1. Thermodynamics
AP Physics
Chapter 15

2. 13.3 Zeroth Law of Thermodynamics

3. 13.3 Zeroth Law of Thermodynamics
If two objects of different temperatures are placed in thermal contact, they will eventually reach the same temperature.
They reach Thermal Equilibrium
Energy flowing in equals the energy flowing out
Thermal Eq Animation
13.3

4. 13.3 Zeroth Law of Thermodynamics
Zeroth Law of Thermodynamics – if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other
Allows for a definition of temperature
(two objects have the same temperature when they are in thermal equilibirum)
13.3

5. Thermodynamics
13.4 Thermal Expansion

6. Objects usually expand when heated and contract when cooled.
13.4 Thermal Expansion
Objects usually expand when heated and contract when cooled.
This can lead to some problems
So we include expansion joints
13.4

7. Change in length is proportional to temperature
13.4 Thermal Expansion
Change in length is proportional to temperature
So the equation is
a is called the coefficient of linear expansion T0 T L0 DL
13.4

8. Imagine an infinitely thin ring
13.4 Thermal Expansion
Common Misconception
When a object with a hole in it is heated, does the hole get larger or smaller?
Imagine an infinitely thin ring
If it is heated length (circumference) of the ring increase
13.4

9. Volume follows the same relationship
13.4 Thermal Expansion
Volume follows the same relationship
is the coefficient of volume expansion
is usually equal to approximately 3a
13.4

10. Thermodynamics
13.7 The Ideal Gas Law

11. P – pressure in Pa (absolute pressure) V – volume in m3
13.7 The Ideal Gas Law
P – pressure in Pa (absolute pressure)
V – volume in m3
R = 8.314J/molK (in standard units)
T – temperature in K
n – quantity in mol
13.7

12. If the equation is written in terms of molecules
13.7 The Ideal Gas Law
If the equation is written in terms of molecules
The number of molecules (or atoms) in one mole is
So if
And the number of molecules is
The ideal gas law can be written
13.7

13. This is known as Boltzmann’s constant Our equation becomes
13.7 The Ideal Gas Law
The quantity
This is known as Boltzmann’s constant
Our equation becomes
The constant has a value
13.7

14. 14.1 Heat as Energy Transfer
Thermodynamics
14.1 Heat as Energy Transfer

15. 14.1 Heat as Energy Transfer
Heat – energy transferred from one object to another because of a difference in temperature
Unit – joule
14.1

16. Thermodynamics
14.2 Internal Energy

17. Internal Energy – total energy of all the particles in the object
Temperature (K)– measurement of average kinetic energy of the particles
Internal Energy – total energy of all the particles in the object
Heat - transfer
Temperature
14.2

18. Internal Energy equation
First internal Energy (U) is equal to the number of particles (N) times average kinetic energy
Since
The equation changes to
And since
14.2

19. 14.3 Specific Heat and Calorimetry
Thermodynamics
14.3 Specific Heat and Calorimetry

20. 14.3 Specific Heat and Calorimetry
Remember from chemistry
Q – heat transfer in Joules
m – mass in kg
C – specific heat (Cp – constant pressure, Cv – constant volume)
DT – change in temperature (oC or K)
14.3

21. 14.3 Specific Heat and Calorimetry
Also when energy is transfered
Expands to
14.3

22. 14.6 Heat Transfer: Conduction
Thermodynamics
14.6 Heat Transfer: Conduction

23. 14.6 Heat Transfer: Conduction
Conduction – by molecular collisions
If heat is transferred through a substance
The rate of heat transfer (Q/t) depends on A l Tc TH
14.6

24. 14.6 Heat Transfer: Conduction
Q = heat (J)
t = time (s)
k = thermal conductivity (J/smCo)
T = temperature (K or Co)
l = length
14.6

25. 15.1 The First Law of Thermodynamics

26. 15.1 The First Law of Thermodynamics
The change in internal energy of a closed system will be equal to the energy added to the system by heat minus the work done by the system on the surroundings.
A broad statement of the law of conservation of energy
15.1

27. 15.1 The First Law of Thermodynamics
Internal energy, U, is a property of the system
Work and heat are not
First Law proven by Joule in an experiment
The work done by
The weight as if fell
(done by gravity)
Equaled the energy increase of the liquid in the sealed chamber
15.1

28. 15.1 The First Law of Thermodynamics
The first law can be expanded
If the system is moving and has potential energy, then
Remember
Q is positive when work flows in
W is positive when the system does work
15.1

29. 15.2 Thermodynamic Processes & the First Law
Thermodynamics
15.2 Thermodynamic Processes & the First Law

