Heat, Temperature and Thermodynamics

1
Heat, Temperature and Thermodynamics
2
What is Temperature?

• We all know some things about temperature (at
  least I hope you do)
• Scientifically, what is it?
• Temperature tells us the average internal energy
  of the constituent particles of a substance

3
Which Energy Types?

• The constituent particles in a substance have two
  main types of energy
• Kinetic energy associated with any movement
  (vibrational, rotational, translational)
• Potential Energy stored in bonds
• We lump all this energy together and call it
  internal (thermal) energy

4

• http//www.edinformatics.com/interactive_molecules
  /ice.htm

5
Energy Distribution

• For gases, temperature corresponds to how fast
  the particles move
• For solids, it relates to both vibrational motion
  (think about solids as old folks in rocking
  chairs) and the potential energy stored in bonds
• Liquids are somewhere in between

6
Average vs. Total

• Temperature tells us the average internal energy
  of an object, not the total
• Two boxes have the same composition and
  temperature, but one is three times as large.
  Which has more total internal energy?
• If the two boxes have the same total internal
  energy, which has the higher temperature?

7
Temperature Scales?

• The Barcelona Fiasco
• The US is the only place in the world that still
  uses the Fahrenheit scale
• 0 point based upon the lowest freezing point of
  water

8
Everyone else

• The rest of the world uses the Celsius scale
• This scale uses the boiling and freezing points
  of pure water as reference points

9
SI Units

• While many scientists do use Celsius in the lab,
  the SI temperature base unit is Kelvin (not
  degrees Kelvin)
• 1 K 1 C

10
Differences

• The two scales differ only in starting point
• While Celsius uses water as its zero point, the
  Kelvin scale uses something different
• ABSOLUTE ZERO!!

11
Absolute What?

• If temperature tells us how much energy a
  particle has, then what would absolute zero
  represent?
• Right, the point at which it had no energy
• In a word, it would be totally motionless
• 0K -273C -459.7 F

12
Heat Temperature ?

• In the common vernacular, the words heat and
  temperature are interchangeable
• In the scientific community, we draw a
  distinction between these ideas
• So the question arises what is heat?

13
Refining Our Definition

• Heat is a dynamic process which involves the
  movement of energy
• When does heat move?
• When two objects at different temperatures
  exchange energy, we call that exchange heat flow

14
(No Transcript)
15
The Force that Binds Us

• For heat to flow, the objects must have some sort
  of connection
• We call this connection Thermal Contact
• Examples air, water, or any other type of
  matter

16
Stopping Point?

• How long will heat move?
• When heat ceases to flow, we say the objects are
  in thermal equilibrium

17
Examples?

• A) A candle
• B) A pot of boiling water
• C) Touch the back of your chair (metal) with your
  hand. In which direction does energy move?

18
Units?

• You can think of heat as energy in transit
• That being the case, you might then think of
  heats units as an energy rate (J/s Watts)
• Turns out that the units of heat the units of
  energy, Joules

19
The Calorie Connection

• Calories are another unit of heat
• 1 Calorie 4.184 J
• Its the amount of energy necessary to raise the
  temperature of 1g of water by 1C

20
What Happens to the Heat?

• When it arrives at an object, it becomes part of
  the objects internal energy
• It can speed up individual particles, rotate
  them, stretch them further apart, etc

21
How Does Heat Move?

• Heat can flow in three ways conduction,
  convection, and radiation
• In certain situations, it only flows in one way
  in others, all three

22
Conduction

• In conduction, particles collide with their
  neighbors, transferring energy through the
  collisions
• Its a bit like the domino effect

23
Particle Movement?

• The particles themselves do not move with the
  heat
• Like dominoes, they remain in their original
  location (with the exception of local movement)

24
Conductors and Insulators

• Conduction works better in some materials than
  others
• Materials which allow conduction are called
  conductors
• Materials which resist conduction are called
  insulators

25
Questions

• Why can you stick your hand inside an oven
  without getting burned?
• Why does the metal part of your chair feel colder
  than your desktop?
• Beyond obvious cultural assumptions, why do we
  wear clothing?
• A metal spoon is placed in one of two coffee
  cups. Which cup will be cooler in a few
  minutes?

26
Conduction Rate

• The rate of heat conduction depends upon a few
  factors
• The type of material, the area through which heat
  can flow, temperature difference, and the heat
  displacement

27

• http//mutuslab.cs.uwindsor.ca/schurko/animations/
  heatcapacitymetals/heat_metal.htm

28
Some k Values
29

• Rank the heat flow rate in each of the objects
  below
• A glass tube of radius R and length L
• A metal tube of radius R and length L
• A metal tube of radius R/2 and length L
• A glass tube of radius R and length 2L

30
(No Transcript)
31

• Heat flows through a metal container (k 250
  J/sKm) with a cross-sectional area of 0.5m2
• If the metal connects two regions with a
  temperature difference of 20K, and the heat flows
  5m, what is the flow rate?
• DQ/Dt kADT/Dx 500J/s

32
Convection

• Think about the air surrounding a candle
• As its temperature increases, it rises
• This process of warm matter rising is called
  convection
• It can occur in solids, liquids, or gases

33
(No Transcript)
34
Why Does It Rise?

