MCQ ON SECOND LAW OF THERMODYNAMICS
1. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output for this heat engine.
a) 30 MW
b) 40 MW
c) 50 MW
d) 60 MW
Solution: Net power output = 80 – 50 MW = 30 MW.
2. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the thermal efficiency for this heat engine.
a) 47.5 %
b) 27.5 %
c) 37.5 %
d) 30.5%
Solution: The thermal efficiency of heat engine = net work output / heat input
= 30/80 = 0.375 = 37.5 %.
3. A car engine with a power output of 50 kW has a thermal efficiency of 24 percent. Determine the fuel consumption rate of this car if the fuel has a heating value of 44,000 kJ/kg .
a) 0.00273 kg/s
b) 0.00373 kg/s
c) 0.00473 kg/s
d) 0.00573 kg/s
Solution: Q = 50/0.24 = 208.3 kW,
hence fuel consumption rate = 208.3 kW / 44000 kJ/kg = 0.00473 kg/s.
4. The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2kW, determine the coefficient of performance of the refrigerator.
a) 4
b) 3
c) 2
d) 1
Solution: COP = (360/2)(1/60) = 3.
5. The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2kW, determine the rate of heat rejection to the room that houses the refrigerator.
a) 450 kJ/min
b) 460 kJ/min
c) 470 kJ/min
d) 480 kJ/min
Solution: Q = 360 + (2)(60/1) = 480 kJ/min.
MCQ ON SECOND LAW OF THERMODYNAMICS
6. A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day when the outdoor air temperature drops to 2°C, the house is estimated to lose heat at a rate of 80,000 kJ/h. If the heat pump under these conditions has a COP of 2.5, determine the power consumed by the heat pump.
a) 32000 kJ/h
b) 33000 kJ/h
c) 34000 kJ/h
d) 35000 kJ/h
Solution: W = Q/COP = 80000 kJ/h / 2.5 = 32000 kJ/h.
7. A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day when the outdoor air temperature drops to 2°C, the house is estimated to lose heat at a rate of 80,000 kJ/h. If the heat pump under these conditions has a COP of 2.5, determine the rate at which heat is absorbed from the cold outdoor air.thermodynamics-questions-answers-second-law-thermodynamics
a) 32000 kJ/h
b) 48000 kJ/h
c) 54000 kJ/h
d) 72000 kJ/h
Solution: The rate at which heat is absorbed = 80000 – 32000 = 48000 kJ/h.
8. An air-conditioner provides 1 kg/s of air at 15°C cooled from outside atmospheric air at 35°C. Estimate the amount of power needed to operate the air-conditioner.
a) 1.09 kW
b) 1.19 kW
c) 1.29 kW
d) 1.39 kW
Solution: Q = m*cp*(temperature change) = 20.08 kW
COP = (15+273)/(35-15) = 14.4
hence power needed = 20/14.4 = 1.39 kW.
9. A cyclic machine, as shown below, receives 325 kJ from a 1000 K energy reservoir. It rejects 125 kJ to a 400 K energy reservoir and the cycle produces 200kJ of work as output. Is this cycle reversible, irreversible, or impossible?
a) reversible
b) irreversible
c) impossible
d) none of the mentioned
Solution: The Carnot efficiency = 1 – (400/1000) = 0.6 and real efficiency = (300/325) = 0.615 which is greater than the Carnot efficiency hence cycle is impossible.
10. In a cryogenic experiment you need to keep a container at -125°C although it gains 100 W due to heat transfer. What is the smallest motor you would need for a heat pump absorbing heat from the container and rejecting heat to the room at 20°C?
a) 97.84 kW
b) 98.84 kW
c) 99.84 kW
d) 95.84 kW
Solution: COP = 1.022 and thus power required = 100/1.022 = 97.84 kW.
MCQ ON SECOND LAW OF THERMODYNAMICS
11. A car engine operates with a thermal efficiency of 35%. Assume the air-conditioner has a coefficient of performance of 3 working as a refrigerator cooling the inside using engine shaft work to drive it. How much fuel energy should be spend extra to remove 1 kJ from the inside?
a) 0.752 kJ
b) 0.952 kJ
c) 0.852 kJ
d) none of the mentioned
Solution :W = thermal efficiency * Q(fuel) thus Q(fuel) = 1/(0.35*3) = 0.952 kJ.
12. According to Kelvin-Planck statement, it is ____ for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at ____
a) impossible, single fixed temperature
b) possible, changing temperature
c) impossible, changing temperature
d) possible, single fixed temperature
Solution: This is the basic definition of Kelvin-Planck statement.
13. If heat rejected from the system Q2 is zero, then
a) net work=Q1 and efficiency=1.00
b) heat is exchanged only with one reservoir
c) it violates the Kelvin-Planck statement
d) all of the mentioned
Solution : Such a heat engine is called a perpetual motion machine of the second kind(PMM2).
