Science 111

1
Science 111

• Chapter 7
• Thermal Energy and Thermodynamics
• Chapter 8
• Heat Transfer and Change of Phase

2
Atoms and Molecules

• Substances are made out of atoms.
• Sometimes the atoms are bound into units called
  molecules (like the H2O molecules that make up
  water).
• We will use the term molecules to mean either
  molecules or single atoms.

3
What an attractive molecule

• Molecules usually attract each other (because
  they tend to have ends that are positively and
  negatively charged and the opposite charges
  attract).
• But molecules are also moving around and the
  attractive forces may not be enough to hold them
  together.

4
Solids, Liquids, and Gases

• When the molecules have very little kinetic
  energy, they stick together tightly and form into
  a solid.
• More kinetic energy, molecules moving faster,
  they squirm around but dont pull apart from each
  other, the liquid phase.
• Still more energy, they break free from each
  other and fly around independently, a gas.

5
This is figure 17-5 from the text. In
the solid, the atoms may vibrate but dont move
around. In the liquid, the molecules move
around but dont break away from each other, like
bees in a beehive. In the gas, molecules all
fly their separate directions, like birds in the
sky.
6
Thermal Energy

• Molecules will have kinetic energy and other
  forms of energy (electrical potential energy,
  nuclear energy, ).
• The total energy within a substance is called the
  thermal energy.
• The thermal energy is all the energy hidden
  within an object, it does not include energies
  due to the collective motion or position of the
  object (bulk kinetic energy or gravitational PE).

7
Sec. 7.2 Temperature

• Why are some things hot and others cold?
• We can feel when an object is hot or cold but
  what makes it so?
• Aristotle thought it was because hot objects
  contained more of the pointy fire atoms.
• Are some types or shapes of molecules inherently
  hot?

8
High Speed is Hot

• Hot and cold has nothing to do with the type or
  shapes of molecules.
• It is the speed of motion of molecules that make
  them hotter or colder.
• Hot substances have very active molecules that
  transfer lots of energy (do work) when in contact
  with cooler objects.

9
What temperature is

• Specifically, temperature is a measure of the
  average kinetic energy of the molecules within a
  substance.
• This is the kinetic energy of the motion or
  vibration of the molecules within the substance.
• Molecules can have a wide variety of speeds which
  is why we use the average.

10
A Glimpse at the Math

• The kinetic energy of a molecule (or anything) is
  calculated by 1/2mv2.
• The average of all the kinetic energies will be
  some amount (KE)avg.
• The temperature of that object is the average
  kinetic energy times two-thirds divided by
  Boltzmanns constant k,
• T (2/3) (KE)avg/k k 1.38 x 10-23 J/K

11
Sorry about that

• Sorry, you didnt need to see that.
• You wont be doing those calculations or using
  Boltzmanns constant.
• I just wanted to emphasize that temperature
  really is determined by the average kinetic
  energy of molecules, that is what it really
  represents.

12
Temperature Scales

• Celsius (C) Fahrenheit (F)
• Kelvin (K)
• C F K
• Room Temp 24 76 297
• H2O Freezing 0 32 273
• H2O Boiling 100 212 373

13
Sec. 7.3 Absolute Zero

• Temperature measures the average kinetic energy
  of molecules.
• There is no limit to how much energy a molecule
  can have, so there is no limit to how high the
  temperature can be.
• There is a lower limit to kinetic energy.

14
Absolute Zero

• The least kinetic energy a molecule can have is
  zero (not moving at all).
• The lowest possible temperature is when the
  average kinetic energy is zero
• This occurs at a temperature of
• -273C -459F 0 K
• This temperature is called absolute zero.

15
Sec. 7.4 Heat

• A force acting on a moving body does work.
• The work done represents a transfer of energy,
  one body gains energy and the other loses an
  equal amount of energy.
• When a hot body is put next to a cold one, the
  hot one cools and the cold one warms.

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• There was a transfer of energy between the hot
  and cold bodies.
• There is work done, but at a microscopic level
  due to the collisions of molecules moving at
  different speeds.
• Lots of little forces doing work, not some large,
  obvious force doing work.
• Energy exchanged in this fashion is called heat.

17
Heat and Work

• Ultimately, all energy exchanges are due to work
  done by forces.
• When the energy transfer occurs because of the
  different temperatures (different average kinetic
  energies of the molecules), we call it a heat
  flow rather than work.

