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Introduction to Thermodynamics - First Law Essentials

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116 diapositives · 1.3K Views · Jan 02, 2020

Explore the fundamental concepts and applications of the first law of thermodynamics, including work, heat transfer, and energy equations. Learn about heat measurement units and ideal gas processes. Gain insights into energy transfer mechanisms through work and heat interactions. Enhance your understanding of thermal behavior and energy conservation principles.

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  1. Chapter 17 Lecture

  2. Chapter 17 Work, Heat, and the First Law of Thermodynamics Chapter Goal: To develop and apply the first law of thermodynamics. Slide 17-2

  3. Chapter 17 Preview Slide 17-3

  4. Chapter 17 Preview Slide 17-4

  5. Chapter 17 Preview Slide 17-5

  6. Chapter 17 Preview Slide 17-6

  7. Chapter 17 Preview Slide 17-7

  8. Chapter 17 Reading Quiz Slide 17-8

  9. Reading Question 17.1 What quantities appear in the first law of thermodynamics? • Force, mass, acceleration. • Inertia, torque, angular momentum. • Work, heat, thermal energy. • Work, heat, entropy. • Enthalpy, entropy, heat. Slide 17-9

  10. Reading Question 17.1 What quantities appear in the first law of thermodynamics? • Force, mass, acceleration. • Inertia, torque, angular momentum. • Work, heat, thermal energy. • Work, heat, entropy. • Enthalpy, entropy, heat. Slide 17-10

  11. Reading Question 17.2 What was the original unit for measuring heat? • BTU. • Watt. • Joule. • Pascal. • Calorie. Slide 17-11

  12. Reading Question 17.2 What was the original unit for measuring heat? • BTU. • Watt. • Joule. • Pascal. • Calorie. Slide 17-12

  13. Reading Question 17.3 What is the name of an ideal-gas process in which no heat is transferred? • Isochoric. • Isentropic. • Isothermal. • Isobaric. • Adiabatic. Slide 17-13

  14. Reading Question 17.3 What is the name of an ideal-gas process in which no heat is transferred? • Isochoric. • Isentropic. • Isothermal. • Isobaric. • Adiabatic. Slide 17-14

  15. Reading Question 17.4 Heat is • The amount of thermal energy in an object. • The energy that moves from a hotter object  to a colder object. • A fluid-like substance that flows from a  hotter object to a colder object. • Both A and B. • Both B and C. Slide 17-15

  16. Reading Question 17.4 Heat is • The amount of thermal energy in an object. • The energy that moves from a hotter object  to a colder object. • A fluid-like substance that flows from a  hotter object to a colder object. • Both A and B. • Both B and C. Slide 17-16

  17. Reading Question 17.5 The thermal behavior of water is characterized by the value of its • Heat density. • Heat constant. • Specific heat. • Thermal index. Slide 17-17

  18. Reading Question 17.5 The thermal behavior of water is characterized by the value of its • Heat density. • Heat constant. • Specific heat. • Thermal index. Slide 17-18

  19. Chapter 17 Content, Examples, and QuickCheck Questions Slide 17-19

  20. Energy Review • The total energy of a system consists of the macroscopic energy plus the microscopic thermal energy. • The final energy statement of Chapter 11 was: The total energy of an isolated system, for which Wext= 0, is constant. Slide 17-20

  21. Energy Transfer by Work • Doing work on a system increases its energy. • Consider lifting a block with a rope at a steady speed. • The rope’s tension is an external force doing work Wext. • Energy transferred into the system goes entirely into the macroscopic potential energy Ugrav. Slide 17-21

  22. Energy Transfer by Work • Doing work on a system increases its energy. • Consider dragging a block with a rope at a steady speed. • The rope’s tension is an external force doing work Wext. • Energy transferred into the system goes entirely into the thermal energy of the object + surface system Eth. Slide 17-22

  23. Energy Transfer by Heat • Work is energy transferred in a mechanical interaction. • Energy can also be transferred between the system and the environment if they have a thermal interaction. • The energy transferred in a thermal interaction is called heat. • The symbol for heat is Q. • The complete energy equation is now: Slide 17-23

  24. QuickCheck 17.1 A steady force pushes in the piston of a well-insulated cylinder. In this process, the temperature of the gas Increases. Stays the same. Decreases. There’s not enough information to tell. Slide 17-24

  25. No heat flows (well insulated) ... ... but work is done on the gas. First law: Q + W = ΔEth Work increases the gas’s thermal energy and with it the temperature. QuickCheck 17.1 A steady force pushes in the piston of a well-insulated cylinder. In this process, the temperature of the gas Increases. Stays the same. Decreases. There’s not enough information to tell. Slide 17-25

