynamics, if two bodies are in thermal equilibrium with each other, they must have the same temperature.
31. The first law of thermodynamics is mathematically expressed as?
A) ΔU=Q+W\Delta U = Q + WΔU=Q+W
B) ΔU=Q−W\Delta U = Q – WΔU=Q−W
C) ΔU=W−Q\Delta U = W – QΔU=W−Q
D) ΔU=W+Q\Delta U = W + QΔU=W+Q
Answer: B) ΔU=Q−W\Delta U = Q – WΔU=Q−W.
Explanation: The first law of thermodynamics is given by ΔU=Q−W\Delta U = Q – WΔU=Q−W, where ΔU\Delta UΔU is the change in internal energy, QQQ is the heat added to the system, and WWW is the work done by the system.
32. In an isobaric process, the work done is?
A) Zero
B) Positive
C) Negative
D) Equal to the heat added
Answer: B) Positive.
Explanation: In an isobaric process (constant pressure), the work done is given by W=PΔVW = P \Delta VW=PΔV, where ΔV\Delta VΔV is the change in volume. The work done is positive if the volume increases.
33. In a thermodynamic system, when the work done by the system is positive, it means that?
A) The system is expanding
B) The system is being compressed
C) The internal energy of the system increases
D) Heat is being added to the system
Answer: A) The system is expanding.
Explanation: When the work done by the system is positive, it means the system is doing work on its surroundings by expanding. This results in the system losing energy.
34. Which process occurs without any change in the entropy of the system?
A) Isothermal process
B) Adiabatic process
C) Reversible adiabatic process
D) Irreversible adiabatic process
Answer: C) Reversible adiabatic process.
Explanation: In a reversible adiabatic process, there is no change in entropy of the system, as there is no heat exchange and the process is reversible.
35. What does the Carnot cycle represent in thermodynamics?
A) A theoretical engine that is 100% efficient
B) An ideal heat engine operating between two heat reservoirs
C) A system with constant volume
D) A system undergoing constant pressure expansion
Answer: B) An ideal heat engine operating between two heat reservoirs.
Explanation: The Carnot cycle represents an idealized heat engine that operates between two heat reservoirs, with maximum efficiency defined by the temperatures of the reservoirs.
36. Which of the following is not a state function?
A) Internal energy
B) Pressure
C) Work
D) Entropy
Answer: C) Work.
Explanation: Work is not a state function because it depends on the path taken during the process, while internal energy, pressure, and entropy are state functions, meaning they depend only on the state of the system.
37. The efficiency of a heat engine is always less than 100% due to?
A) Heat loss to the surroundings
B) Frictional losses in the engine
C) Entropy increase
D) The irreversible processes involved
Answer: C) Entropy increase.
Explanation: According to the second law of thermodynamics, the efficiency of a heat engine is less than 100% because entropy increases in the process, leading to some energy being lost as heat.
38. What is the relationship between heat and temperature change for an ideal gas?
A) Q=nCvΔTQ = nC_v \Delta TQ=nCvΔT
B) Q=nCpΔTQ = nC_p \Delta TQ=nCpΔT
C) Q=mLQ = mLQ=mL
D) Q=ΔU+WQ = \Delta U + WQ=ΔU+W
Answer: B) Q=nCpΔTQ = nC_p \Delta TQ=nCpΔT.
Explanation: For an ideal gas, the heat added to the system during a process that occurs at constant pressure is given by Q=nCpΔTQ = nC_p \Delta TQ=nCpΔT, where CpC_pCp is the specific heat capacity at constant pressure, nnn is the number of moles, and ΔT\Delta TΔT is the change in temperature.
39. The value of the adiabatic index γ\gammaγ for a monatomic ideal gas is?
A) 1
B) 5/3
C) 3/2
D) 2
Answer: B) 5/3.
Explanation: For a monatomic ideal gas, the adiabatic index γ=CpCv=5/3\gamma = \frac{C_p}{C_v} = 5/3γ=CvCp=5/3.
40. The change in the entropy of a system is defined as?
A) ΔS=QT\Delta S = \frac{Q}{T}ΔS=TQ
B) ΔS=TQ\Delta S = \frac{T}{Q}ΔS=QT
C) ΔS=WT\Delta S = \frac{W}{T}ΔS=TW
D) ΔS=PV\Delta S = \frac{P}{V}ΔS=VP
Answer: A) ΔS=QT\Delta S = \frac{Q}{T}ΔS=TQ.
Explanation: The change in entropy ΔS\Delta SΔS is defined as the heat added to the system QQQ divided by the temperature TTT at which the process occurs, assuming the process is reversible.
This concludes the Unit 7: Thermodynamics MCQs. These questions cover a wide range of topics from the laws of thermodynamics to the principles of heat engines, processes, and concepts such as entropy and heat capacity.
Also Read: Unit 8: Behaviour of Perfect Gas and Kinetic Theory
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