APSC AE Mechanical Exam MCQ: Thermodynamics, Heat Transfer, Refrigeration

1. What is the first law of thermodynamics also known as?

a) Law of Entropy b) Law of Energy Conservation c) Law of Heat Transfer d) Law of Equilibrium

Answer: b) Law of Energy Conservation
Explanation: The first law of thermodynamics states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. This is also known as the law of energy conservation.

2. A gas expands in a piston-cylinder device from 2 L to 5 L at constant pressure of 100 kPa. Calculate the work done by the gas.

a) 300 J b) 300 kJ c) 0.3 kJ d) 30 kJ

Answer: c) 0.3 kJ
Explanation: Work done by a gas at constant pressure is given by W = P × ΔV. Here, P = 100 kPa = 100 × 10³ Pa, ΔV = (5 - 2) L = 3 L = 3 × 10⁻³ m³. So, W = 100 × 10³ × 3 × 10⁻³ = 300 J = 0.3 kJ.

3. Which of the following is true for an isolated system?

a) It can exchange both heat and work with surroundings. b) It cannot exchange heat or matter with surroundings. c) It can exchange only heat with surroundings. d) It can exchange only matter with surroundings.

Answer: b) It cannot exchange heat or matter with surroundings.
Explanation: An isolated system is completely sealed from its surroundings, meaning neither heat, work, nor matter can be exchanged. This distinguishes it from open and closed systems.

4. The efficiency of a Carnot engine operating between 600 K and 300 K is:

a) 25% b) 50% c) 75% d) 100%

Answer: b) 50%
Explanation: Carnot efficiency is given by η = 1 - (T_C / T_H), where T_C = 300 K and T_H = 600 K. So, η = 1 - (300 / 600) = 0.5 = 50%.

5. Which statement is correct about the second law of thermodynamics?

a) It states that energy can be created. b) It introduces the concept of entropy. c) It states that heat and work are interchangeable. d) It applies only to open systems.

Answer: b) It introduces the concept of entropy.
Explanation: The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. It introduces entropy as a measure of disorder and governs the direction of heat flow.

6. A refrigerator absorbs 500 J of heat from a cold reservoir and rejects 800 J to a hot reservoir. What is its coefficient of performance (COP)?

a) 1.67 b) 0.625 c) 2.5 d) 3.2

Answer: a) 1.67
Explanation: COP of a refrigerator = Q_C / W, where Q_C = heat absorbed = 500 J, W = work input = Q_H - Q_C = 800 J - 500 J = 300 J. So, COP = 500 / 300 = 1.67.

7. In a heat exchanger, 2 kg/s of water at 20°C is heated to 60°C using hot gases. If the specific heat of water is 4.18 kJ/kg·K, calculate the heat transfer rate.

a) 167.2 kW b) 83.6 kW c) 334.4 kW d) 41.8 kW

Answer: a) 167.2 kW
Explanation: Heat transfer rate = m × c × ΔT, where m = 2 kg/s, c = 4.18 kJ/kg·K, ΔT = (60 - 20) = 40°C. So, Q = 2 × 4.18 × 40 = 167.2 kW.

8. Which mode of heat transfer does not require a medium?

a) Conduction b) Convection c) Radiation d) All require a medium

Answer: c) Radiation
Explanation: Radiation involves the transfer of heat through electromagnetic waves and can occur in a vacuum, unlike conduction and convection, which require a material medium.

9. The refrigerant commonly used in domestic refrigerators is:

a) Ammonia b) R-134a c) Carbon Dioxide d) Freon-12

Answer: b) R-134a
Explanation: R-134a is widely used in domestic refrigerators due to its low environmental impact and suitable thermodynamic properties. Freon-12 was phased out due to ozone depletion concerns.

10. Which of the following is a statement of the zeroth law of thermodynamics?

a) Energy cannot be created or destroyed. b) If two systems are in thermal equilibrium with a third, they are in equilibrium with each other. c) Entropy of a system increases over time. d) Heat flows from cold to hot naturally.

Answer: b) If two systems are in thermal equilibrium with a third, they are in equilibrium with each other.
Explanation: The zeroth law establishes the concept of temperature and thermal equilibrium, allowing the use of thermometers to measure temperature.

11. What is the entropy change of a system when 1000 J of heat is added reversibly at 500 K?

a) 2 J/K b) 0.5 J/K c) 500 J/K d) 1000 J/K

Answer: a) 2 J/K
Explanation: For a reversible process, entropy change ΔS = Q/T. Here, Q = 1000 J, T = 500 K. So, ΔS = 1000 / 500 = 2 J/K.