18
Thermal Contact

• Bodies that can have a heat flow between them are
  said to be in thermal contact.
• All energy transfers are due to either heat or
  work.
• When the energy exchange requires a difference in
  temperatures, it is heat, otherwise it is
  work.

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Thermal Equilibrium

• Heat will flow from the hot object to the cold
  one until they reach the same temperature.
• Objects at the same temperature are said to be in
  thermal equilibrium and there is no heat flow
  between them.

20
Sec. 7.5 Quantity of Heat

• Heat is energy, so we can measure heat in joules.
• In the past, scientists didnt know that heat was
  energy.
• They thought there was a substance (called
  caloric) which could move from body to body.

21
Caloric

• Objects gaining more caloric would get hotter,
  cooler when losing caloric.
• Eventually the caloric idea was disproven when it
  was found that temperatures of objects could be
  changed by doing work on them (like banging them
  with a hammer) which was just adding energy
  caloric must also just be energy.

22
Units of Heat

• Special units were used to measure the amount of
  caloric in a heat flow.
• Calories and BTUs (British Thermal Units).
• Now we know that these units represent an amount
  of energy.

23
Some new energy units

• 1 cal 4.187 J
• 1 BTU 1055 J
• 1 food-calorie 1 Calorie 1 big calorie
  1000 cal 1 kilocalorie 4187 J
• A food contains 100 Calories, that means it
  contains 100,000 calories of available chemical
  energy.

24
Some Questions

• How, exactly, does heat (energy) get from a hot
  object to a cold object?
• Why does heat always flow from hot to cold and
  never from cold to hot?
• We now skip ahead to chapter 8 to answer this
  first question.

25
Sec. 8.1 Conduction

• There are three main ways in which heat can move
  from body to body
• conduction, convection, and radiation
• Conduction occurs when two objects are put
  physically right against each other.

26
Conduction

• Two bodies are physically touching.
• The molecules in one can collide with the
  molecules in the other.
• The collisions can cause an exchange of energy
  (electromagnetic forces do work, but again, this
  microscopic work is usually called heat rather
  than work).

27
Direction of heat flow

• One body gains energy (kinetic energy of its
  molecules).
• The other body will lose an equal amount of
  energy.
• The hotter body always gets colder (loses energy)
  while the cold gets hotter. Why?

28
Why heat goes hot to cold

• When a fast-moving molecule collides with a
  slow-moving molecule, the usual result is two
  medium-speed molecules.
• It can happen that the fast ends up moving faster
  and the slow slower, but that is extremely rare.
• So, the hot body with its faster moving molecules
  tends to lose energy overall due to these
  collisions, and the colder gains an equal amount
  of energy.

29
Conduction across an object

• Imagine a metal rod at room temperature except
  that one end is being heated.
• That hot end will pass energy on to the adjacent
  sections and on down the rod.
• In a metal rod, heat (energy) moves quickly.
• Metals are good conductors of heat.
• Why is that?

30
Why metals are good conductors

• Metals all contain free electrons, electrons
  which move far and fast within the metal.
• Far is a relative term, still microscopic on
  human terms but much further than the usual
  distances moved by other electrons and molecules
  within the metal.
• Collisions between these electrons is responsible
  for the rapid conduction of heat in metals.

31
Insulators

• Gases, or substances that contain pockets of
  gases, are poor conductors of heat, poor
  conductors are called insulators.
• Insulators slow the flow of heat, they cant
  entirely prevent it.

32
Heat Conduction Notes

• In the conduction of heat, no exchange of
  material occurs.
• Consider, say, the conduction between your butt
  and the chair youre sitting in.
• Heat flows from you to the chair, but no part of
  you does.
• Neighboring atoms interact and energy is
  exchanged (the more vigorous oscillations of your
  hotter molecules induce faster vibrations in
  those molecules).

33
Heat Notes Continued

• Its like when you kick a ball.
• You do work and transfer energy but there is not
  exchange (usually) of any of your substance.
• Heat conduction is like the kicking of billions
  of microscopic adjacent balls.

34
Question

• You wake in the morning.
• The tile floor feels very cold to your bare feet
  but the carpeting or wood feels warm.
• The tile must be colder than the carpet, right?

35
Tile must be colder? No!

• Wrong, both the carpet and the tile are at the
  same temperature!
• The tile and carpet (and air and wood and walls
  and silverware and ) are in thermal equilibrium,
  theyve exchanged heat all night until they are
  all at the same temperature.
• So why does the tile feel colder?