13a. A PMM2 is possible.
a) true
b) false
14. A heat engine has to exchange heat with ___ energy reservoir at ___ different temperatures to produce net work in a complete cycle.
a) one, one
b) one, two
c) two, two
d) none of the mentioned
15. The second law is not a deduction of the first law.
a) true
b) false
MCQ ON SECOND LAW OF THERMODYNAMICS
16. The continual operation of a machine that creates its own energy and thus violates the first law is called
a) PMM2
b) PMM1
c) PMM0
d) none of the mentioned
17. Which of the following is true?
a) heat always from a high temperature body to a low temperature body
b) heat always from a low temperature body to a high temperature body
c) heat can flow from both low to high and high to low temperature body
d) none of the mentioned
18. According to Clausius statement
a) it is impossible to construct a device than can transfer heat from a cooler body to a hotter body without any effect
b) it is impossible to construct a device than can transfer heat from a hotter body to a cooler body without any effect
c) it is possible to construct a device than can transfer heat from a cooler body to a hotter body without any effect
d) none of the mentioned
19. If the second law were not true
a) a ship could be driven by extracting heat from the ocean
b) run a power plant by extracting heat from the air
c) both of the mentioned
d) none of the mentioned
20. The operation of a machine that utilizes the internal energy of only one TER, thus violating second law is called
a) PMM0
b) PMM1
c) PMM2
d) none of the mentioned
MCQ ON SECOND LAW OF THERMODYNAMICS
21. Which device maintains a body at a temperature lower than the temperature of the surroundings?
a) PMM1
b) PMM2
c) refrigerator
d) heat pump
22. What does a refrigerant do?
a) absorbs the heat leakage into body from surroundings
b) evaporates in the evaporator
c) absorbs latent heat of vaporization form the body which is cooled
d) all of the mentioned
23. Coefficient of performance(COP) is defined as
a) heat leakage/work input
b) work input/heat leakage
c) latent heat of condensation/work input
d) work input/latent heat of condensation
24. Which device maintains a body at a temperature higher than the temperature of the surroundings?
a) PMM1
b) PMM2
c) refrigerator
d) heat pump
25. In a heat pump, there is heat leakage from the body to the surroundings.
a) true
b) false
MCQ ON SECOND LAW OF THERMODYNAMICS
26. What is the relation between COP of heat pump and refrigerator?
a) COP of pump=COP of refrigerator – 1
b) COP of pump=COP of refrigerator + 1
c) COP of pump=COP of refrigerator – 2
d) COP of pump=COP of refrigerator + 2
27. Heat leakage from a heat pump to surroundings is always greater than work done on pump.
a) true
b) false
28. Which of the following statements are true?
a) a heat pump provides a thermodynamic advantage over direct heating
b) COP for both refrigerator and pump cannot be infinity
c) work input for both refrigerator and pump is greater than zero
d) all of the mentioned
29. Kelvin-Planck’s and Clausius’ statements are
a) not connected to each other
b) virtually two parallel statements of second law
c) violation of one doesn’t violate the other
d) none of the mentioned
30. If one of the Kelvin-Planck’s or Clausius’ statement is violated, then other is also violated.
a) true
b) false
MCQ ON SECOND LAW OF THERMODYNAMICS
31. Integral of dQ/T is independent of reversible path connecting between two points.
a) true
b) false
32. Integral of dQ/T of a reversible path is given by
a) Si-Sf
b) Sf-Si
c) Si+Sf
d) -Si-Sf
33. Entropy is a
a) path function, intensive property
b) path function, extensive property
c) point function, intensive property
d) point function, extensive property
34. Specific entropy is given by( where m is the mass)
a) Sm
b) m/S
c) S/m
d) none of the mentioned
35. For any process which is undergone by a system
a) dQ/T>=ds
b) dQ/T<=ds
c) dQ/T≠ds
d) none of the mentioned
MCQ ON SECOND LAW OF THERMODYNAMICS
36. For a reversible process,
a) dS=dQ/T
b) dS>dQ/T
c) dS<dQ/T
d) none of the mentioned
37. For an irreversible process,
a) dS=dQ/T
b) dS>dQ/T
c) dS<dQ/T
d) none of the mentioned
38. For two different paths between same two points, entropy change is
a) depends on path taken
b) different
c) same
d) none of the mentioned
39. For the general case, we can write
a) S2-S1<=dQ/T for a path
b) S2-S1>=dQ/T for a path
c) S2-S1≠dQ/T for a path
d) none of the mentioned
40. When two equilibrium states are infinitesimally near,
a) dQ/T=dS
b) dQ/T>dS
c) dQ/T<dS
d) none of the mentioned