12. Which of the following devices is used as a throttling device in a refrigeration cycle?

a) Compressor b) Condenser c) Capillary tube d) Evaporator

Answer: c) Capillary tube
Explanation: A capillary tube is used as a throttling device in refrigeration cycles, especially in domestic refrigerators, to reduce the pressure of the refrigerant.

13. The Fourier’s law of heat conduction is given by:

a) Q = kA(dT/dx) b) Q = kA(T²) c) Q = k/A(dT/dx) d) Q = kA(dT²/dx)

Answer: a) Q = kA(dT/dx)
Explanation: Fourier’s law states that the rate of heat conduction (Q) is proportional to the thermal conductivity (k), area (A), and the temperature gradient (dT/dx).

14. A wall of thickness 0.2 m and thermal conductivity 0.5 W/m·K has a temperature difference of 50°C across it. If the area is 2 m², calculate the heat transfer rate.

a) 250 W b) 500 W c) 1000 W d) 125 W

Answer: b) 500 W
Explanation: Using Fourier’s law, Q = kA(ΔT/Δx). Here, k = 0.5 W/m·K, A = 2 m², ΔT = 50°C, Δx = 0.2 m. So, Q = 0.5 × 2 × (50 / 0.2) = 500 W.

15. Which of the following is true for a reversible process?

a) It produces maximum entropy. b) It is an idealized process with no irreversibilities. c) It violates the second law of thermodynamics. d) It cannot occur in real systems.

Answer: b) It is an idealized process with no irreversibilities.
Explanation: A reversible process is an idealized process where no irreversibilities (like friction or heat loss) occur, and the system can return to its initial state without affecting the surroundings.

16. The COP of a Carnot refrigerator operating between 300 K and 250 K is:

a) 5 b) 6 c) 4 d) 7

Answer: a) 5
Explanation: COP of a Carnot refrigerator = T_C / (T_H - T_C), where T_C = 250 K, T_H = 300 K. So, COP = 250 / (300 - 250) = 250 / 50 = 5.

17. Which component in a refrigeration system removes heat from the refrigerant?

a) Compressor b) Condenser c) Evaporator d) Expansion valve

Answer: b) Condenser
Explanation: The condenser removes heat from the high-pressure refrigerant, causing it to condense from a gas to a liquid, rejecting heat to the surroundings.

18. A heat engine produces 2000 J of work while rejecting 3000 J of heat to a sink. What is its efficiency?

a) 40% b) 28.6% c) 66.7% d) 50%

Answer: a) 40%
Explanation: Efficiency η = W / Q_H, where W = 2000 J, Q_H = W + Q_C = 2000 + 3000 = 5000 J. So, η = 2000 / 5000 = 0.4 = 40%.

19. Which of the following is a characteristic of an adiabatic process?

a) Heat transfer occurs. b) No heat transfer occurs. c) Temperature remains constant. d) Pressure remains constant.

Answer: b) No heat transfer occurs.
Explanation: An adiabatic process is one in which no heat is exchanged between the system and its surroundings, though work may be done, causing changes in temperature or pressure.

20. The Stefan-Boltzmann law relates to which mode of heat transfer?

a) Conduction b) Convection c) Radiation d) All of the above

Answer: c) Radiation
Explanation: The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body is proportional to the fourth power of its temperature, applicable to radiation heat transfer.

21. A heat pump operates between 300 K and 400 K. What is its maximum COP?

a) 3 b) 4 c) 5 d) 6

Answer: b) 4
Explanation: Maximum COP of a heat pump (Carnot) = T_H / (T_H - T_C), where T_H = 400 K, T_C = 300 K. So, COP = 400 / (400 - 300) = 4.

22. Which of the following is NOT a primary refrigerant?

a) R-134a b) Ammonia c) Brine d) R-22

Answer: c) Brine
Explanation: Brine is a secondary refrigerant, used to transfer heat after being cooled by a primary refrigerant like R-134a, ammonia, or R-22, which undergo phase changes in the refrigeration cycle.

23. The heat transfer coefficient in convection depends on:

a) Fluid properties only b) Surface area only c) Fluid properties and flow characteristics d) Temperature difference only

Answer: c) Fluid properties and flow characteristics
Explanation: The convective heat transfer coefficient depends on fluid properties (e.g., viscosity, density) and flow characteristics (e.g., laminar or turbulent flow), which affect the rate of heat transfer.

24. A system undergoes a process where 500 J of work is done on it, and it absorbs 300 J of heat. What is the change in internal energy?

a) -200 J b) 200 J c) 800 J d) -800 J

Answer: c) 800 J
Explanation: By the first law, ΔU = Q + W, where Q = +300 J (heat added), W = +500 J (work done on the system). So, ΔU = 300 + 500 = 800 J.

