Lecture Outline Chapter 12 College Physics, 7 th Edition Wilson / Buffa / Lou

1. Lecture Outline Chapter 12 College Physics, 7th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc.

2. Units of Chapter 12 Thermodynamic Systems, States, and Processes The First Law of Thermodynamics Thermodynamic Processes for an Ideal Gas The Second Law of Thermodynamics and Entropy Heat Engines and Thermal Pumps The Carnot Cycle and Ideal Heat Engines © 2010 Pearson Education, Inc.

3. 12.1 Thermodynamic Systems, States, and Processes What is thermodynamics? Systems and Surroundings If heat transfer is impossible, the system is thermally isolated. Work may be done on a thermally isolated system. (Work is a way to transfer energy) A heat reservoir transfers heat into or out of a system that is not thermally isolated. Its assumed a heat reservoir will have an unlimited heat capacity. © 2010 Pearson Education, Inc.

4. 12.1 Thermodynamic Systems, States, and Processes A thermodynamic system is described by an equation of state, such as the ideal gas law. We think about systems in terms of state variables. These include: The “location” of the state can be plotted on a p–V diagram, as at left. © 2010 Pearson Education, Inc.

5. 12.1 Thermodynamic Systems, States, and Processes A process changes the state of a system. For example… A process may be reversible (3–4) or irreversible (1–2); if it proceeds through a sequence of equilibrium states it is reversible. © 2010 Pearson Education, Inc.

6. 12.2 The First Law of Thermodynamics The first law is a statement of conservation of energy in thermodynamic processes: Here, Q is the heat transferred, ΔU is the change in internal energy, and W is the work. © 2010 Pearson Education, Inc.

7. 12.2 The First Law of Thermodynamics It is important to get the signs of Q, ΔU, and W correct. The diagram below illustrates the sign conventions. © 2010 Pearson Education, Inc.

8. 12.2 The First Law of Thermodynamics • A 65 kg worker shovels coal for 3.0 hours. During the shoveling, the worker did work at an average rate of 20 Watts and lost heat to the environment at an average rate of 480 Watts. Ignoring the loss of water by the evaporation of perspiration from his skin, how much fat will the worker lose? The energy value of fat (Ef) is 9.3 kcal/g.

9. 12.2 The First Law of Thermodynamics It can be shown that the work done in expanding or through contraction of a gas is given by: Work will be positive when the gas expands, and it will be negative when the gas contracts. © 2010 Pearson Education, Inc.

10. 12.2 The First Law of Thermodynamics Generalizing, we see that: The work done by a system is equal to the area under the process curve on a p–V diagram. © 2010 Pearson Education, Inc.

11. 12.2 The First Law of Thermodynamics The heat transferred and the work done both depend on the path taken from state 1 to state 2; the change in internal energy depends only on the end points. © 2010 Pearson Education, Inc.

12. 12.3 Thermodynamic Processes for an Ideal Gas An isothermal process is one in which the temperature does not change. Why did the internal energy drop out of this? © 2010 Pearson Education, Inc.

13. 12.3 Thermodynamic Processes for an Ideal Gas The work done by an isothermal systems is: Be aware of this, I don’t see it on your FRQ formula papers, and I don’t think they will have you calculating natural logs on your multiple choice. © 2010 Pearson Education, Inc.

14. 12.3 Thermodynamic Processes for an Ideal Gas An isobaric process is one in which the pressure does not change. The work done is: © 2010 Pearson Education, Inc.

15. 12.3 Thermodynamic Processes for an Ideal Gas • Two moles of a monatomic ideal gas, initially at 0 degrees Celsius and 1 atm, are expanded to twice their original volume, using 2 different processes. They are expanded isothermally or isobarically, both starting in the same initial state. • A.) Does the gas do more work in the isothermal system, the isobaric system, or are they the same? Why? • B.) To prove your answer, determine the work done in each process.

16. 12.3 Thermodynamic Processes for an Ideal Gas An isometric process is one in which the volume is constant. In this case, no work is done, so © 2010 Pearson Education, Inc.

17. 12.3 Thermodynamic Processes for an Ideal Gas An adiabatic process is one where there is no heat transfer. In this case, © 2010 Pearson Education, Inc.

18. 12.3 Thermodynamic Processes for an Ideal Gas © 2010 Pearson Education, Inc.

19. 12.4 The Second Law of Thermodynamics and Entropy The second law of thermodynamics: Heat will not flow spontaneously from a colder body to a warmer body. If we left liquid water out in the middle of a room, would it suddenly turn to ice by itself? © 2010 Pearson Education, Inc.

