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Chapter Eleven

Chapter Eleven. Energy in Thermal Processes. Read and take notes on pages : 352-353. Read and take notes on pages : 366-367. Energy Transfer.

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Chapter Eleven

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  1. Chapter Eleven Energy in Thermal Processes

  2. Read and take notes on pages: 352-353

  3. Read and take notes on pages: 366-367

  4. Energy Transfer • When two objects of different temperatures are placed in thermal contact, the temperature of the warmer decreases and the temperature of the cooler increases • The energy exchange ceases when the objects reach thermal equilibrium • The concept of energy was broadened from just mechanical to include internal • Made Conservation of Energy a universal law of nature Introduction

  5. Heat Compared to Internal Energy • Important to distinguish between them • They are not interchangeable • Heat involves a transfer of energy Section 11.1

  6. Internal Energy • Internal Energy, U, is the energy associated with the atoms and molecules of the system • Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the particles that make up the system • Also includes any potential energy bonding the particles together Section 11.1

  7. Link to Bright storm on Heat

  8. Heat • Heat is the transfer of energy between a system and its environment because of a temperature difference between them • The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment Section 11.1

  9. Units of Heat • Calorie • An historical unit, before the connection between thermodynamics and mechanics was recognized • A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C . • A Calorie (food calorie) is 1000 cal Section 11.1

  10. Units of Heat, cont. • US Customary Unit – BTU • BTU stands for British Thermal Unit • A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F • 1 cal = 4.186 J • This is called the Mechanical Equivalent of Heat Section 11.1

  11. James Prescott Joule • 1818 – 1889 • British physicist • Conservation of Energy • Relationship between heat and other forms of energy transfer Section 11.1

  12. Link to Bright storm on Heat Transfer

  13. Methods of Heat Transfer • Need to know the rate at which energy is transferred • Need to know the mechanisms responsible for the transfer • Methods include • Conduction • Convection • Radiation Section 11.5

  14. Conduction • The transfer can be viewed on an atomic scale • It is an exchange of energy between microscopic particles by collisions • Less energetic particles gain energy during collisions with more energetic particles • Rate of conduction depends upon the characteristics of the substance Section 11.5

  15. Conduction example • The molecules vibrate about their equilibrium positions • Particles near the stove coil vibrate with larger amplitudes • These collide with adjacent molecules and transfer some energy • Eventually, the energy travels entirely through the pan and its handle Section 11.5

  16. Conduction, cont. • The rate of conduction depends on the properties of the substance • In general, metals are good conductors • They contain large numbers of electrons that are relatively free to move through the metal • They can transport energy from one region to another • Conduction can occur only if there is a difference in temperature between two parts of the conducting medium Section 11.5

  17. Conduction, equation • The slab of material allows energy to transfer from the region of higher temperature to the region of lower temperature • A is the cross-sectional area Section 11.5

  18. Conduction, equation explanation • A is the cross-sectional area • Through a rod, Δx = L • P is in Watts when Q is in Joules and t is in seconds • k is the thermal conductivity of the material • See table 11.3 for some conductivities • Good conductors have high k values and good insulators have low k values Section 11.5

  19. Read and take notes on page: 368

  20. Read and take notes on pages: 371-374

  21. Convection • Energy transferred by the movement of a substance • When the movement results from differences in density, it is called natural convection • When the movement is forced by a fan or a pump, it is called forced convection Section 11.5

  22. Convection example • Air directly above the flame is warmed and expands • The density of the air decreases, and it rises • The mass of air warms the hand as it moves by Section 11.5

  23. Convection applications • Boiling water • Radiators • Upwelling • Cooling automobile engines • Algal blooms in ponds and lakes Section 11.5

  24. Convection Current Example • The radiator warms the air in the lower region of the room • The warm air is less dense, so it rises to the ceiling • The denser, cooler air sinks • A continuous air current pattern is set up as shown Section 11.5

  25. Radiation • Radiation does not require physical contact • All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules • Rate of radiation is given by Stefan’s Law Section 11.5

  26. Radiation example • The electromagnetic waves carry the energy from the fire to the hands • No physical contact is necessary • Cannot be accounted for by conduction or convection Section 11.5

  27. Radiation equation • P = σ A e T4 • The power is the rate of energy transfer, in Watts • σ = 5.669 6 x 10-8 W/m2.K4 • Called the Stefan-Boltzmann constant • A is the surface area of the object • e is a constant called the emissivity • e varies from 0 to 1 • T is the temperature in Kelvins Section 11.5

  28. Energy Absorption and Emission by Radiation • The rate at which the object at temperature T with surroundings at To radiates is • Pnet = σ A e (T4 - To4) • When an object is in equilibrium with its surroundings, it radiates and absorbs at the same rate • Its temperature will not change Section 11.5

  29. Ideal Absorbers • An ideal absorber is defined as an object that absorbs all of the energy incident on it • e = 1 • This type of object is called a black body • An ideal absorber is also an ideal radiator of energy Section 11.5

  30. Ideal Reflector • An ideal reflector absorbs none of the energy incident on it • e = 0 Section 11.5

  31. Applications of Radiation • Clothing • Black fabric acts as a good absorber • White fabric is a better reflector • Thermography • The image of the pattern formed by varying radiation levels is called a thermogram • Body temperature • Radiation thermometer measures the intensity of the infrared radiation from the eardrum Section 11.5

  32. Resisting Energy Transfer • Dewar flask/thermos bottle • Designed to minimize energy transfer to surroundings • Space between walls is evacuated to minimize conduction and convection • Silvered surface minimizes radiation • Neck size is reduced Section 11.5

  33. Global Warming • Greenhouse example • Visible light is absorbed and re-emitted as infrared radiation • Convection currents are inhibited by the glass • Earth’s atmosphere is also a good transmitter of visible light and a good absorber of infrared radiation Section 11.6

  34. Chapter Twelve The Laws of Thermodynamics

  35. Read and take notes on pages: 385-387

  36. First Law of Thermodynamics • The First Law of Thermodynamics tells us that the internal energy of a system can be increased by • Adding energy to the system • Doing work on the system • There are many processes through which these could be accomplished • As long as energy is conserved Introduction

  37. Video # 1 First Law of Thermodynamics & Internal Energy Khan Academy The next four videos are long but a MUST WATCH!!!!!! The concepts being covered I find to be very difficult, these videos when watched together, do a great job explaining the first law of Thermodynamics

  38. Video # 2 More on Internal Energy (Khan Academy)

  39. Video # 3 Work from Expansion (Khan Academy)

  40. Video # 4 PV Diagrams and Expansion Work (Khan Academy)

  41. Second Law of Thermodynamics • Constrains the First Law • Establishes which processes actually occur • Heat engines are an important application Introduction

  42. Work in Thermodynamic Processes – Assumptions • Dealing with a gas • Assumed to be in thermodynamic equilibrium • Every part of the gas is at the same temperature • Every part of the gas is at the same pressure • Ideal gas law applies Section 12.1

  43. Work in a Gas Cylinder • The gas is contained in a cylinder with a moveable piston • The gas occupies a volume V and exerts pressure P on the walls of the cylinder and on the piston Section 12.1

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