36
Why the tile feels colder

• The carpet and tile are at the same temperature,
  both are colder than your body temperature.
• Heat flows from your body to both.
• The tile is a better conductor, heat flows into
  it more quickly.
• The carpet is an insulator (air pockets).
• What you feel is the loss of heat from your body,
  not the temperature of the other object.

37
Sense of Touch

• On the tile, your feet lose a lot of heat and
  feel cold.
• On the carpet, little loss, doesnt feel cold.
• Your sense of touch detects the temperature of
  your skin, not the temperature of the thing you
  are touching.
• This is common theme of exam questions.

38
Sec. 8.2 Convection

• I carry a hot cup of coffee across the room.
• Heat (energy) has moved from here to there.
• This is an example of convection.
• Hot smoke from a fire rises up into the sky.
• Heat is carried upwards.
• This is another example of convection.

39
Forced vs Natural

• The rising of the hot smoke is natural
  convection because the hot gas expands, becomes
  less dense, and rises due to buoyant forces.
• No one forced the heat to rise.
• My carrying the hot coffee would be forced
  convection because it was not movement caused by
  its being hot.

40
Mix It Up

• Opening a door during summer and letting warm air
  into the house is another heat exchange due to
  convection.
• Unlike conduction, heat exchange via convection
  can involve the exchange of molecules.

41
Convection Summary

• Convection is when material containing thermal
  energy moves.
• That movement can cause one place to get warmer
  and another to get colder.
• Natural convection will always cause a hot region
  to get cooler while a cold region gets hotter.

42
Sec. 8.3 Radiation

• Light is electromagnetic waves or
  electro-magnetic radiation.
• Radiation is light.
• And light contains energy.

43
All objects emit light

• Everything, everywhere, all the time emits light.
• Very hot objects will emit visible light (like
  light bulbs, the Sun, or flames).
• Cooler objects emit light as well, usually
  infrared radiation that our eyes dont detect.

44
Different types of light

• This is figure 12.3 from section 12.1 which will
  be covered later in the quarter - but learning
  the names (radio, microwave, infrared, visible,
  ultra-violet, X-ray, gamma ray) now wouldnt hurt.

45
Light and Energy

• Light waves carry energy.
• Radio and microwave are low-energy light.
• Infrared and visible are medium-energy.
• Ultraviolet, X-ray, Gamma ray are high-energy.
• Emitting light is emitting energy.
• Absorbing light is gaining energy.

46
Emission and Absorption

• Everything emits radiation (light) continuously.
• Human bodies, walls, stars, trees, pencils,
• Everything absorbs light continuously.
• There can be a net gain or loss of energy (heat)
  depending on the difference between the total
  energy it emits or absorbs.

47
Radiation Emission

• Hotter objects emit more radiation than otherwise
  identical objects that are cooler.
• Larger objects emit more radiation than otherwise
  identical objects that are smaller.
• Actually, the larger or smaller needs to be
  judged in terms of the amount of surface area.
• Emission also depends on the objects color.

48
The Color Black

• Objects that are more black in color emit more
  radiation than otherwise identical objects that
  are more white in color.
• Really!
• Black objects may not reflect much light, but
  they do emit more light.
• An infrared camera would show people wearing
  black to glow more brightly than those wearing
  white.

49
Radiation Absorption

• Hotter and colder objects will absorb at the same
  rate if they are otherwise identical.
• Larger (in surface area) will absorb more light
  than smaller (if otherwise identical).
• Dark objects absorb more light than light
  (whiter) objects - which reflect light instead of
  absorbing.

50
Summary Emitting Radiation

• All objects do it.
• Lose energy faster by radiation when
• They are hotter
• They are bigger
• They are darker

51
Summary Absorbing Radiation

• All objects do it.
• Gain energy faster by radiation when
• They are bigger
• They are darker
• And, obviously, when there is more radiation
  directed towards them.

52
Question 1

• Why are black shirts hotter than white shirts on
  sunny days?
• The black shirt absorbs much more sunlight than
  the white.
• The black shirt will emit more radiation but its
  equilibrium temperature is higher.
• Similarly, black cars, blacktop streets,
  dark-painted houses will all get hotter in the
  sunlight than their lighter-colored counterparts.

53
Question 2

• Why are cloudy nights warmer than clear nights?
• Whether there are clouds or not, the ground emits
  radiation upwards, losing energy.
• On cloudy nights, the clouds emit radiation
  downwards which the ground absorbs.
• On clear nights, there is virtually no radiation
  coming down from (cold, empty) outer space.
• Larger net loss of energy for ground when clear.