30. 15.2 Thermodynamic Processes & the First Law
You need to remember the names of these processes
Isothermal – constant temperature
If temperature is held constant, then
A graph would look like
The curves are called
isotherms
15.2

31. 15.2 Thermodynamic Processes & the First Law
If DT is zero, then DU is zero because
We can then show
The work done by the gas in an isothermal process equals the heat added to the gas
15.2

32. 15.2 Thermodynamic Processes & the First Law
Adiabatic – no heat is allowed to flow into or out of the system
That leaves First Law as
If the gas expands, the internal energy decreases, and so does the temperature
15.2

33. 15.2 Thermodynamic Processes & the First Law
Isobaric – pressure is constant
Work = PV
Isovolumetric – volume is
Constant
Work = 0
15.2

34. 15.2 Thermodynamic Processes & the First Law
Example 1: Isobaric Process
A gas is placed in a piston with an area of .1m2. Pressure is maintained at a constant 8000 Pa while heat energy is added. The piston moves upward 4 cm. If 42 J of energy is added to the system what is the change in internal energy?
15.2

35. 15.2 Thermodynamic Processes & the First Law
Example 1: Isobaric Process
A gas is placed in a piston with an area of .1m2. Pressure is maintained at a constant 8000 Pa while heat energy is added. The piston moves upward 4 cm. If 42 J of energy is added to the system what is the change in internal energy?
15.2

36. 15.2 Thermodynamic Processes & the First Law
Example 2: Adiabatic Expansion
How much work is done the adiabatic expansion of a car piston if it contains 0.10 mole an ideal monatomic gas that goes from 1200 K to 400 K?
15.2

37. 15.2 Thermodynamic Processes & the First Law
Example 2: Adiabatic Expansion
How much work is done the adiabatic expansion of a car piston if it contains 0.10 mole an ideal monatomic gas that goes from 1200 K to 400 K?
Adiabatic Q=0
15.2

38. 15.2 Thermodynamic Processes & the First Law
Example 3: Isovolumetric Process
Water with a mass of 2 kg is held at a constant volume in a container, while 10 kJ of energy is slowly added. 2 kJ of energy leaks out to the surroundings. What is the temperature change of the water?
15.2

39. 15.2 Thermodynamic Processes & the First Law
Example 3: Isovolumetric Process
Water with a mass of 2 kg is held at a constant volume in a container, while 10 kJ of energy is slowly added. 2 kJ of energy leaks out to the surroundings. What is the temperature change of the water?
Constant volume and we don’t have Cv
15.2

40. 15.2 Thermodynamic Processes & the First Law
When a process is cyclical
And the work done is the area bound by the curves
15.2

41. 15.2 Thermodynamic Processes & the First Law
Example 4: First law in a Cyclic Process
An ideal monatomic gas is confined in a cylinder by a movable piston. The gas starts at A with P = kPa, V = .005 m3 and T = 300 K.
The cycle is
A  B is isovolumetric and raises P to 3 atm.
BC Isothermal Expansion (Pave = kPa)
CA Isobaric
Calculate DU, Q, and W for each step and for the entire cycle
15.2

42. 15.2 Thermodynamic Processes & the First LawB A C
15.2

43. 15.2 Thermodynamic Processes & the First Law
Example 4: First law in a Cyclic Process
An ideal monatomic gas is confined in a cylinder by a movable piston. The gas starts at A with P = kPa, V = .005 m3 and T = 300 K.
The cycle is
A  B is isovolumetric and raises P to 3 atm.
15.2

44. 15.2 Thermodynamic Processes & the First Law
Example 4: First law in a Cyclic Process
An ideal monatomic gas is confined in a cylinder by a movable piston. The gas starts at A with P = kPa, V = .005 m3 and T = 300 K.
The cycle is
BC Isothermal Expansion (Pave = kPa)
15.2

45. 15.2 Thermodynamic Processes & the First Law
Example 4: First law in a Cyclic Process
An ideal monatomic gas is confined in a cylinder by a movable piston. The gas starts at A with P = kPa, V = .005 m3 and T = 300 K.
The cycle is
CA Isobaric
15.2

46. 15.2 Thermodynamic Processes & the First Law
Example 4: First law in a Cyclic Process
An ideal monatomic gas is confined in a cylinder by a movable piston. The gas starts at A with P = kPa, V = .005 m3 and T = 300 K.
The cycle is
Totals
15.2