20. 12.4 The Second Law of Thermodynamics and Entropy • Another statement of the second law of thermodynamics: • In a thermal cycle, heat energy cannot be completely transformed into mechanical work. • What is a thermal cycle? • Nothing is 100% efficient..some heat energy will always be lost. © 2010 Pearson Education, Inc.

21. 12.4 The Second Law of Thermodynamics and Entropy Entropy: Is a measure of a system’s ability to do useful work Is a measure of disorder Is increasing in the universe © 2010 Pearson Education, Inc.

22. 12.4 The Second Law of Thermodynamics and Entropy We cannot measure entropy directly, only changes in it. Units are J/K The sign convention for entropy is positive if the system absorbs heat, it is negative when the system loses heat. © 2010 Pearson Education, Inc.

23. 12.4 The Second Law of Thermodynamics and Entropy • While doing physical exercise at a temperature of 34 degrees Celsius, an athlete loses 0.400 kg of water per hour by the evaporation of perspiration from his skin. Estimate the change in entropy of the water as it vaporizes. The latent heat of vaporization of perspiration is about 24.2 x 105 J/kg.

24. 12.4 The Second Law of Thermodynamics and Entropy • A metal spoon at 24 degrees Celsius is immersed in 1 kg of water at 18 degrees Celsius. The system (spoon and water) is thermally isolated and comes to equilibrium at a temperature of 20 degrees Celsius. Find the change in the entropy of the system.

25. 12.4 The Second Law of Thermodynamics and Entropy The universe is the ultimate isolated system; therefore, the total energy of the universe increases during every natural process. All naturally occurring processes move toward a state of greater disorder. Larger entropy = more disorder It is a lot easier to break things than it is to put them back together again! © 2010 Pearson Education, Inc.

26. 12.5 Heat Engines and Thermal Pumps A heat engine converts heat energy into work. According to the second law of thermodynamics, however, it cannot convert *all* of the heat energy supplied to it into work. Basic heat engine: hot reservoir, cold reservoir, and a machine to convert heat energy into work. Thermal Cycle . . . . . . © 2010 Pearson Education, Inc.

27. 12.5 Heat Engines and Thermal Pumps This is a simplified diagram of a heat engine, along with its thermal cycle. © 2010 Pearson Education, Inc.

28. 12.5 Heat Engines and Thermal Pumps Thermal efficiency of a heat engine: Tells us how much useful work an engine does For an ideal gas heat engine Wnet = Qh - Qc © 2010 Pearson Education, Inc.

29. 12.5 Heat Engines and Thermal Pumps • The small, gasoline-powered engine of a leaf blower absorbs 800J of heat energy from a high temperature reservoir and exhausts 700 J to a low temperature reservoir. What is the engine’s thermal efficiency?

30. 12.5 Heat Engines and Thermal Pumps • A gasoline powered water pump can pump 7.6 x 103 kg of water from a basement floor to the ground outside the house in each hour while consuming 1.3 x 106 J of gasoline. Assume the basement floor is 3.0 m below the ground. • What is the thermal efficiency of the water pump? • How much heat is wasted to the environment in 1 hour?

31. 12.5 Heat Engines and Thermal Pumps A thermal pump is the opposite of a heat engine: it transfers heat energy from a cold reservoir to a hot one. As this will not happen spontaneously, work must be done on the pump as well. Examples of thermal pumps: refrigerator, air conditioner © 2010 Pearson Education, Inc.

32. 12.5 Heat Engines and Thermal Pumps An air conditioner removes heat from the house and exhausts it outside, where it is hotter. © 2010 Pearson Education, Inc.

33. 12.5 Heat Engines and Thermal Pumps The purpose of a refrigerator or air conditioner is to keep a cool place cool; we describe such a device by its coefficient of performance (COP). © 2010 Pearson Education, Inc.

34. 12.6 The Carnot Cycle and Ideal Heat Engines The Carnot cycle consists of two isotherms and two adiabats. © 2010 Pearson Education, Inc.

35. 12.6 The Carnot Cycle and Ideal Heat Engines An ideal Carnot engine—with perfect isotherms and adiabats—has the maximum efficiency a heat engine can have. This leads to the third law of thermodynamics: It is impossible to reach absolute zero in a finite number of thermal processes. © 2010 Pearson Education, Inc.

36. 12.6 The Carnot Cycle and Ideal Heat Engines • An engineer is designing a cyclic heat engine to operate between the temperatures of 150 degrees Celsius and 27 degrees Celsius. • What is the maximum theoretical efficiency that can be achieved? • Suppose the engine when built does 1000J of work per cycle for every 5000J of input heat per cycle. What is its efficiency and how close is it to the Carnot efficiency?