Chapter 12
Thermal Energy
Heat and Temperature
Light
Law of Conservation of Energy?
When rub your hands together
• What is the Net KE?
– Zero, the motion is in opposite directions.
• What is the change is PE?
– Zero, your hands are at the same height.
• You did work against friction, where did the
energy go?
• You did work against friction, where did the
energy go?
Thermal Energy
• Thermal Energy: The total internal Energy
• Internal Energy: The sum of the kinetic
and potential energies of the internal
motion of particles that make up an object.
Kinetic-molecular Theory
• All matter is made up of molecules and
atoms
• Molecules are in constant motion
• Objects in motion have Kinetic Energy.
Kinetic-molecular Theory
• Molecules are in constant motion
• Objects in motion have Kinetic Energy.
• When particles get hotter, they move faster
(e.g. higher Kinetic Energy)
Heat
Heat Applet
The Nature of Matter
• Molecules in liquids and gases move freely.
• Molecules in solids simply vibrate.
• This means that all molecules possess their own
kinetic energy (KE), the energy of motion.
Solid
Liquid/Gas
Temperature
• Molecules move at different speeds.
• Now let’s add some heat?
– Now they are all moving faster!
• Temperature relates to the average kinetic
energy of the molecules in a substance.
The higher the temperature,
the faster the molecules
move.
Temperature Scales
• Celsius
– Water boils at
– Water at Freezes
100oC
0oC
C
F
100
212
• Fahrenheit
– Water boils at
– Water Freezes at
212oF
32oF
0
• Kelvin
– Absolute Zero
– Water boils at
– Water Freezes at
-40
0ºK
373oK
273oK
32
-40
Temperature Scales conversions
Fahrenheit
Celsius
Range 180
18 9
 
10 5
Range 100
Fahrenheit scale freezes 32o above Celsius
9
TF  TC  32
5
Temperature Scales conversions
Celsius Range 100
Fahrenheit
10 5
 
Range 180 18 9
Celsius scale freezes 32o below Fahrenheit
5
TC  TF  32 
9
Temperature Scales
• There are three major temperature scales that we will
deal with.
– The Fahrenheit Scale (°F)
– The Celsius Scale (°C)
– The Kelvin Scale (°K)
• Conversion formulas are shown below.
TF  95 TC  32
TC  95 TF  32
TK  TC  273.15
TC  TK  273.15
Sample Temperature Conversion
• A beaker of water at room temperature is
measured to be at 21°C.
• What is the Fahrenheit and Kelvin temperature?
TF  95 TC  32
TK  TC  273.15
TF  95  21  32 TK  21  273.15
TK  294.15K
TF  69.8F
Temperature Scales conversions
Example 1
• Convert 45ºF to Celsius
5
TC  TF  32 
9
5
TC   45o  32 
9
5
TC  13 
9
TC  7.2o C
Temperature Scales conversions
Example 1
• Convert 32ºC to Fahrenheit
9
TF  TC  32
5
9
TF  (32)  32
5
TF  57.6  32
TF  89.6
Homework
• WS1 1-5
• WS 1a 1-3
Absolute Zero
• The final temperature to note is absolute zero, 0K.
• This is the lowest possible temperature.
• Here molecules are in a complete state of rest, which
means that there is no kinetic energy.
• Absolute zero has never been reached.
• However, scientists have come within 0.1K
• Pressure versus Temp graphs
0K
Thermal Contact
• KE transfer through thermal contact
COLD
WARM
KE
KE
Warm
KE
KE
HOT
Thermal Equilibrium:
– Both objects have the same average Kinetic
Energy
What is Heat?
• Heat is energy in transfer from an object of
higher temperature to one of lower temperature.
• The quantity of energy transfer from one object
to another because of a difference in
temperature.
Heat Flow
Cold
Warm
Warm
Hot
Thermal Energy
• The sum of all the kinetic energies within a material is
known as thermal energy.
• Both full cups of coffee are at the same temperature.
– Which cup contains greater thermal energy? B
– Which cup contains a higher average kinetic energy within the
molecules? Both are the same!
A
B
Heat & Kinetic Energy
• Both Brass blocks are at 115oC.
Brass
Brass
• Which block has the higher KE?
• Which block has the higher average KE?