25. Which law governs the operation of a thermocouple?

a) Fourier’s law b) Seebeck effect c) Newton’s law of cooling d) Stefan-Boltzmann law

Answer: b) Seebeck effect
Explanation: The Seebeck effect describes the generation of voltage in a thermocouple due to a temperature difference between two different metals, used for temperature measurement.

26. In a vapor compression refrigeration cycle, the process in the evaporator is:

a) Isenthalpic b) Isobaric c) Isentropic d) Isothermal

Answer: b) Isobaric
Explanation: In the evaporator, the refrigerant absorbs heat at constant pressure (isobaric process), changing from a liquid-vapor mixture to a saturated or superheated vapor.

27. A black body emits radiation at a rate proportional to:

a) T² b) T³ c) T⁴ d) T

Answer: c) T⁴
Explanation: According to the Stefan-Boltzmann law, the power radiated by a black body is proportional to the fourth power of its absolute temperature (T⁴).

28. The Kelvin-Planck statement is related to:

a) First law of thermodynamics b) Second law of thermodynamics c) Zeroth law of thermodynamics d) Third law of thermodynamics

Answer: b) Second law of thermodynamics
Explanation: The Kelvin-Planck statement says it’s impossible for a heat engine to convert all absorbed heat into work without rejecting some heat, a key concept of the second law.

29. A refrigeration system requires 1 kW of power to remove 5 kW of heat. What is its COP?

a) 5 b) 4 c) 6 d) 3

Answer: a) 5
Explanation: COP of a refrigerator = Q_C / W, where Q_C = 5 kW, W = 1 kW. So, COP = 5 / 1 = 5.

30. Which of the following is true for a closed system?

a) It can exchange both mass and energy. b) It can exchange energy but not mass. c) It cannot exchange energy or mass. d) It can exchange mass but not energy.

Answer: b) It can exchange energy but not mass.
Explanation: A closed system allows the exchange of energy (heat or work) with its surroundings but not mass, unlike an open system which allows both.

31. The heat transfer rate through a composite wall depends on:

a) Thermal conductivity only b) Thickness and thermal conductivity c) Area only d) Temperature only

Answer: b) Thickness and thermal conductivity
Explanation: Heat transfer through a composite wall depends on the thermal conductivity and thickness of each layer, as well as the temperature difference and area.

32. A Carnot engine has an efficiency of 40%. If the sink temperature is 300 K, what is the source temperature?

a) 500 K b) 450 K c) 600 K d) 400 K

Answer: a) 500 K
Explanation: Carnot efficiency η = 1 - (T_C / T_H). Given η = 0.4, T_C = 300 K, we get 0.4 = 1 - (300 / T_H). Solving, T_H = 300 / (1 - 0.4) = 500 K.

33. The primary function of the compressor in a refrigeration system is:

a) To remove heat from the refrigerant b) To increase the pressure of the refrigerant c) To reduce the pressure of the refrigerant d) To absorb heat from the surroundings

Answer: b) To increase the pressure of the refrigerant
Explanation: The compressor increases the pressure and temperature of the refrigerant vapor, enabling it to release heat in the condenser.

34. Which of the following is a unit of thermal conductivity?

a) W/m²·K b) W/m·K c) J/kg·K d) W/K

Answer: b) W/m·K
Explanation: Thermal conductivity measures the ability of a material to conduct heat, with units of power per unit length per unit temperature difference, i.e., W/m·K.

35. An ideal gas undergoes an isothermal expansion. Which statement is true?

a) Internal energy increases. b) Internal energy remains constant. c) Internal energy decreases. d) No work is done.

Answer: b) Internal energy remains constant.
Explanation: For an ideal gas, internal energy depends only on temperature. In an isothermal process, temperature is constant, so internal energy remains constant.

36. A heat exchanger transfers 100 kW of heat to water flowing at 1 kg/s. If the inlet temperature is 20°C, what is the outlet temperature? (Specific heat of water = 4.18 kJ/kg·K)

a) 43.9°C b) 35.5°C c) 50.2°C d) 28.7°C

Answer: a) 43.9°C
Explanation: Q = m × c × ΔT. Given Q = 100 kW = 100 kJ/s, m = 1 kg/s, c = 4.18 kJ/kg·K, we get 100 = 1 × 4.18 × (T_out - 20). Solving, T_out = 20 + (100 / 4.18) ≈ 43.9°C.

37. The Clausius statement is associated with:

a) First law of thermodynamics b) Second law of thermodynamics c) Zeroth law of thermodynamics d) Third law of thermodynamics

Answer: b) Second law of thermodynamics
Explanation: The Clausius statement says it’s impossible to transfer heat from a colder to a hotter body without external work, a principle of the second law.