54
Question 3

• Why isnt the Earth as cold as outer space?
• First, outer space isnt always cold, the very
  thin gases filling space are hotter than the
  Earth in some places and cooler elsewhere.
• The Earth, and gases near the Earth, both gain
  energy from the Suns radiation equal to the
  energy they radiate out into space.
• The Earths temperature is just that needed to
  maintain that balance (thermal equilibrium).

55
Sec. 8.4 Newtons Law of Cooling

• Objects that are hotter than their surroundings
  will lose more heat than they gain (whether the
  exchange is due to conduction, convection,
  radiation, or some combination).
• Objects cooler than their surroundings will gain
  heat energy and warm up.

56
Objects and Their Surroundings

• If the object and the surroundings are at the
  same temperature (thermal equilibrium), there
  will be no net heat flow.
• If parts of the surroundings are hotter than the
  body and other parts cooler, whether the object
  gains or loses heat will depend on the details of
  all those interactions.

57
Newtons Law of Cooling

• Newtons law of cooling deals with the rate of
  heat flow between an object and its surroundings
  (where we will assume the surroundings are at a
  uniform temperature).
• The rate at which an object will gain or lose
  heat is (approximately) proportional to the
  temperature difference between the object and its
  surroundings.

58
Applications

• Another stupid, pointless law?
• No, this is easily used to answer many questions.
• Your soda is warm and you want it cold as soon as
  possible. Will it cool faster in the freezer than
  the refrigerator or will it be the same either
  way?

59
Soda Cooling Problem

• According to Newtons law of cooling, the soda
  will lose heat the fastest when the difference
  between its temperature and the surroundings is
  the most,
• It will cool faster in the freezer!
• Lets put in some numbers to better see how this
  would work.

60
Soda Cooling Numbers

• Soda Temperature 70F, Refrigerator 40F, Freezer
  25F
• Refrigerator ?T 30, Freezer ?T 45
• Rate of heat loss is proportional to this
  temperature difference so the soda will lose heat
  50 faster (45/301.50) in the freezer than the
  refrigerator.
• Wait, it gets better.

61
Soda Numbers (cont.)

• Once the soda has cooled to 50F, refrigerator ?T
  10, freezer ?T 25.
• So the soda will be cooling 2 1/2 times faster
  once it reaches this temperature.
• Just how long will it take the soda in the
  refrigerator to reach the temperature of the
  refrigerator (40F)?

62
Time to Cool

• Mathematically, it will take infinite time for
  the soda to fully cool.
• The refrigerator stays at 40F, the soda cans
  temperature drops 72 to 56 to 48 to 44 to 42 to
  41 to 40.5 to 40.25 etc.
• As the temperature difference decreases, the rate
  of cooling slows.

63
Warming of objects similar

• A cold soda left in the open will also approach
  room temperature at ever slower and slower rates.
• The time is not infinite because there are always
  variations in temperature, if the temperature in
  the refrigerator momentarily rises to 41F, the
  soda then catches up and is the same temperature
  as everything else.

64
Household Radiator

• Radiators are common
• in houses and buildings.
• They work by running hot
• water or steam through the pipes which then heat
  the surrounding room.
• Q. Do they work primarily by conduction,
  convection, or radiation?
• And why are they always white or silver?

65
Best Answer Convection

• The pipes become very hot because of the hot
  water or steam passing through them.
• The air next to the pipes is heated by
  conduction, but the air is a pretty good
  insulator so the heat does not get into the rest
  of the room by conduction.
• The hot air does rise due to convection and most
  of the heating of the room is due to the
  convecting air.

66

• The hot air rises, drawing in cooler air and that
  process repeats continuously.
• Note how radiators are always placed on the floor
  and the pipes are always designed to allow for
  vertical movement of air.
• Maybe they should be called convectors rather
  than radiators!
• What about radiation and why are the pipes
  painted white?

67

• The pipes will radiate, mostly infrared.
• Darker pipes would radiate more than white pipes,
  so painting them white is to reduce the amount of
  radiation.
• By limiting the heat loss by radiation, the pipes
  will be a little hotter and more heat loss will
  occur by conduction/convection.
• Painting the pipes white is done to maximize
  convection (intense infrared radiation can feel
  unpleasant).

68
How does a Thermos bottle work

• A vacuum (Thermos) bottle is designed to
  minimized all three forms of heat transfer a
  vacuum reduces conduction, mirrored sides
  minimize radiation, and a tight lid minimizes
  cooling of the air above the liquid's surface by
  convection.