Homework
• WS #1 4-9
• WS #1 4-13
Specific Heat Video Clip
Specific Heat
Heat: The energy that flows as a result of a
difference in temperature
Q: The symbol for heat. Measured in Joules
Specific Heat (C): The amount of energy needed
to raise a unit of mass one temperature unit.
(J/kg K)
Heat gained
Q  mCT
Q: Heat
m: mass
ΔT: Change in Temperature
Sample Heat Problem
• How much heat is required to raise the temperature of a
2.4kg gold ingot (c = 129J/kgK) from 23°C to 45°C?
T  TF  TI
T  45C  23C
T  22C
Q  mcT
J
Q   2.4kg  129 kgK
Q  6811.2 J

Light
2.4kg
  22C 
Heat Transfer
Example 1
A 0.40kg block of iron is heated from 295K
to 325K. How much heat is absorbed by
the iron?
C  450 KgJ K
Q  mCT
Q  (.4kg)(450 KgJ K )(325K  295K )
Q  5400 J
Heat Transfer
WS3 #1
How much heat is absorbed by 60.0g of
copper when it is heated from 20ºC to 80ºC
60 g  .06kg
C  385 KgJ K
80o C  353K
Q  mCT
Q  (.06kg )(385 KhJ K )(353K  293K )
Q  1386 J
Practice Problems
• WS 3a
– #’s 1-3
Specific heat
Calorimerty
Calorimeter: A device used to measure
changes in thermal energy.
Calorimerty Utilizes on the conservation of
energy.
Qgained + Qlost = Zero
Calorimerty Example
A .4kg block of zinc
@115°C is placed in
.5kg of water @15°C.
Find the final
temperature.
C  388 kgJ K
Qgained  Qlost  0
maCa Ta  mbCb Tb  0
maCa (Tf  Tai )  mbCb (Tf  Tbi )  0
J
J
.4kg(388 kgK
)(t f 115 oC)  .5kg(4180 kgK
)(t f 15 oC)  0
Tf  21.9 C
o
Calorimerty Example
A .1kg block of brass @90°C is placed in .2kg
of water @20°C. Find the final temperature.
maCa Ta  mbCb Tb  0
.1kg (376 kg Jo C )(t f  90 oC )  .2kg (4180 kg JoC )(t f  20 oC )  0
Tf  23.0o C
Practice Problems
• Book Page 252
– #’s 9-11Calorimetery
• WS4
– # 7-11 Calorimetery
WS #3
– Problem # 1 a-d
– Problem #2 a-d
• Quiz
Latent Heat (Enthalpy)
• Describe what Q=mCΔT means
• Is it possible to add heat without changing the
temperature?
• A heat transfer is also required to change phase.
• Heat must be added to go to a more energetic phase.
• Heat must be removed to go to a less energetic phase.
+Q
Solid
+Q
Vapo
r
Liquid
-Q
-Q
Substances in Phase Transfer
C
F
100
212
0
40
32
40
Heats of Fusion and Vaporization
• Heat of fusion (Hf): the amount of heat energy required
change 1kg of substance in the solid state into the liquid
state.
• The equation is as follows:
Q  mLf
• The heat of vaporization (Hv) is the amount of heat
energy required to change 1kg of substance in the liquid
state into the vapor state.
• The equation is as follows:
Q  mLv
Sample Heating Across Phase
• A block of ice (m = 1.2kg) at -10°C being heated
to water at 80°C. How much heat is used?
3 Steps
Heating
Ice -10°C
Ice 0°C
Q  mcT
Q  mcT  mLf  mcT


Melting
Q  mH f

Water 0°C
Q  1.2kg  2060 kgJC 10C   1.2kg  3.34 105
J
kg
Heating
Q  mcT
Water 80°C
  1.2kg   4180
Q  826800 J
• These problems have 1-5 steps.
J
kg C
 80C 
The Heating Curve (H2O)
• As H2O is heated from ice below freezing to steam
above boiling, the temperature can be plotted with
respect to time.
• Where are the flat spots located?
Temperature remains
constant during
phase changes.
Heating Curve of Water
Boiling
Heating
Melting
Heat
Heat
140
Once each phase
change is complete,
temperature can rise
again.
