1. Energy in Thermal Processes
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

7. 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

9. Link to
Bright storm on
Heat

10. 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

11. 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

12. 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 = J
This is called the Mechanical Equivalent of Heat
Section 11.1

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

14. Link to
Bright storm on
Heat Transfer

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

25. Read and take notes on page: 368

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

27. 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

28. 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

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

30. 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

31. 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

32. 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

33. Radiation equation P = σ A e T4
The power is the rate of energy transfer, in Watts
σ = 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

34. 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

35. 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

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

37. 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

38. 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

39. 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

40. The Laws of Thermodynamics
Chapter Twelve
The Laws of Thermodynamics

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

42. 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

43. 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

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

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

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

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

48. 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

50. 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

51. Work in a Gas Cylinder, cont.
A force is applied to slowly compress the gas
The compression is slow enough for all the system to remain essentially in thermal equilibrium
W = - P ΔV
This is the work done on the gas where P is the pressure throughout the gas
Section 12.1

52. More about Work on a Gas Cylinder
When the gas is compressed
ΔV is negative
The work done on the gas is positive
When the gas is allowed to expand
ΔV is positive
The work done on the gas is negative
When the volume remains constant
No work is done on the gas
Section 12.1

53. Work By vs. Work On
The definition of work, W, specifies the work done on the gas
This definition focuses on the internal energy of the system
Wenv is used to denote the work done by the gas
The focus would be on harnessing a system’s internal energy to do work on something external to the gas
W = - Wenv
Section 12.1

54. Notes about the Work Equation
The pressure remains constant during the expansion or compression
This is called an isobaric process
The previous work equation can be used only for an isobaric process
Section 12.1

58. PV Diagrams
Used when the pressure and volume are known at each step of the process
The work done on a gas that takes it from some initial state to some final state is equal in magnitude to the area under the curve on the PV diagram
This is true whether or not the pressure stays constant
Section 12.1

59. PV Diagrams, cont.
The curve on the diagram is called the path taken between the initial and final states
The work done depends on the particular path
Same initial and final states, but different amounts of work are done
Section 12.1

60. First Law of Thermodynamics
Energy conservation law
Relates changes in internal energy to energy transfers due to heat and work
Applicable to all types of processes
Provides a connection between microscopic and macroscopic worlds
Section 12.2

61. First Law, cont. Energy transfers occur
By doing work
Requires a macroscopic displacement of an object through the application of a force
By heat
Due to a temperature difference
Usually occurs by radiation, conduction and/or convection
Other methods are possible
All result in a change in the internal energy, DU, of the system
Section 12.2

62. First Law, Equation
If a system undergoes a change from an initial state to a final state, then DU = Uf – Ui = Q + W
Q is the energy transferred between the system and the environment
W is the work done on the system
DU is the change in internal energy
Section 12.2

63. First Law – Signs Signs of the terms in the equation Q W DU
Positive if energy is transferred into the system
Negative if energy is removed from the system W
Positive if work is done on the system
Negative if work is done by the system DU
Positive if the temperature increases
Negative if the temperature decreases
Section 12.2

64. Notes About Work
Positive work increases the internal energy of the system
Negative work decreases the internal energy of the system
This is consistent with the definition of mechanical work
Section 12.2

70. Read and take notes on pages: 388-389

75. Read and take notes on pages: 390-391