47. 15.4 The Second Law of Thermodynamics-Intro

48. 15.3 The Second Law of Thermodynamics-Intro
The first law deals with conservation of energy.
However there are situations that would conserve energy, but do not occur.
Falling objects convert from Ug to K to Q
Never Q to K to Ug
Heat flows from TH to TC
Never TC to TH
Falling and Energy
15.4

49. 15.3 The Second Law of Thermodynamics-Intro
The second law explains why some processes occur and some don’t
In terms of heat, the second law could be stated
-Heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object
15.4

50. Thermodynamics
15.5 Heat Engines

51. The Heat Engine Heat flows into the engine Energy is convert-
15.5 Heat Engines
The Heat Engine
Heat flows into
the engine
Energy is convert-
ed to work
Remaining heat is
exhausted to cold
15.5

52. Steps in an internal combustion engine
15.5 Heat Engines
Steps in an internal combustion engine
1. The intake valve is open, and fuel and air are drawn past the valve and into the combustion chamber and cylinder from the intake manifold located on top of the combustion chamber (Intake Stroke)
15.5

53. Steps in an internal combustion engine
15.5 Heat Engines
Steps in an internal combustion engine
2. With both valves closed, the combination of the cylinder and combustion chamber form a completely closed vessel containing the fuel/air mixture. As the piston is pushed to the right, volume is reduced & the fuel/air mixture is compressed (Compression Stroke)
15.5

54. Steps in an internal combustion engine
15.5 Heat Engines
Steps in an internal combustion engine
3. the electrical contact is opened. The sudden opening of the contact produces a spark in the combustion chamber which ignites the fuel/air mixture. Rapid combustion of fuel releases heat, produces exhaust gases in the combustion chamber. (Power Stroke)
15.5

55. Steps in an internal combustion engine
15.5 Heat Engines
Steps in an internal combustion engine
4. The purpose of the exhaust stroke is to clear the cylinder of the spent exhaust in preparation for another ignition cycle. (Exhaust Stroke)
15.5

56. 15.5 Heat Engines
Complete cycle
15.5

57. Looking at a steam engine If the steam were the same temperature
15.5 Heat Engines
Looking at a steam engine
If the steam were the
same temperature
throughout
-exhaust pressure
would be the same
as the intake pressure
-then exhaust work would be the same as intake work
For net work there must be a DT
15.5

58. Actual Efficiency of an engine is defined as
15.5 Heat Engines
Actual Efficiency of an engine is defined as
The ratio of work to heat input
Since
We can write the efficiency as
15.5

59. Carnot Engine (ideal) – no actual Carnot engine A four cycle engine
15.5 Heat Engines
Carnot Engine (ideal) – no actual Carnot engine
A four cycle engine
Isothermal expansion (DT=0, Q=W)
Adiabatic expansion (Q=0, DU=-W)
Isothermal compression
Adiabatic compression
Each process was considered reversible
That is that each step is done very slowly
Real reactions occur quickly – there would be turbulence, friction - irreversible
15.5

60. The Carnot efficiency is defined as
15.5 Heat Engines
The Carnot efficiency is defined as
15.5

61. 15.6 Refrigerators, Air Conditioners
Thermodynamics
15.6 Refrigerators, Air Conditioners

62. 15.6 Refrigerators, Air Conditioners
The reverse of Heat Engines
Work must be done – because heat flows from hot to cold
15.6

63. 15.7 Entropy and the 2nd Law of Thermodynamics

64. 15.7 Entropy and the 2nd Law of Thermodynamics
Entropy – a measure of the order or disorder of a system
Change in entropy is defined as
Q must be added as a reversible process at a constant temperature
15.7

65. 15.7 Entropy and the 2nd Law of Thermodynamics
The Second Law of Thermodynamics – the entropy of an isolated system never decreases. It can only stay the same or increase.
Only idealized processes have a DS=0
Or – the total entropy of any system plus that of its environment increases as a result of any natural process
15.7

66. 15.7 Entropy and the 2nd Law of Thermodynamics
Example – A sample of 50 kg of water at 20oC is mixed with 50 kg at 24oC. The final temperature is 22oC. Estimate the change in Entropy.
The reaction does not occur at constant temperature, so use the average temperature to estimate the entropy change.
15.7

67. 15.7 Entropy and the 2nd Law of Thermodynamics
Example – A sample of 50 kg of water at 20oC is mixed with 50 kg at 24oC. The final temperature is 22oC. Estimate the change in Entropy.
Cold Water
Hot Water
15.7

68. Thermodynamics
15.8 Order to Disorder

69. 15.8 Order to Disorder
2nd Law can be stated – natural processes tend to move toward a state of greater disorder
15.7