• Objects expand when heated
• As a hot gas expands, its density decreases
• Think back to Archimedes if its density
  decreases, it is now less dense than air
• Hence, it rises

35
Conduction Comparison?

• Like conduction, convection is a heat transport
  mechanism
• Like conduction, convection requires a medium
• Unlike conduction, the medium moves in convection

36
Examples

• Warm air rising above a fire
• Convection currents in a pot of boiling water

37
Convection in the Mantle
38
Plate Tectonics?
39
Onshore/Offshore winds at the coast
40
Gulf Stream
41
Matter Matters

• With conduction and convection, matter is
  required
• Is there a way to move heat/energy without
  matter?
• How does energy get to us from space?

42
Blinded by the Light

• Light allows energy to move from one place to
  another without a medium
• We call this process radiation

43
Space Death?

• Many movies portray a horrible fate to those that
  take off their protective helmets in space
• Take a moment to think about this
• What would happen to you if you took off your
  helmet in space?

44
Heat Effects?

• So we know what heat is and how it moves
• The next question is what happens to objects as
  they absorb/release heat?

45
Heat/Temperature Connection

• As weve said, heat flowing into an object will
  increase its internal energy
• This will increase its temperature
• Different materials will have different changes
  in temperature
• Q mcDT

46
Other Changes?

• What else can happen to an object when it
  absorbs/releases heat?
• Thermal Expansion or Contraction
• Chemical Changes

47
Expansion Examples

• Bridge Gaps
• Challenger
• WTC Collapse
• Bimetallic Strip
• The Hot Water/Jar Trick

48
Calculating Heat Amount

• Its sometimes helpful to know how much heat it
  takes to produce a certain temperature change
• To accomplish this, what do we need to know?
• Object mass, object composition/type, and the
  temperature change

49

• We summarize this information in the following
  equation
• Q mcDT
• Q Heat released/absorbed
• m mass
• c specific heat
• DT temperature change

50
What is specific heat?

• Specific heat gives us some insight into the ease
  with which we can change an objects temperature
• Objects with a low c value heat up easily/quickly
  (metals)
• Objects with high c values heat up slowly (glass,
  water)

51
Water The Ultimate Heat Sponge

• Water is great at storing energy, which gives it
  a very high c value (4184J/kgC)
• If you cool down 1 km3 of seawater by 1C, how
  much heat is released?
• r 1025 kg/m3

52

• You release about 4x1015 J
• This is the same amount of energy released by a
  nuclear device

53

• After Katrina passed through the Gulf of Mexico,
  the average temperature dropped by 5C
• Think about how much water is in the Gulf

54
Laws of Thermodynamics

• Thermodynamics is the branch of science that
  describes how heat and temperature interact with
  matter
• It has three basic laws

55
The First Law

• The first law is something weve already seen
• The Law of Energy Conservation
• We do phrase it somewhat different, however

56
Heat, Work, and Internal Energy

• Imagine a cylinder, filled with gas

57

• If you do work to compress the cylinder, what
  happens?
• If you add heat to the cylinder, what happens?
• How about if the gas inside the cylinder loses
  heat?
• What happens if the gas does work by pushing the
  cylinder outwards?

58
Energy Conservation, Remixed

• DU Q W
• DU change in gass internal energy
• Q heat exchanged with the environment
• W work done on the gas

59
Sign Conventions?

• Heat added to the system is , heat released is
• Work done on the gas is , work done by the gas
  is
• An increase in internal energy is , a decrease
  is

60

• http//phet.colorado.edu/web-pages/simulations-bas
  e.html

61
Questions

• The fuel/air mixture inside a car engine receives
  10J of heat from the spark plug, ignites, and
  does 50J of work. What is its internal energy
  change?
• A gass internal energy increases by 20J after I
  do 40J of work on it. How much heat does the gas
  exchange with its environment? Is the heat
  gained or lost?

62
Work Done By Gases

• How can a gas do work on its environment?
• By applying a force and pushing its surroundings
  outward, changing its volume
• Work Force x distance
• Work Pressure x Change in Volume
• (F/A x V F x d)

63

• http//auto.howstuffworks.com/engine.htm

64
Examples

• Consider a fuel/air mixture in an engine piston.
  It combusts at a pressure of 200,000Pa, changing
  the piston volume from .001m3. How much work
  does the fuel/air mixture do on the piston?
• W PDV 200,000Pa x .001m3 200J

65

• At atmospheric pressure, a gas expands from 3E-4
  m3 to 8E-4 m3. If this gas loses 10J of heat
  during the process, what happens to its internal
  energy?
• DU Q W - Q - PDV
• DU -10J 1.01E5Pa ( 5E-4 m3)
• DU -10J 50J -60J