Temperature (C)
120
100
80
60
40
20
0
-20 0
5
10
Time (min)
15
20
The Cooling Curve (H2O)
• The cooling curve is very similar to the heating curve
• What can you conclude about temperatures when cooling
between phases?
The temperature can
continue to drop after
all of the substance is
converted into the next
phase.
Cooling Curve of Water
Condensing
Solidifying
Cooling
Cool
140
120
Temperature (C)
Cooling also requires
the temperature to be
held constant while in
the midst of a phase
change.
100
80
60
40
20
0
-20 0
5
10
Time (min)
15
20
Phase Change WS7 #1
•
•
•
•
Between A-B: The ice is warming to 0oC
Between B-C: Thermal energy melts the ice at 0oC
Between C-D: The water is warm to 100oC
Between D-E: The water boils and changes to vapor at
100oC
• After E:
The temperature of the vapor increases
Latent Heat
Temperature (C)
150
D
100
C
50
B
0
-50
A
Time
E
Practice Problems
• WS #7
– # 1 b-g
– # 2,3
Heat of fusion WS8 #1
If 5,000J is added to ice at 0oC, how much ice is
melted?
Q=mLf
Water
Water
Q=mLv
Hf=3.34x105J/kg
Hv=2.26x106J/kg
m  Q / Lf
m  5000J / 3.34 x105 J / kg
m  .015kg
Heat of fusion WS8 #2
How much heat must be transferred to 100g of ice
at 00C until the ice melts and the temperature of
the resulting water rises to 200C?
Water
Hf=3.34x105J/kg
Qmelt ice  mH f
Q  .1kg (3.34x105 kgJ )
Q  3.34x10 J
4
Qheat water  mCT
Q  .1kg (4180
Q  8360 J
Qtotal  3.34 x104 J  8360 J
Qtotal  41760 J
J
kg 0 C
0
)20 C
Homework
WS #8
– #’s 3-5
• Book Page 255
– # 16 Heat of fusion
• WS #7 (5 heats)
Thermal Energy Transfer
• Conduction:
– Transfer of Kinetic Energy by contact
• Convection:
– Heat transfer by the motion of a fluid (e.g. air)
• Radiation:
– Electromagnetic waves carry energy
Note: Conduction and Convection require matter
Thermal Energy Transfer video Clip
Conduction
• Conduction is the transfer of heat through molecular
collisions.
• This form of heat transfer best occurs in solids where
molecules are closely packed.
• Materials that conduct heat well are called conductors.
(Eg. metals such as copper and iron)
• Materials that conduct heat poorly are called insulators.
(Eg. foam, air, and asbestos)
Magnification
Convection
• Convection is the transfer of heat though moving fluids.
• A fluid is any substance that flows, which includes all
liquids and gases.
• Examples include convection ovens and cloud formation.
Convection
?
The Heating Water Pot
• Have you ever watched a pot of water when it is being
heated, especially while boiling? What do you notice?
• You will notice that there are currents within the pot.
The process of convection
transfers heated water from
the bottom of the pot to the
top, where it is exchanged
for cooler water.
Convection
• Gulf stream current
• http://rads.tudelft.nl/gulfstream/#fig2
Convection
It was remarkable how weightlessness
affected things: I found that if I stayed
perfectly still, my own body heat built up
around me like a tenuous blanket, because
there was no convection to carry it away.
But my slightest motion dispelled this
warmth and let me cool.
– Jim Lovell, Apollo 13
Radiation (not radioactivity)
•
•
•
•
Radiation is the transfer of heat via electromagnetic waves.
These waves include visible light, but are mostly infrared.
No matter is required for this type of heat transfer.
Examples include the sun’s heat and warmth felt from a
flame.
Open Space
Radiation
?
More on Radiation
• All objects emit heat in the form of radiation (radiant heat).
• Hotter objects emit more energetic waves.
• Some extremely hot objects can emit visible light, a form
of electromagnetic radiation.
• In fact, all forms of radiation travel at the speed of light.
 2.998 10 
8 m
s
Objects in thermal
equilibrium will emit
the same amount of
radiation that they
receive from other
objects.
Heat Transfer Question
• Consider a camp fire burning vigorously.
– How is heat normally transferred while warming its viewers?
• Radiation
– How is heat transferred when you put a hand in the smoke?