69
How can a person fire walk?

• The hot coals can be poor conductors so that the
  rate of heat flow is slow and your feet dont
  burn in the time it takes to walk across.
• It also helps if your feet are wet or damp for
  reasons well learn about later.

70
Melting of snow on a sunny day

• Heat transfer from Sun due to radiation (all our
  energy from the Sun is by radiation).
• Snow gains energy and melts.
• If a dark powder is spread over the snow making
  it darker, it absorbs more of the sunlight and
  melts faster.

71
Why does a candle flame go up?

• Convection, the burning of wax at the wick heats
  air which rises carrying the flame upward.
• On the space station (free fall), candles just
  make little flame balls which quickly go out
  because in the weightless (or should I say free
  fall?) condition there is no natural convection
  of hot gases.

72
Heating a house with a fireplace

• Mostly by radiation.
• Hot gases mostly go up the chimney due to
  convection.

73
Heating a house with a furnace

• Mostly by (forced) convection.
• The natural gas (or whatever) is used to heat air
  which is pushed into the rooms while cooler air
  is drawn in elsewhere.
• Enough examples, back to the text.

74
Back to chapter 7 Sec. 7.6 The Laws of
Thermodynamics

• The first law of thermodynamics is
• Whenever heat flows into or out of a system,
  the gain or loss of thermal energy equals the
  amount of heat transferred.
• The first law is also known by another name
  conservation of energy.
• Why not just call it conservation of energy
  rather than the first law of thermodynamics?

75
Why its called the first law

• Because the the first law was originally
  formulated for caloric, before caloric was
  discovered to be energy.
• The basic rule is straightforward,
• However much heat is added or removed from an
  object must equal the change in the total energy
  content of that object.

76
The First Law

• Little needs to be said about the first law.
• We have already learned about energy conservation
  and we have already been applying the first law
  regularly when talking about thermodynamics
  systems.
• On to the second law

77
The second law of thermodynamics

• Heat never spontaneously flows from a cold
  substance to a hot substance.
• It doesnt, but why doesnt heat ever flow
  spontaneously from cold to hot?
• And is it okay for heat to flow from cold to hot
  as long as it is not spontaneous?

78
What if it could?

• Before answering those other questions, lets
  consider what it would mean if, when cold and hot
  objects are put together, the cold could become
  colder and the hot hotter.
• That would be amazing!
• It would change everything.

79
Violation of the Second Law

• It would allow
• Perpetual motion machines
• Cars that need no fuel
• Refrigerators that dont have to be plugged in
• That would be good stuff.
• So, why cant heat naturally flow from cold to
  hot?

80
Explanation for Conduction

• As we discussed before.
• When fast (hot) molecules collide with slow
  (cold) molecules, the usual result is two
  medium-speed molecules.
• Only if the majority of the billion billion
  collisions were the extremely unlikely result
  would heat go the wrong way.

81
Convection and Radiation

• There are similar arguments for convection and
  radiation.
• The second law is really a statistical law, heat
  can go cold to hot but the probability of that
  happening is so overwhelmingly unlikely that you
  are best off thinking it strictly impossible.

82
Things were going to skip

• Probabilities and temperatures can be connected
  using entropy but we wont be doing that in
  this class.
• The third law of thermodynamics
• No system can reach absolute zero.
• Not important for us, skip.

83
Sec. 7.7 Specific Heat Capacity

• Add 4187 joules of heat to one kilogram of water
  and the temperature will go up 1C.
• Add 900 joules of heat to one kilogram of
  aluminum and the temperature goes up 1C.
• Each substance is different.
• Some require more heat than others to change
  their temperature.

84
Specific Heat

• The heat needed to change the temperature of 1 kg
  of a substance by 1C is called the specific
  heat capacity (or just specific heat).
• The specific heat of water is 4187.
• The specific heat of aluminum is 900.
• Both are 1 kg, why different amounts?

85
Why different specific heats

• Why do different materials have different
  specific heats?
• There are two main reasons
• One kilogram will be different numbers of
  molecules for different substances.
• The heat added can go into other forms of energy
  besides kinetic energy.

86
Different Numbers of Molecules

• A single aluminum molecule (atom) has more mass
  than a single H2O (water) molecule.
• So one kilogram of aluminum is fewer molecules.
• The added heat is shared among fewer molecules
  and each gains more kinetic energy, hence a
  higher temperature.
• This, in part, explains why it takes less energy
  to increase the temperature of aluminum as
  compared to water.