  26. Work in Ideal-Gas Processes • Consider a gas cylinder sealed at one end by a movable piston. • The external force does work on the gas as the piston moves. Slide 17-26

  27. The Sign of Work Slide 17-27

  28. Work in Ideal-Gas Processes • On a pV diagram, the work done on a gas W has a nice geometric interpretation. • W = the negative of the area under the pV curve between Viand Vf. Slide 17-28

  29. QuickCheck 17.2 The work done on the gas in this process is 8000 J. 4000 J. 0 J. –4000 J. –8000 J. Slide 17-29

  30. QuickCheck 17.2 The work done on the gas in this process is 8000 J. 4000 J. 0 J. –4000 J. –8000 J. W = –(area under pV curve) Slide 17-30

  31. QuickCheck 17.3 Three possible processes A, B, and C take a gas from state i to state f. For which process is the magnitude of the work the largest? Process A. Process B. Process C. The work is the same for all three. Slide 17-31

  32. QuickCheck 17.3 Three possible processes A, B, and C take a gas from state i to state f. For which process is the magnitude of the work the largest? Largest area under the curve. Process A. Process B. Process C. The work is the same for all three. Slide 17-32

  33. Problem-Solving Strategy: Work in a Ideal-Gas Process Slide 17-33

  34. Example 17.1 The Work Done on an Expanding Gas Slide 17-34

  35. Example 17.1 The Work Done on an Expanding Gas Slide 17-35

  36. Example 17.1 The Work Done on an Expanding Gas Slide 17-36

  37. Work Done on an Ideal Gas In an isochoric process, when the volume does not change, no work is done. Slide 17-37

  38. Work Done on an Ideal Gas In an isobaric process, when pressure is a constant and the volume changes by V = Vf−Vi, the work done during the process is: Slide 17-38

  39. Work Done on an Ideal Gas In an isothermal process, when temperature is a constant, the work done during the process is: Slide 17-39

  40. Example 17.2 The Work of an Isothermal Compression Slide 17-40

  41. QuickCheck 17.4 Dragging an object across a rough surface makes it warm, or even hot. The temperature increase occurs because of Work. Heat. Thermal energy. Both work and heat. None of these. Slide 17-41

  42. QuickCheck 17.4 Dragging an object across a rough surface makes it warm, or even hot. The temperature increase occurs because of Work. Heat. Thermal energy. Both work and heat. None of these. Slide 17-42

  43. Work in Ideal-Gas Processes • Figure (a) shows two different processes that take a gas from an initial state i to a final state f. • The work done during an ideal-gas process depends on the path followed through the pVdiagram. • During the multistep process of figure (b), the work done is not the same as a process that goes directly from 1 to 3. Slide 17-43

  44. Heat In the 1840s James Joule showed that heat and work, previously regarded as completely different phenomena, are simply two different ways of transferring energy to or from a system. Slide 17-44

  45. Heat, Temperature, and Thermal Energy • Thermal energyEth is an energy of the system due to the motion of its atoms and molecules. • HeatQis energy transferred between the system and the  environment as they interact. • TemperatureT is a state variable that quantifies the “hotness” or “coldness” of a system. A temperature difference is required in order for heat to be transferred between the system and the environment. Slide 17-45

  46. The Sign of Heat Slide 17-46

  47. Understanding Work and Heat Slide 17-47

  48. Units of Heat • The SI unit of heat is the Joule. • Historically, a unit for measuring heat, the calorie, had been defined as: 1 calorie = 1 cal = the quantity of heat needed to change the temperature of 1 g of water by 1°C • In today’s SI units, the conversion is: 1 cal = 4.186 J • The calorie you know in relation to food is not the same as the heat calorie. 1 food calorie = 1 Cal = 1000 cal = 1 kcal = 4186 J Slide 17-49

  49. The First Law of Thermodynamics Work and heat are two ways of transferring energy between a system and the environment, causing the system’s energy to change. If the system as a whole is at rest, so that the bulk mechanical energy due to translational or rotational motion is zero, then the conservation of energy equation is: Slide 17-49

  50. QuickCheck 17.5 A cylinder of gas has a frictionless but tightly sealed piston of mass M. Small masses are placed onto the top of the piston, causing it to slowly move downward. A water bath keeps the temperature constant. In this process: Q > 0. Q = 0. Q < 0. There’s not enough information to say anything about the heat. Slide 17-50