Heat
Section 1
Preview
Section 1 Temperature and Thermal Equilibrium
Section 2 Defining Heat
Section 3 Changes in Temperature and Phase
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Heat
TEKS
Section 1
The student is expected to:
6E describe how the macroscopic properties
of a thermodynamic system such as
temperature, specific heat, and pressure are
related to the molecular level of matter,
including kinetic or potential energy of atoms
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Heat
Section 1
What do you think?
• Suppose you have two cups of water. One is hot
and the other is cold.
– How is the cold water different from the hot water?
• Describe the motion of the molecules in each.
– What changes would occur if the hot water was
changed into steam?
• What are the common scales used to measure
temperature?
– When is each scale generally used?
– All scales use degrees to measure temperature.
Which scale has the largest degrees? Explain.
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Heat
Section 1
Temperature
• Temperature measures the average kinetic
energy of the particles.
– Average speed is used because all particles do not
have the same speed, and speeds change as the
particles collide.
• Internal energy is the energy a substance has
due to the motion of the particles (kinetic energy)
and the position of the particles (potential
energy).
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Heat
Section 1
Forms of Internal Energy
Click below to watch the Visual Concept.
Visual Concept
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Heat
Section 1
Temperature
• Do thermometers change the temperature of the
substance being measured?
– If so, how?
– How can you minimize the problem?
• Temperature must be measured when thermal
equilibrium is reached.
• Always read a thermometer after it has stopped
rising or falling.
– At this point, equilibrium has been reached between
the thermometer and the substance.
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Heat
Section 1
Thermal Expansion
Click below to watch the Visual Concept.
Visual Concept
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Heat
Section 1
Thermometers
• The expansion of mercury changes the reading in this
thermometer.
• How does such a small change in the volume of the
mercury (see circled segments) result in such a large
rise inside the thermometer?
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Heat
Section 1
Thermometers
• Calibration depends on fixed temperatures.
• Three common temperature scales used:
– Fahrenheit for weather and medicine (U.S.)
– Celsius for work in science
– Kelvin or absolute for many scientific laws
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Heat
Comparison of Temperature Scales
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Section 1
Heat
The Absolute Temperature Scale
• What is meant by absolute
zero?
– Absolute zero = 0 K
• Suppose the pressure and
temperature of a gas are
plotted as shown, and the
graph is extrapolated to 0 K.
What does this suggest
about P at absolute zero
(-273.15°C or 0 K)?
– P = 0 at absolute zero
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Section 1
Heat
Temperature Conversions
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Section 1
Heat
Section 1
Classroom Practice Problems
• One day it was -40°C at the top of Mont Blanc
and -40°F at the top of Mount Whitney. Which
place was colder?
– Answer: Neither (-40°C = -40°F)
• What is the Fahrenheit temperature equivalent
to absolute zero?
– Answer: -459.67°F
• What is the Celsius temperature on a hot
summer day when the temperature is 100. °F?
– Answer: 37.8°C
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Heat
Section 1
Now what do you think?
• Suppose you have two cups of water. One is hot
and the other is cold.
– How is the cold water different from the hot water?
• Describe the motion of the molecules in each.
– What changes would occur if the hot water was
changed into steam?
• What are the common scales used to measure
temperature?
– When is each scale generally used?
– All scales use degrees to measure temperature.
Which scale has the largest degrees? Explain.
© Houghton Mifflin Harcourt Publishing Company
Heat
TEKS
Section 2
The student is expected to:
6E describe how the macroscopic properties
of a thermodynamic system such as
temperature, specific heat, and pressure are
related to the molecular level of matter,
including kinetic or potential energy of atoms
6F contrast and give examples of different
processes of thermal energy transfer,
including conduction, convection, and
radiation
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Heat
Section 2
What do you think?
• Internal energy is the energy due to the kinetic
and potential energy of the particles.
– Does the ice water or an equal
quantity of hot chocolate have
greater internal energy? Why?
– Which has more internal
energy, a gallon of cold water
or a drop of hot chocolate?
– How will the internal energy of
the water and hot chocolate
change over time?
• How will this change occur?
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Heat
Section 2
Internal Energy and Heat
• Internal Energy (U) is the energy contained
within the particles of a substance.
• Heat (Q) is the internal energy transferred
between objects.
– Heat always moves from a higher-temperature
object to a lower-temperature object.
– The rate of transfer depends on the difference in
temperature.
• The greater the temperature difference, the greater the rate
of energy transfer (if other factors are equal).
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Heat
Section 2
Internal Energy and Heat
• Which way does heat flow if you place a warm
canned beverage in cold water?
• How does this occur, on a molecular level?
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Heat
Section 2
Temperature and Heat
Click below to watch the Visual Concept.
Visual Concept
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Heat
Heat has Units of Energy (joules)
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Section 2
Heat
Section 2
Heat Transfer
• In what three ways can internal energy be transferred
from a hot object to a colder object?