87
Other forms of energy

• H2O molecules can have kinetic energy but can
  also have energy of vibrations and rotations.
• The temperature of a substance depends only on
  the (average) kinetic energy.
• When heat is added to water, only a fraction goes
  into kinetic energy, the rest goes into those
  other types of motions.

88
Waters High Specific Heat

• Water has a very high specific heat.
• In part because H2O molecules are light and there
  are a lot of them in one kilogram.
• And because a lot of the added heat goes into
  vibrations and rotations of the molecules and not
  into kinetic energy.
• This is why wet feet help for fire walking.

89
Example Using Math

• One kilogram of aluminum at 100C is put into two
  kilograms of water at 20C, what will be the
  equilibrium temperature of the mixture?
• First Law The heat lost by the aluminum will
  equal the heat gained by the water (assuming no
  other heat exchanges occur).

90

• We are trying to find the final temperature, call
  it T.
• The formula for the heat required to cause a
  temperature change is
• Q m c ?T
• Q heat m mass c specific heat ?T change in
  temperature
• For the aluminum, the heat lost is
• (1) (900) (100 T)

91

• For the water, the heat gained is
• (2) (4187) (T 20)
• Now we set these equal,
• 900 (100 T) 8374 (T 20)
• 90,000 900T 8374T 167,480
• 257,480 9274T
• T 257,480/9274 27.8 C
• There is no homework like this, but you may do
  similar calculations in lab.

92
A Heated Debate?

• You are in a restaurant and your coffee was just
  served but it will be another 5 or 10 minutes
  before you get your food, which is when you will
  drink the coffee. In order that it be as hot as
  possible when you drink it, should you pour in
  the room-temperature cream right away or when you
  are ready to drink the coffee? Discussion? Vote?

93
Answer

• The energy lost from the coffee to warm the cream
  is basically the same in both cases.
• So maybe waiting or not doesnt matter?
• No, to maximize the temperature of the coffee
  when you drink it, you should pour the cream in
  right away.
• There are a couple reasons why.

94
Newtons Law of Cooling

• We learned that the greater the temperature
  difference between object and surroundings, the
  faster the heat will flow.
• So, the hot coffee will lose heat faster than the
  cooler coffeecream will.
• That pretty much proves it right there, add the
  cream right away.

95

• Newtons Law of Cooling is only an approximation
  of behaviors.
• Lets consider all heat transfer mechanisms to
  double check our guess.
• The energy lost as radiation by the coffee is
  less when the cream is added because
• the coffee is less hot
• the coffee is less dark

96
Maybe I Dont Want Any Cream

• Conduction of heat through the cup and to the
  table will be more when the temperature
  difference is more, adding cream sooner is
  better.
• The greatest heat losses will probably be due to
  evaporation/convection. And that will definitely
  cool the coffee faster when it is hotter.
• All the analysis supports adding cream sooner.

97
Sec. 7.8 Thermal Expansion

• Molecules are always moving. Bumping against each
  other and beating out a space for themselves.
• When temperature increases, the molecules have
  more kinetic energy. They are moving faster and
  push more on their neighbors.
• At higher temperatures, each molecule (and hence
  the entire object) occupies more space.

98
Heat it and it grows

• This is called thermal expansion.
• As materials get hotter, they get bigger!
• Heat a metal rod and it will grow a little longer
  (and a little wider).
• Railroad tracks can ex-
• pand and buckle on a
• very hot day.

99
Expansion Joints
Expansion joints (connections that expand shorter
or longer) are ubiquitous, youll see them all
over the place.
100
Thermal Expansion of Water

• Water has very unusual behavior.
• Frozen water (ice) is less dense and bigger than
  the liquid water.
• This occurs because ice forms into crystals that
  have a hexagonal arrangement of molecules. This
  takes up more space than a normal, random
  arrangement.

101
Ice Water

• Because of ices bloated shape, water expands
  when it freezes.
• Water can shrink when heated and expand when
  cooled.
• An example

102
Cooling Water

• Imagine a beaker of water at room temperature
  (about 20C).
• Cool it (remove heat).
• Water shrinks, this is normal behavior.
• At 4C, little hexagons of ice start to form and
  the volume of water now increases as it is
  cooled, unusual behavior.

