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  1. 1. Thermodynamics Chapter 4 Work and Heat 國立成功大學工程科學系
  2. 2. Work and heat are energy transfer from onesystem to another and thus play a crucial rolein most thermodynamic system on devices. Aswe want to analyze such systems, we need tomodel heat and work as functions of propertiesand parameters characteristic of the system orhow they function. 國立成功大學工程科學系
  3. 3. 4.1Definition of Work 國立成功大學工程科學系
  4. 4. In elementary mechanics, the work done by aforce F on a body displaced a distance inthe direction of the force is:However, when treating thermodynamicsfrom a macroscopic point of view, it isadvantageous to tie in the definition of workwith the concepts of system, properties, andprocesses. 國立成功大學工程科學系
  5. 5. Work is done by a system if the sole effect on the surrounding could be the raising of a weight. Work done by a system is positive; energy leaves the system. Work done on a system is negative; energy is added to the system. In general, work is a form of energy in transit, that is , energy being transferred across a system boundary. 國立成功大學工程科學系
  6. 6. Figure 4.1 Example of work crossing the boundary of a system. 國立成功大學工程科學系
  7. 7. Figure 4.2 Example of work crossing the boundary of a systembecause of a flow of an electric current across the system boundary. 國立成功大學工程科學系
  8. 8. 4.2Unit for Work 國立成功大學工程科學系
  9. 9. Raising of a weight Work Power 國立成功大學工程科學系
  10. 10.  Shaft workFigure 4.3 Forceacting Specific workat radius r gives atorque T = Fr. 國立成功大學工程科學系
  11. 11. 4.3Work Done at the Moving Boundary of a Simple Compressible System. 國立成功大學工程科學系
  12. 12. Considering Fig. 4.4 as a quasi-equilibrium process,Figure 4.4 Example of work done at the moving boundary of asystem in a quasi-equilibrium process. 國立成功大學工程科學系
  13. 13.  Work done on the system during the process from state 1 to state 2  (∵ on ∴ is negative .)Figure 4.5 Use of pressure-volume diagram to show work done atthe moving boundary of a system in a quasi-equilibrium process. 國立成功大學工程科學系
  14. 14. Why the work done during each process is a path function (inexact differentials) not a point function (exact differentials)?Figure 4.6 Various quasi-equilibrium processes between two givenstates, indicating that work is a path function. 國立成功大學工程科學系
  15. 15.  Remarks: A work done during a process depends not only on the end states of the process but also on its path (e.g. the path from states “1” to “2” in Fig. 4.6) too.
  16. 16. A: The area underneath each curve in Fig. 4.6 represents the work for each process. The differentials of point functions are exact differentials. Thermodynamic properties are, of course, point functions. The differential of path functions are inexact differentials 國立成功大學工程科學系
  17. 17. Two classes of problems: 1. on graphical form (e.g. experiments). Evaluate by graphical or numerical integration. 2. Analytical relations between P and V, e.g., polytrophic process: . 國立成功大學工程科學系
  18. 18. Remarks: Eqs. (4.5) or (4.6) are mathematicalresults, because there are cases in which work isnot given by Eq. (4.4)! -- see Ex. 4.5. 國立成功大學工程科學系
  19. 19. Students read examples 4.1~4.5(Students read examples 3.1~3.7) -- recall 國立成功大學工程科學系
  20. 20. 4.4Other Systems That Involve Work 國立成功大學工程科學系
  21. 21.  Consider as a system a stretched wire that is under a given tension. The work done by the system when the length of the wire is changed by an amount :The minus sign above is due to work done by the system when is negative See Example 4.6 Electric power is volts time ampere. (Electric) power See P=Vi H.W#4.63, P88 國立成功大學工程科學系
  22. 22. 4.5Concluding Remarks Regarding Work 國立成功大學工程科學系
  23. 23.  Work, in general, is given by the integral of the product of an intensive property (e.g. P) and the change of an extensive property (e.g. ) 國立成功大學工程科學系
  24. 24.  The identification of work in a process is an important aspect of many thermodynamic problems.