66
Thermodynamic Processes

• Any process that involve heat, internal energy,
  and work done by/on a system is called a
  thermodynamic process
• The four main types are adiabatic, isothermal,
  isovolumetric, and isobaric

67
Adiabatic

• An adiabatic process is a very rapid
  thermodynamic process
• It occurs so quickly, there is no heat transfer
• Q 0, therefore
• DU W PDV

68
Adiabatic Examples

• Car Combustion
• Rising Air Masses ( 10C per km)
• Blow air your hand

69
Isothermal

• Iso is a Greek word meaning equal
• thermal refers to temperature
• Therefore
• An isothermal process is one in which temperature
  remains constant

70

• If T doesnt change, then DU 0
• Therefore
• Q -W -PDV
• Heating up a gas causes it to expand
• Extracting heat causes it to contract
• Examples boiling water

71
Isovolumetric

• An isovolumetric process involves a constant
  volume
• This means that W 0 (the gas cant expand nor
  can it be compressed)
• Therefore
• DU Q

72
Examples?

• If you add heat to a gas, its temperature
  increases
• Examples aerosol cans in fires, the collapsing
  soda can

73
Isobaric

• An isobaric process occurs at constant pressure
• None of the quantities are necessarily zero
• DU Q W Q PDV

74
Process Summary
75
Seeing The Process

• Think back to motion, and how position/time
  graphs allowed us to analyze an objects speed,
  acceleration, and displacement
• We can play the same game with thermodynamic
  processes, using graphical analysis to derive
  information

76
PV Graphs

• The most common graph for thermodynamic processes
  is a pressure/volume graph
• As it is usually easier to manipulate a
  containers volume, that is our independent
  variable

77
Which Cycles?
78
(No Transcript)
79
Isotherms

• Isothermal processes lie upon what are called
  isotherms, lines of constant temperature on a PV
  graph
• These are analogous to lines of constant
  elevation on a map

80
PV NkT ? P NkT/V
81
Adiabatic Processes

• As adiabatic process involve changes in P, V, and
  T, they show up on a PV graph as curved lines
  that jump between isotherms

82

• Describe whats happening to the gas in the
  following PV graph
• For each step, write down whether DU, W, and Q
  are ,-, or 0

83
Amount of Work?

• Like using F/t graphs to compute impulse, we can
  use PV graphs to figure out the amount of work
  done on/by a gas
• We simply need the area under the curve (W PDV)

84
Cyclical Processes

• Weve seen that a systems internal energy can
  change during a process
• However, the net change in internal energy is
  zero for any cyclical process (it starts and ends
  at the same temperature)

85
The Second Law

• Very generally, the 2nd law of thermodynamics
  describes the efficiency of natural processes
• It is written in a few different ways

86
Version 1

• Why does a pendulum always stop?
• Friction
• The second law of thermodynamics says that no
  process is ever 100 efficient because it always
  has some waste energy

87
Efficiency?

• Remember, the efficiency of a process is the
  ratio of the work you get out to the energy input

______________________
Efficiency
88
Efficiency of Natural Processes

• Modern Cars 20
• Power Plants 13
• Photosynthesis 2
• Human Body 20

89
Heat Engines

• The second law becomes very important when
  describing heat engines
• A heat engine is a device which converts heat
  flow into work
• Examples steam engines, Stirling engine

90
(No Transcript)
91

• http//auto.howstuffworks.com/stirling-engine1.htm
  
• http//science.howstuffworks.com/steam1.htm

92
Ideal Engine Efficiency

• As energy flows from the high temperature to the
  low temperature region, the engine extracts work
• QH W QL
• Efficiency W/QH
• Efficiency (QH QL)/QH
• 1 QL/QH

93
Temperature?

• As the amount of heat flowing from one region to
  the next is proportional to the regions
  temperature, we can rewrite the efficiency
  equation as follows
• Efficiency (TH TL)/TH
• 1 TL/TH

94
Increasing Efficiency?

• What happens to the engine efficiency as the
  difference between TH and TL increases (the
  engine burns hotter)?
• What implications does this have for heat
  engines?
• Example Indy cars, ceramic car engines

95
Version 2 Entropy

• The second law is also useful when discussion an
  interesting concept entropy
• In a nutshell, a systems entropy measures its
  randomness/disorder

96
Entropy Rankings
97
Calculations

• Entropy Change DS
• DS Q/T

98
The Ultimate Room Cleaning Defense?

• According to the second law, the entropy in the
  universe never decreases
• In otherwords, the universe is becoming more
  random over time
• Examples waste heat, natural corrosion
• A room defense?

99
The Paradox of Life

• According to the Big Bang Model, the universe
  initially consisted of Hydrogen plasma (protons)
• So how are we here?

? ?
100
The Third Law

• The third law of thermodynamics establishes and
  describes the state known as absolute zero

101
(No Transcript)
102
(No Transcript)