– Conduction is the transfer of heat through a substance
by molecule to molecule contact.
• Metals are good conductors.
• Styrofoam is a good insulator.
– Convection is the transfer of energy by the movement
of a fluid.
• Hot air in a room rises and cold air moves in to replace it.
– Radiation is the transfer of energy by electromagnetic
waves.
• No matter is transferred, only energy.
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Heat
Section 2
Comparing Convection, Conduction, and
Radiation
Click below to watch the Visual Concept.
Visual Concept
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Heat
Section 2
Heat and Work
• Work can be changed into internal energy (U).
– Rub your hands together and you’ll feel the increase in
internal energy produced by your work.
– Pull a nail from a piece of wood and the nail is hot.
• Mechanical energy (PE + KE) is conserved when
there is no friction.
• Total energy, including internal energy, is always
conserved.
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Heat
Section 2
Energy Conservation
• Any loss of one type is balanced by a gain in the other
types of energy.
• Predict the sign (+, -, or 0) for the change in each
quantity when:
– A child slides down a plastic playground slide
– A car applies the brakes to stop on a level road
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Heat
Section 2
Now what do you think?
– Does the ice water or an equal
quantity of hot chocolate have
greater internal energy? Why?
– Which has more internal
energy, a gallon of cold water
or a drop of hot chocolate?
– How will the internal energy of
the water and hot chocolate
change over time?
• How will this change occur?
© Houghton Mifflin Harcourt Publishing Company
Heat
TEKS
Section 3
The student is expected to:
6E describe how the macroscopic properties
of a thermodynamic system such as
temperature, specific heat, and pressure are
related to the molecular level of matter,
including kinetic or potential energy of atoms
© Houghton Mifflin Harcourt Publishing Company
Heat
Section 3
What do you think?
• What property of water makes it so useful as a
coolant in automobiles, nuclear reactors, and
other machinery?
• How does it differ from other liquids regarding its
ability to cool substances?
• Why do you feel cool when getting out of a warm
swimming pool on a hot day?
• How do you feel if it is windy out? Why?
• How do you feel if it is an indoor pool? Why?
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Heat
Section 3
Specific Heat Capacity
• Specifc heat capacity (cp) measures the amount of heat
required to raise the temperature by 1°C for 1 kg of a
substance.
– It is different for every substance.
• SI Units: J/kg•°C
• Alternate form of this equation: Q = cpmT
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Heat
Section 3
• Suppose each metal shown above absorbs 100 J of
energy.
– Which will show the greatest increase in temperature? the least?
– How does water compare to iron with regard to heat capacity?
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Heat
Calorimetry
• Calorimetry is a problemsolving approach to heat
transfer problems.
– Conservation of energy
– Qgained = -Qlost
• A calorimeter is an
insulated cup with water
used for the experiment.
– Ignore heat gained or lost by
the cup, as it is small.
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Section 3
Heat
Section 3
Classroom Practice Problem
• In a student’s experiment, a 0.125 kg metal ball
is placed in a calorimeter filled with 0.150 kg of
water at 21.0°C. The initial temperature of the
ball is 98.5°C. After reaching equilibrium, the
temperature is 27.3°C. Find the specific heat
capacity of the metal and use the table to
determine the type of metal.
– Answer: 444 J/kg•°C , very close to iron
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Heat
Section 3
Classroom Practice Problem
• A bathtub has 20.0 kg of water at 60.0°C and
the bather wants the temperature to be 30.0°C.
How much 20.0°C water must be added to the
bath water to achieve the desired temperature?
– Answer : 60.0 kg
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Heat
Section 3
Latent Heat
• Latent heat is heat gained or lost during phase
changes.
– When substances melt, freeze, boil, condense, or
sublime, the temperature does not change during the
phase change.
– Heat absorbed changes the potential energy of the
particles.
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Heat
Section 3
Latent Heat
Click below to watch the Visual Concept.
Visual Concept
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Heat
Section 3
Latent Heat
• Heat of fusion (Lf) is
the heat required to
melt 1 kg of a
substance.
– Also equals the
heat released
when 1 kg freezes
– Which graph
segment
represents this?
• Heat of vaporization (Lv) is the heat required to change 1 kg of a
substance from a liquid to a gas.
– Which graph segment represents this?
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Heat
Section 3
Latent Heats of Fusion and Vaporization
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Heat
Section 3
Now what do you think?
• What property of water makes it so useful as a
coolant in automobiles, nuclear reactors and
other machinery?
• How does it differ from other liquids regarding its
ability to cool substances?
• Why do you feel cool when getting out of a warm
swimming pool on a hot day?
• How do you feel if it is windy out? Why?
• How do you feel if it is an indoor pool? Why?
© Houghton Mifflin Harcourt Publishing Company