The Second Law of Thermodynamics

1. The Second Law of Thermodynamics Chapter 6

2. The Second Law • The second law of thermodynamics states that processes occur in a certain direction, not in just any direction. • Physical processes in nature proceed toward equilibrium spontaneously:

3. Water always flows downhill

4. Gases always expand from high pressure to low pressure

5. Heat always flows from high temperature to low temperature

6. We can reverse these processes • It requires the expenditure of work • The first law gives us no information about the direction in which a process occurs – it only tells us that energy must balance • The second law tells us in what direction processes occur

7. Work Always Converts Directly and Completely to Heat, But not the Reverse Work and Heat are not interchangeable forms of energy We need a device to convert heat to work

8. Some Definitions • Heat Reservoir • Source • Sink • Provides heat energy, but does not change temperature

9. Some Definitions • Thermodynamic Cycle • System returns to the initial conditions • The initial state is the same as the final state

10. Some Definitions • Heat Engine • A heat engine is a thermodynamic system operating in a thermodynamic cycle to which net heat is transferred and from which net work is delivered This is the device that converts heat to work

11. A steam power plant is a heat engine

12. Some Definitions • Thermal Efficiency • Index of performance of a device • What you want over what you have to pay for

13. For a heat engine

14. Sign Conventions • When we talk about engines, we generally dispense with our sign conventions • All the energy terms are positive • We determine the direction of flow based on the situation, and label carefully

15. Cyclic heat engines operate between a high temperature source and a low temperature sink

16. For a cyclic heat engine

17. Even the Most Efficient Heat Engines Reject Most Heat as Waste Heat Steam power plants run at about 40% efficiency

19. 5-5 A Heat-Engine Cycle Must Reject Some Heat to a Low-Temperature Sink A heat-engine cycle cannot be completed without rejecting some heat to a low-temperature sink

20. It is impossible for any device that operates on a cycle, to receive heat from a single reservoir and produce a net amount of work.  Kelvin-Planck Statement of the Second Law

21. The Kelvin-Planck statement of the second law of thermodynamics states that no heat engine can produce a net amount of work while exchanging heat with a single reservoir only. In other words, the maximum possible efficiency is less than 100%.

22. Heat always flows spontaneously from hot to cold • Some Mechanical Devices accept work as input, and move heat from low to high temperatures • Heat Pump • Refrigerators and Air Conditioners

23. Heat pumps and refrigerators differ in their intended use. They work the same

24. For a heat pump For a refrigerator or air conditioner

25. COP • We use coefficient of performance (COP) instead of efficiency (h) for heat pumps and refrigerators • COP can be greater than 1 for refrigerators • It is always greater than 1 for heat pumps

26. COP Refrigerators But what is Wnet equal to? Substituting in… Since QH is always greater than QL, COP can be greater than one, but only when QH /QL is less than 2

27. COP Heat Pumps But what is Wnet equal to? Substituting in… Since QL is always less than QH, COP is always greater than 1

28.  It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. Clausius Statement of the Second Law

29. Energy from the surroundings in the form of work or heat has to be expended to force heat to flow from a low-temperature media to a high-temperature media. Thus, the COP of a refrigerator or heat pump must be less than infinity.

30. Perpetual Motion Machines • Any device that violates the first or second law is called a perpetual motion machine • If it violates the first law, it is a perpetual motion machine of the first type (PMM1) • If it violates the second law, it is a perpetual motion machine of the second type (PMM2)

31. Perpetual Motion Machines are not possible

32. Reversible Processes • Heat pumps, refrigerators and heat engines all work best reversibly • Reversible processes don’t have any losses such as • Friction • Unrestrained expansion of gases • Heat transfer through a finite temperature difference • Mixing of two different substances • Any deviation from a quasistatic process

33. Internally Reversible Process The internally reversible process is a quasiequilibrium process, which once having taken place, can be reversed and in so doing leave no change in the system. This says nothing about what happens to the surroundings around the system.

34. Totally or Externally Reversible Process The externally reversible process is a quasiequilibrium process, which once having taken place, can be reversed and in so doing leaves no change in the system or surroundings.

35. All real processes are irreversible!! So why should we worry about reversible processes? Reversible processes represent the best that we can do.

36. Carnot Cycle • Named for French engineer Nicolas Sadi Carnot (1769-1832) • One example of a reversible cycle

37. Nicolas Sadi Carnot • French Physicist • Developed the theory of heat engines http://scienceworld.wolfram.com/biography/CarnotSadi.html

38. Carnot Cycle • Composed of four reversible processes • 2 adiabatic expansion or compression • 2 reversible isothermal heat transfer

39. Carnot Cycle • Process 1-2 Reversible isothermal heat addition at high temperature • Process 2-3 Reversible adiabatic expansion during which the system does work and the working fluid temperature changes from TH to TL • Process 3-4 Reversible isothermal heat exchange takes place while work of compression is done on the system. • Process 4-1 A reversible adiabatic compression process increases the working fluid temperature from TL to TH

40. Process 1-2 Reversible isothermal heat addition at high temperature, TH > TL to the working fluid in a piston-cylinder device which does some boundary work. 1 TH=constant QH 2 Pressure Specific Volume

41. 1 TH=constant QH 2 Pressure 3 Specific Volume Process 2-3 Reversible adiabatic expansion during which the system does work as the working fluid temperature decreases from TH to TL.

42. 1 TH=constant QH 2 Pressure QL 4 3 TL=constant Specific Volume Process 3-4 The system is brought in contact with a heat reservoir at TL < TH and a reversible isothermal heat exchange takes place while work of compression is done on the system.

43. 1 TH=constant QH 2 Pressure QL 4 3 TL=constant Specific Volume Process 4-1 A reversible adiabatic compression process increases the working fluid temperature from TL to TH

44. Execution of the Carnot Cycle in a Closed System Isothermal Isothermal The Carnot cycle is a reversible heat engine Adiabatic Adiabatic The area inside the figure represents the work

45. Reversed Carnot Cycle A reversed Carnot Cycle is a refrigerator or a heat pump

46. Only true for the reversible case Efficiency of a Carnot Engine • For a reversible cycle the amount of heat transferred is proportional to the temperature of the reservoir

47. COP of a Reversible Heat Pump and a Reversible Refrigerator Only true for the reversible case

48. How do Reversible and Real Systems Compare? • The efficiency of a reversible heat engine, such as a Carnot engine, is always higher than a real engine • The COP of a reversible heat pump is always higher than a real heat pump • The COP of a reversible refrigerator is always higher than a real refrigerator

49. Carnot Principle • The thermal efficiency of all reversible heat engines operating between the same two reservoirs is the same • No heat engine is more efficient than a reversible heat engine

50. SummarySecond Law • The second law of thermodynamicsstates that processes occur in a certain direction, not in any direction. • A process will not occur unless it satisfies both the first and the second laws of thermodynamics. • Bodies that can absorb or reject finite amounts of heat isothermally are called thermal energy reservoirs or heat reservoirs.