  25. 25.  Q1: What is the work of the system comprising of both gas and vacuum shown below? A: No work done because no work can be identified at the system boundary.Figure 4.17 Example of process involving a change of volume forwhich the work is zero. 國立成功大學工程科學系
  26. 26.  Q2: Work of the system comprising of the gas alone? A: No work done in this processes of filling the vacuum, because 1. there is no resistance at the system boundary as V↑ 2. this is not a quasi-eq’m process, and therefore the work can not be calculated from . 國立成功大學工程科學系
  27. 27. 4.6Definition of Heat 國立成功大學工程科學系
  28. 28. Heat is defined as the form of energy that istransferred across the boundary of a system ata given temperature to another system (or thesurroundings) at a lower temperature by virtueof the temperature difference between thesetwo systems. 國立成功大學工程科學系
  29. 29. Another aspect of this definition of heat is that abody never contains heat. Rather, heat can beidentified only as it crosses the boundary.Thus, heat is a transient phenomenon. Heat, likework, is a form of energy transfer to (positive)or from (negative) a system. The unit for it isthe same as that for work.SI unit for heat is Joule. 國立成功大學工程科學系
  30. 30. From a mathematical perspective, heat is apath function, and is an inexact differential : heat transferred during the process from“1” to state “2”. Later, e.g. in Fig. 8.8 & 8.9 the rate of heat transfer specific heat transfer 國立成功大學工程科學系
  31. 31. 4.7Heat Transfer Modes 國立成功大學工程科學系
  32. 32.  Conduction: energy exchange between molecules. Energy is given out by molecules having more (energy) in the average (high temperature) to those having less (energy) in the average (low temperature). This is Fourier law of conduction. The minus sign gives the direction of the heat transfer from a higher temperature to a low temperature. 國立成功大學工程科學系
  33. 33.  Convection: the transfer of energy between a solid surface and the adjacent flowing fluid. This is the Newton’s law of cooling. 國立成功大學工程科學系
  34. 34.  Radiation: the transfer of energy due to the emission of electromagnetic waves in space (do not require any substances in space) 國立成功大學工程科學系
  35. 35. 4.8Comparisons of Heat and Work 國立成功大學工程科學系
  36. 36. Students read 4.8 and Summary 國立成功大學工程科學系
  37. 37. Figure 4.20 An example showing the difference betweenheat and work. 國立成功大學工程科學系
  38. 38. Figure 4.7 Sketch for Example 4.1.Chapter 4. Work and Heat.
  39. 39. Figure 4.8 Pressure-volume diagram showing work done in thevarious processes of Example 4.1. Chapter 4. Work and Heat.
  40. 40. Figure 4.9 Sketch of physical system for Example 4.2.Chapter 4. Work and Heat.
  41. 41. Figure 4.12 Sketch for Example 4.4. Chapter 4. Work and Heat.
  42. 42. Figure 4.13 Sketch for Example 4.4. Chapter 4. Work and Heat.
  43. 43. Figure 4.14 Example of a nonequilibrium process. Chapter 4. Work and Heat.
  44. 44. Figure 4.18 Example showing how selection of the systemdetermines whether work is involved in a process. Chapter 4. Work and Heat.
  45. 45. Figure 4.20 An example showing the difference betweenheat and work. Chapter 4. Work and Heat.
  46. 46. Figure 4.21 The effects of heat addition to a control volume thatalso can give out work. Chapter 4. Work and Heat.
  47. 47. 【Homework Problems】 Figure P4.56 Chapter 4. Work and Heat.
  48. 48. Figure 3.1 Constant-pressure change from liquid to vapor phasefor a pure substance. Chapter 3. Properties of a Pure Substance.
  49. 49. Figure 3.5 Pressure-temperature diagram for a substance suchas water. Chapter 3. Properties of a Pure Substance.
  50. 50. What processes (or cycles?)什 p=c麼 V=c ? T=c過程 Q=0? 也可能 Q ≠0
  51. 51. 看懂題目的初始條件與過程 (變化) 條件, 配合查表或應用物質性質變化公式可知道物質(工作流體)的性質及其變化。對第四章言,主要計算功及工作流體的狀態變化。第五章則是應用1st Law…求(其中一個是第四章沒有計算的H.T.)系統之功、熱傳或性質的變化。