2. Processes proceed spontaneously in certain directions, but reverse is not automatically attainable in real life even though the reversal of the process does not violate the first law.

3. First law provides a necessary but not sufficient condition for a process to occur.

6. Macroscopic kinetic energy: Consider a fluid element of mass m having the mass velocity V (as shown in figure). The macroscopic kinetic energy EK of the fluid element by virtue of its motion is given by

7. EK=mV22

8. Macroscopic potential energy: If the elevation of the fluid element from an arbitrary datum is z, then the macroscopic potential energy EP by virtue of its position is given by

9. EP=mgz

10. where g is acceleration due to gravity.

11. Microscopic energy mode : It refers to the energy stored in the molecular and atomic structure of the system, which is called the molecular internal energy or internal energy, it is denoted by U.

12. Matter is composed of molecules. Molecules are in random thermal motion (for a gas) with some average velocity, v, constantly colliding with each other and the walls (as shown in figure). Due to collision, the molecules may be subjected to rotation as well as vibration. They can have translation kinetic energy, rotational kinetic energy, vibrational energy, electronic energy, chemical energy and nuclear energy. If ε represents the energy of one molecule, then

13. ε=εtran+εrot+εvib+εchem+εelect+εnuclear

14. If N is the total number of molecules in the system, then the total internal energy is given by

15. U=Nε

16. In an ideal gas there are no intermediate forces of attraction and repulsion, and the internal energy depends only on temperature. Thus,

17. U=f(T)

18. for an ideal gas.

19. Other forms of energy which can be possessed by a system are magnetic energy, electrical energy and surface tension energy. In the absence of these forms, the total energy E of a system is given by

20. E= Emacro+Emicro=EK+EP+U

21. In the absence of motion and gravity; EK = 0, EP = 0

22. ->E=U

23. From the first law of thermodynamics,

24. Q=∆E+W ......(1)

25. Where Q is heat transfer,

26. W = work transfer

27. Since E=U

28. The first law of thermodynamics reduces to

29. Q=∆U+W .....(2)

30. U is an extensive property of the system and specific internal u (=U/m) is intensive.

31. Equation (1) and (2) in differential form can be written as

32. dQ=dE+dW ......(3)

33. dQ=dU+dW ......(4)

34. where dW=dWpdV + dWshaft + dWelectrical +...................

35. considering the different forms of work transfer which may be present. When only pdV work is present, the equations (3) and (4) becomes

36. dQ=dE+pdV ......(5)

37. dQ=dU+pdV ......(6)

38. On integrating

39. Q=∆E+pdV ......(7)

40. Q=∆U+pdV ......(8)

41. ENTHALPY

42. The enthalpy of a substance, h, is defined as

43. h=u+pv

44. where u is specific internal energy, kJ/kg.

45. p is pressure, KPa.

46. v is specific volume, m3/kg.

47. It is an extensive property of the system (kJ/kg).

48. Internal energy change is equal to the heat transferred in a constant volume process involving no work other than pdV work. In such a process in a closed stationary system of unit mass of a pure susbstance

49. dQ=du+pdv

50. where Q = Heat transfer, kJ

51. At constant pressure

52. pdv=d(pv)

54. The fluid is uniform in state, composition and velocity both at entrance and exit of the control volume.

55. The heat and work interactions at the control surface are at a constant rate.

56. The state of fluid at any point is same at all times.

57. No change in chemical composition of the fluid within the control volume; change in the chemical energy is not involved.Flow process is unsteady when the conditions vary with respect to time. Unsteadiness refers to changing parameters with the passage of time at a position in the control volume. Symbolically, ∂P∂t≠0 defines unsteady flow process.<br />REVERSIBLE AND IRREVERSIBLE PROCESSES<br /> A reversible process is defined as a process that can be reversed without leaving any trace on the surroundings (Fig. 6–30). That is, both the system and the surroundings are returned to their initial states at the end of the reverse process. This is possible only if the net heat and net work exchange between the system and the surroundings is zero for the combined (original and reverse) process. Processes that are not reversible are called irreversible processes.<br />29051251104265 It should be pointed out that a system can be restored to its initial state following a process, regardless of whether the process is reversible or irreversible. But for reversible processes, this restoration is made without leaving any net change on the surroundings, whereas for irreversible processes, the surroundings usually do some work on the system and therefore does not return to their original state. <br /> Reversible processes actually do not occur in nature. They are merely idealizations of actual processes. Reversible processes can be approximated by actual devices, but they can never be achieved. That is, all the processes occurring in nature are irreversible. But, there are some reasons because of which we study reversible process. There are two reasons. First, they are easy to analyze, since a system passes through a series of equilibrium states during a reversible process; second, they serve as idealized models to which actual processes can be compared.<br />Engineers are interested in reversible processes because work-producing devices such as car engines and gas or steam turbines deliver the most work, and work-consuming devices such as compressors, fans, and pumps consume the least work when reversible processes are used instead of irreversible ones.<br /> Reversible processes can be viewed as theoretical limits for the corresponding irreversible ones. Some processes are more irreversible than others. We may never be able to have a reversible process, but we can certainly approach it. The more closely we approximate a reversible process, the more work delivered by a work-producing device or the less work required by a work-consuming device. <br />