103
Water at 4C is the most dense water.

• In a very cold pond, the coldest water (0C) is
  at the top while the densest water (4C) is at
  the bottom.
• This is why water (ice cubes, ponds, pools,
  lakes) freeze starting at the top.

104
Heated Ring

• A metal ring is heated, does the central hole
  become larger, smaller, or stay the same size?
• What do you think? Vote?
• What is your reasoning?

105
The Hole Gets Larger

• Ill try explaining this answer in a variety of
  ways.
• First, imagine heating a complete disk of metal
  (no hole) but with a circle drawn on it.
• The metal expands when heated and that circle
  will get bigger.
• Cut out the metal within that circle both before
  and after and you have exactly the problem we
  were doing, the hole gets larger.

106
Another Argument

• Imagine four long skinny metal rods joined at
  their corners to make a square.
• What will happen when this is heated?
• All the rods expand, each rod will grow more
  lengthwise than across.
• The square will have longer sides and a bigger
  area (hole!) in the middle.

107
Still Another Argument

• Everyone agrees that cooling the ring will make
  the hole smaller.
• So heating it must undo that, make the hole
  larger again.
• Otherwise repeatedly heating and cooling would
  keep making the hole smaller and smaller and
  smaller.

108
The Practical Application

• If the metal lid on a jar is too tight to
  unscrew, you heat the lid (maybe by running hot
  water over it).
• The lid expands, the hole gets bigger, and the
  lid unscrews easily.

109
Expand This

• A piece of metal is bent into a C-shape.
• When the temperature is increased and the metal
  expands, will the gap between the ends become
  narrower, wider, or remain unchanged?
• Guesses? Why?

110
Wider

• The gap will expand just as much as a metal
  segment in the gap wouldve expanded.
• The marked spots on the left edge will stay
  aligned with ends of the gap, both will expand.

111
Sec 8.5-8.10 Changes of Phase

• Phases of matter
• Solid, liquid, gas, plasma
• (Dont worry about plasma.)
• Molecules in solids all have fixed positions with
  only slight motions (vibrations).
• Attractive forces hold the molecules rigidly
  making it a solid.

112
Solid to Liquid, Melting

• Add heat to a solid and the molecules gain energy
  vibrate more.
• More heat and the molecules can break away from
  their previous positions and move around like
  how the chocolates in a box of candy can move
  about if the box is shaken too hard.
• Melting has occurred.

113
Melting Requires Energy

• Molecules lose energy pulling away from each
  other, the electric attraction that held them
  together does work on the molecules, converting
  their kinetic or vibrational energy into
  electrical potential energy.
• Yet, molecules will have the same average kinetic
  energy just before and just after the melting.
• Huh? Now Im confused.

114
Added Heat Breaks the Bonds

• The kinetic energy of vibrating molecules is not
  enough for them to break out of the bonds holding
  them next to their neighbors.
• Add heat, increase the kinetic energy, and they
  can break free.
• But they lose energy as they pull away and are
  typically back to the same kinetic energy they
  had before breaking free.

115
Melting Occurs at Constant Temp

• If the average kinetic energy of the molecules in
  the solid just before melting and the liquid just
  after melting are the same, that means the
  temperatures are the same.
• When something is melting, the added heat does
  not change the temperature, instead it causes
  bonds to be broken and the phase to change.
• All phase changes occur at constant temperature.

116
Liquid to Solid, Freezing

• You can turn a liquid into a solid by removing
  energy, called freezing.
• The molecules are pulled into their positions in
  the solid, gaining energy from the work of
  electrical forces.
• Although you remove energy to achieve freezing,
  the average kinetic energy of the molecules is
  unchanged.

117
Heat of Fusion

• The heat needed to melt a solid, or the heat that
  must be removed to freeze a liquid, is called the
  heat of fusion.
• For water, the heat of fusion is 334 J per gram
  (about 80 cal).
• To freeze 1 kg 1000 g of water at 0C into
  ice, 334,000 joules of heat must be removed.
  Formula Q mL

118
Liquid to Gas, BoilingGas to Liquid,
Condensation

• The heat added to boil a liquid into a gas is
  energy used to pull the molecules apart.
• Heat must be removed from a gas to condense it
  into a liquid, this equals the energy gained by
  the molecules as they are pulled back next to
  each other by electrical forces.

119
Heat of Vaporization

• The energy that must be added in boiling, or
  removed in condensation, is called the heat of
  vaporization.
• For water it is 2256 joules per gram.
• Like melting/freezing, boiling/condensation are
  constant temperature processes.