• Convection
– How is heat transferred to a stick when it is placed in the hot
coals?
• Conduction
• Some situations involve
multiple heat transfer
types like this.
Homework
• WS #4 1-6
– Thermal Energy Transfer
Homework
• WS #2
– Temperature Conversions
Thermal Expansion
• The change in length of a material due to
change in temperature.
Thermal Expansion Video Clip
• Expansion and Contraction movie clip.
Linear Expansion
• Most objects expand when heated and contract
when cooled
• The change in length of a solid is proportional to
ΔT
• The change in length is proportional to the length
of the object
L  Li   Li (T  Ti )
Linear Expansion animation
• Expansion Joint
Linear Expansion animation
• Expansion joint versus NO joint
Linear Expansion animation
• Expansion in a U joint
Linear Expansion animation
• Expansion in an elbow joint
Linear Expansion WS9 #1
A metal bar is 2.6m long at
210C. The bar is heat to 930C
and the length increases by
3.4mm. What is the coefficient
of linear expansion?
Li  2.6m
Ti  210 C
T  930 C
L  3.4 x103 m
L  Li   Li (T  Ti )
L   Li (T  Ti )
L
3.4 x103 m


Li T 2.6m(720 C )
Heat
  1.8x10 ( C)
5 0
1
Sample Linear Expansion Problem
• A train rail is initially 20m long in the morning when the
temperature outside is 10°C. By how much will the rail
expand in the heat of the day when the temperature
reaches 35°C?
• The coefficient of linear expansion for steel is:
  8.0 106 C 1
• Now find L.
L  LiT
L   20m   8.0 106   35C  10C 
L  0.004m
L  L f  Li
• What is the final length?
L f  Li  L
Lf  20m  0.004m  20.004m
Linear Expansion WS9 #2
• A piece of aluminum siding 3.66m long on a 28.00C winter day is how long on a hot 390C
summer day?
  25 x10
6
0
C
1
L  Li   Li (T  Ti )
6 0
L  3.66m  (25x10
1
C )(3.66m)(39 C  28 C)
L  3.666m
0
0
Practice Problems
• WS 9
– #’s 3-5
Linear Expansion
The Bimetallic Strip
• A bimetallic strip consists of two metal strips
pressed together into a single strip.
• A common strip consists of steel and brass.
• Since different materials expand at different
rates, the strip will bend.
Room Temp
Steel
Brass
High Temp
Steel
Brass
Low Temp
Steel
Brass
The Bimetallic Strip (Cont.)
• Bimetallic strips are most often used in temperature
sensitive instruments as thermostats.
• The strip is wound into a coil.
• As temperature changes, the coil expands or
contracts to activate a switch controlling a
heating/cooling system.
I’m
Cold!!
Hot!!!
Heat
AC
Expansion of Water
•
•
•
•
•
Water initially at 100C that cools to 40C
Water initially at 40C that cools to 00C
Water initially at 00C that freezes to 00C ice
Water initially at 100C that cools to 40C
Water initially at 100C that cools to 40C
Unique Properties of Water (Cont.)
• In liquid water, the molecules move freely in no particular
order or array. This is what allows them to flow.
• When water freezes, the molecules form a hexagonal pattern.
• Why may ice possess less density?
Solid Ice
Liquid Water
O
H
O
H
H
O
H
H
H
O
H
H
O
O
O
H
H
H
H
O
H
H
H
H
Open
Space
O
H
H
Applications of Linear Expansion
• Linear expansion must be taken into consideration for
engineers designing something that will experience a
range of temperatures.
• Here are just a few select examples:
Railroads
Bridges
Metal Roofs
Practice Problems
• WS #5 1-16 Thermal Expansion
End Unit on Heat
Latent Heat
Temperature (C)
150
100
50
0
-50
Time
C
F
100
212
0
40
32
40
Change of State
• States of mater
– Solid
– Liquid
– Gas
examples
• Examples
– Temperature scales
• Examples
– Convert Celsius to Fahrenheit
– Convert Fahrenheit to Celsius
•
•
•
•
Heat transfer types
Calorimeter and specific heat
Thermal expansion
Examples
– Conservation of energy transfer
C
F
100
212
0
40
32
40
Thermal Energy
COLD
HOT