1. CHAPTER 4
Heat and Work
熱與功

2. 2.1 Introduction
Energy cannot be created or destroyed; it can only change forms (the first law)
Energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. (the second law)

3. 2.2 Forms of Energy Ability to work 功
Capacity to move a force through a distance
The total Energy全能: E , kJ/kg
-Kinetic Energy 動能
-Potential Energy 位能
-Internal energy 內能

4. 2.2 Forms of Energy (1) The macroscopic 巨觀的energy;
A system possesses as a whole with respect to some outside reference frame,i.e. kinetic energy, potential energy.
The microscopic 微觀的energy;
Related to the molecular structure of a system and the degree of the molecular activity, and independent of outside reference frames

5. 2.2 Forms of Energy (2)
Sensible energy顯能: Associated with the kinetic energy of the molecules
Latent energy潛能: Associated with the phase of a system
Chemical energy化學能: Associated with the atomic bonds in a molecule
Nuclear energy核能: Associated with the strong bonds within the nucleus of the atom itself
Thermal energy熱能: The forms of energy associated with temperature

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FIGURE 2–5 The various forms of microscopic energies that make up sensible energy.
1-8

7. 2-3 Energy transfer by Heat
Energy can cross the boundaries of a closed system in the form of heat and work.
Heat : The form of energy that is transferred between two systems(or a system and its surroundings) by virtue of a temperature difference.
-Heat is transferred from hot bodies to colder ones) by virtue of a temperature difference.
-Energy is recognized as heat transfer only as it crosses the system boundary.

8. 2-3 Energy transfer by Heat
Adiabatic Process: A process during which there is no heat transfer.
-During an adiabatic process, a system exchanges no heat with its surroundings.
-Energy is recognized as heat transfer only as it crosses the system boundary.

9. FIGURE 2-11 Specifying the directions of heat and work.
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FIGURE 2-11 Specifying the directions of heat and work.
3-1

10. 2-4 Energy Transfer by Work
Energy can cross the boundaries of a closed system in the form of heat and work.
- If the energy cross the boundaries of a closed is not heat, it must be work.
-Work is the energy transfer associated with a force acting through a distance.

11. WORK

12. Processes 過程
Process line, or path路徑
State 1
State 2 P1 P3 P2

13. Interactions System Surroundings Ma f(Pk, k =1...N)=0 Mass Flow Heat
Work
Heat

14. Mechanical work flow Work Flow The turning fan represents the
Motor
Electrical Power
System
Boundary
Work Flow
The turning fan
represents the
result of a
mechanical work
transfer.

15. Mechanical Work　機械功 m

16. Work at a system boundary...

17. Deformation變形 of the system boundary
Note: Pgas > Pambient
Direction of Motion
Piston Wall
System: A gas
at p,V, and T. x

18. Closed system with boundary deformation
Initial System
State 1
Deformed System
Boundary
State 2 x y z P1 1 2 P3 P2
Process Path

19. Work transfer at a boundary
System
Surroundings
W > 0
W< 0
System Boundary

20. Evaluating work at a boundary...

21. pambient pgas Force balance at the boundary on the x
Note: Pgas > Pambient
Direction of Motion x X
pambient
The gas is the system for analysis.
Force balance at the boundary on the
piston, where the boundary deforms.
pgas

22. X
Pambient
Pgas dx
The net force on
the piston.

23. Total work done

24. pambient pgas > pambient Component of work dueX
Component of work due
to expansion of the gas
Work to raise
the piston

25. Electrical Work
Electrical work電功
-the generalized force is the voltage, the generalized displacement is the electrical charge
We = VI
- V the potential difference
- I the current (number of electrical charges flowing per unit time)

26. 2-5 Mechanical forms of Work
Shaft work軸功
Spring work彈簧功
Other mechanical forms of work

27. The expansion膨脹 and compression 壓縮works
2-6 Moving Boundary Work
The work associated with a moving boundary is called moving boundary work.
The expansion膨脹 and compression 壓縮works

28. Work of Expansion: p-dV work

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FIGURE 4-2 A gas does a differential amount of work dWb as it forces the piston to move by a differential amount ds.
3-2

30. Evaluating an equilibrium expansion process
V = Ax V1 V2 p1 p2

31. Evaluating the work Integral for a quasi-static process
V = Ax V1 V2 p1 p2

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FIGURE 4-3 The area under the process curve on a P-V diagram represents the boundary work.
3-3

33. Moving boundary Work
The area under the process curve on a P-V diagram represents the boundary work.
-The boundary work done during a process depends on the path followed as well as the end states.
-The net work done during a cycle is the difference between the work done by the system and the work done on the system.

34. Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FIGURE 4-5 The net work done during a cycle is the difference between the work done by the system and the work done on the system.
3-4

35. Work at a system boundary
Key concepts and terms
Equilibrium process
Kinetic energy
Path-dependent work
Quasistatic process
Work at a system boundary
Work transfer
Work of expansion

36. PROCESSES INVOLVING IDEAL GASES

37. Polytropic processes...多變過程

38. The polytropic process: PVn=Const.
State 1
State 2

39. Work under the curve p Work done is proportional
State 1
State 2
Work done is proportional
to the area under the curve
for a quasi-static process. The.
process can go either forward
(1-2) or the reverse (2-1)

40. The ideal gas PVn=Const

41. Increment of work done
Area = A x

42. Assumptions Changes in KE and PE are zero Quasi-static process
Polytropic process,
PVn=Const
Ideal gas

43. Increment of work done x A

44. Initial and final states
Area = A x x1 x2

45. Calculation of total work done
From the system perspective, work is done by the
system:

46. Expression for work:
Process equation:

47. Evaluating the integral:
Note that n cannot equal one, which is the general case.

48. For the special case when n = 1:
Isothermal Expansion

49. Polytropic processes p n > 1 V1 V2 V T1 T2
Isothermal Process (n = 1)
n > 1 p1 p2

50. Alternative expressions for W1-2

51. Constant pressure processes...

52. Constant pressure process
Consider as a limiting case of the general polytropic process.
P = Constant
Evaluation of the work integral

53. Constant pressure, constant temperature and polytropic processes:1 2 P V
P = Constant
(n = 0)
Isobaric process
Constant pressure, constant temperature
and polytropic processes:

54. Isothermal process Polytropic process Isobaric process
Key concepts and terms
Isothermal process
Polytropic process
Isobaric process

55. Definition of heat...

56. Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

57. Several phrases which are in common use today such as: heat flow, heat addition, heat rejection, heat removal , heat gain, heat loss, heat storage, heat generation, electrical heating, resistance heating, heat of reaction, specific heat, sensible heat, latent heat, waste heat, body heat, are not consistent with the strict thermodynamic meaning of the term heat, which limits its use to the transfer of thermal energy during a process.
In thermodynamics the term heat simply means heat transfer.

58. A process during which there is no heat transfer is called an adiabatic process.

59. Heat has energy units, kJ or Btu.
The amount of heat transferred during the process between two states is denoted by Q12 or just Q.
Heat transfer per unit mass of a system is denoted q and is determined from

60. The sign for heat is as follows: heat transfer to a system is positive, and heat transfer from a system is negative.
Modes of heat transfer
Heat can be transferred in three different ways: conduction (傳導), convection (對流), and radiation (輻射).

61. Heat: The result of non-adiabatic processes
path, i.e., Q1-2 > 0.
Adiabatic path,
i.e., Q1-2 = 0. P1 P2