120
Temp Change vs Phase Change

• Imagine a block of ice at -20C.
• Add heat to the block.
• Its temperature will rise in accordance with its
  specific heat (Q m c ?T).
• When the ice reaches 0C, the added heat stops
  causing temperature changes, instead it causes
  phase changes in accordance with the latent heat
  equation (Q mL).

121

• Once all the ice is melted (into water at 0C),
  added heat will again cause temperature changes.
• Added heat will raise the temperature until the
  water reaches 100C.
• Now boiling occurs, adding heat changes the phase
  without changing the temperature.
• Once all the water is boiled into steam, added
  heat will raise the temperature of the steam
  according to the specific heat of steam (which is
  different than cice and cwater).

122
Question

• Touch the inside of a 200C hot oven and you burn
  yourself. But when the 1800C white hot sparks
  from a 4th-of-July-type sparkler hit your skin,
  youre okay.
• Why?

123
Burning Questions

• You will get burned when touching something only
  if
• It is hot enough (so that heat will flow from it
  to you).
• It contains enough energy (it is the amount of
  energy absorbed that will burn you, it is not the
  temperature of the energy, that doesnt even
  make sense).
• The contact is long enough in duration (how long
  is long enough to burn depends on how readily the
  contact allows heat to flow).

124
Answer

• Why does the 200C oven burn you but the 1800C
  spark doesnt?
• Both are hot enough.
• The oven contains enough energy but the spark
  does not the spark cools to room temperature
  after transferring a measly fraction of a joule,
  not enough to burn you.

125
To Burn or not to Burn

• Why doesnt this burn?
• Touching aluminum foil that was just in the oven.
• Stepping into a hot Jacuzzi.
• The Sun/sunlight.
• Steam rising from boiling water.

126
Aluminum Foil from Oven

• The aluminum foil is hot enough and can quickly
  transfer energy to you.
• But the thin foil and aluminums low specific
  heat mean it just doesnt contain enough energy
  to burn.
• Drops of oil on the foil are a different story,
  they can burn you.

127
Stepping into a hot Jacuzzi

• Enough energy? Yes
• Long enough duration? Yes
• Efficient energy transfer? Yes
• Hot enough? No
• Being only slightly hotter than your skin, you
  can lose heat (sweating) about as fast as the
  Jacuzzi can add heat.

128
The Sun/Sunlight

• Is the Sun hot enough? Yes
• Does it contain enough energy? Yes
• Is the contact long enough or the heat transfer
  method efficient enough? Not usually.
• You can get a sunburn but only if you allow the
  sun to hit your skin for a lengthy time.

129
Steam from Boiling Water

• Hot enough? Yes
• Contains enough energy/contact long enough?
  Usually not.
• You can get burned by steam, especially steam
  that is much hotter than 100C (which you might
  run into in a power plant but not on your stove
  top).
• Only holding your hand in the steam for a lengthy
  time will result in a burn.

130
Evaporation

• Some water at room temperature.
• By chance, some water molecules will be moving
  faster than others.
• Some will be moving so fast that they will break
  out of the liquid state.
• These molecules escape and there is now less
  water remaining.

131
Evaporative Cooling

• Because only the fastest (hottest) molecules are
  the ones that escape, the remaining molecules are
  cooler on average.
• Evaporation causes the water to cool.
• This is called evaporative cooling.

132
Sketch of Evaporation
133
Just the opposite

• A water (steam) molecule that condenses from gas
  to liquid gains energy as it is pulled next to
  the water (liquid) molecules.
• The liquid will gain energy due to this
  condensation, get warmer.
• Depending on conditions, the evaporation and
  condensation may occur at equal rates having no
  net effect on the water or air.

134
Other Phase Changes

• Sublimation solid to gas transition (without
  passing through a liquid phase)
• Example dry ice (frozen CO2)
• Deposition gas to solid phase change
• Example frost (water vapor in air going
  straight to a snow-like solid form)

135
Boiling Point

• For a molecule to boil (escape from the liquid
  phase into the gas phase), it must both pull away
  from the attractive force of the neighboring
  liquid molecules, and push its way into and among
  the surrounding gas molecules.

136
Almost time for exam 3

• If the pressure (density) of gas molecules is
  greater, the liquids molecules need more energy
  to escape and the boiling temperature will be
  higher.
• Lower pressure and the liquid will boil at a
  lower temperature with less energy per molecule.

137
End Chapter 7 8 Lecture

• Another dreaded midterm on the way