38. Which refrigerant has zero ozone depletion potential?

a) R-12 b) R-134a c) R-22 d) Freon-11

Answer: b) R-134a
Explanation: R-134a has zero ozone depletion potential, making it an environmentally friendly alternative to R-12 and R-22, which contribute to ozone depletion.

39. The third law of thermodynamics states that:

a) Entropy of a system approaches infinity at absolute zero. b) Entropy of a pure crystalline substance is zero at absolute zero. c) Heat cannot be transferred at absolute zero. d) Work is zero at absolute zero.

Answer: b) Entropy of a pure crystalline substance is zero at absolute zero.
Explanation: The third law states that the entropy of a perfect crystal at absolute zero (0 K) is zero, as there is no disorder at this temperature.

40. A metal rod of length 1 m and cross-sectional area 0.01 m² has a thermal conductivity of 200 W/m·K. If one end is at 100°C and the other at 20°C, calculate the heat transfer rate.

a) 1600 W b) 2000 W c) 800 W d) 400 W

Answer: a) 1600 W
Explanation: Q = kA(ΔT/L), where k = 200 W/m·K, A = 0.01 m², ΔT = (100 - 20) = 80°C, L = 1 m. So, Q = 200 × 0.01 × (80 / 1) = 1600 W.

41. In a refrigeration cycle, the process in the expansion valve is:

a) Isentropic b) Isenthalpic c) Isobaric d) Isothermal

Answer: b) Isenthalpic
Explanation: The expansion valve in a refrigeration cycle is a throttling device where the process is isenthalpic, meaning enthalpy remains constant as pressure drops.

42. Which of the following affects the rate of convective heat transfer?

a) Surface emissivity b) Fluid velocity c) Thermal conductivity of the solid d) Absolute temperature to the fourth power

Answer: b) Fluid velocity
Explanation: Convective heat transfer depends on fluid velocity, which influences the heat transfer coefficient by affecting the flow regime (laminar or turbulent).

43. A heat engine operates with a heat input of 10,000 J and produces 4000 J of work. What is the heat rejected?

a) 6000 J b) 4000 J c) 8000 J d) 2000 J

Answer: a) 6000 J
Explanation: By the first law, Q_H = W + Q_C. Given Q_H = 10,000 J, W = 4000 J, we get Q_C = 10,000 - 4000 = 6000 J.

44. The purpose of superheating in a refrigeration cycle is to:

a) Increase compressor work b) Ensure complete vaporization before compression c) Reduce refrigerant flow rate d) Decrease COP

Answer: b) Ensure complete vaporization before compression
Explanation: Superheating ensures that the refrigerant entering the compressor is fully vaporized, preventing liquid slugging and damage to the compressor.

45. Which of the following is true for a pure substance in a phase change process?

a) Temperature changes during boiling. b) Temperature remains constant during boiling. c) Pressure changes during melting. d) Internal energy decreases during boiling.

Answer: b) Temperature remains constant during boiling.
Explanation: For a pure substance, during a phase change like boiling or melting at constant pressure, temperature remains constant as heat is used to change the phase (latent heat).

46. A room is maintained at 25°C using a heat pump with a COP of 3. If the heat loss is 10 kW, what is the power required?

a) 3.33 kW b) 5 kW c) 2.5 kW d) 6.67 kW

Answer: a) 3.33 kW
Explanation: For a heat pump, COP = Q_H / W. Given COP = 3, Q_H = 10 kW, we get W = Q_H / COP = 10 / 3 ≈ 3.33 kW.

47. The critical temperature of a refrigerant should be:

a) Lower than the condensing temperature b) Higher than the condensing temperature c) Equal to the condensing temperature d) Zero

Answer: b) Higher than the condensing temperature
Explanation: The critical temperature of a refrigerant must be higher than the condensing temperature to ensure it can condense into a liquid during the refrigeration cycle.

48. Newton’s law of cooling applies to:

a) Conduction b) Convection c) Radiation d) All of the above

Answer: b) Convection
Explanation: Newton’s law of cooling states that the rate of heat loss from a body is proportional to the temperature difference between the body and its surroundings, typically applied to convection.

49. A system absorbs 600 J of heat and does 200 J of work. What is the change in internal energy?

a) 400 J b) 800 J c) 200 J d) -400 J

Answer: a) 400 J
Explanation: By the first law, ΔU = Q - W, where Q = +600 J (heat absorbed), W = +200 J (work done by the system). So, ΔU = 600 - 200 = 400 J.

50. The primary purpose of the evaporator in a refrigeration system is to:

a) Compress the refrigerant b) Absorb heat from the refrigerated space c) Reject heat to the surroundings d) Reduce refrigerant pressure

Answer: b) Absorb heat from the refrigerated space
Explanation: The evaporator allows the refrigerant to absorb heat from the refrigerated space, causing it to evaporate and cool the surroundings.