Chapter 16

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Chapter 16 Thermal Energy and Heat

• Jennie L. Borders
• Edited by Lenny Leta

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Section 16.1 Thermal Energy and Matter

• In the 1700s most scientists thought that heat
  was a fluid called caloric that flowed between
  objects.

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Count Rumford

• Rumford studied the process of drilling holes in
  the barrels of cannons.
• The process occurred in water so that the metal
  would not melt due to the heat produced.
• Rumford discovered that the heat was a result of
  the motion of the drill, not a form of matter.

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Work and Heat

• Some of the work done by the drill does useful
  work, but some energy is lost due to friction.
• Heat is the transfer of thermal energy from one
  object to another because of a temperature
  difference.
• Heat flows spontaneously from hot objects to cold
  objects.

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Temperature

• Temperature is a measure of how hot or cold an
  object is compared to a reference point.
• Temperature is related to the average kinetic
  energy of the particles in an object due to their
  random motions through space.
• As an object heats up, its
• particles move faster, on
• average.

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Thermal Energy

• Thermal energy is the total potential and kinetic
  energy of the particles of an object.
• Thermal energy depends on the mass, temperature,
  and phase (solid, liquid, or gas) of an object.
• Thermal energy, unlike temperature depends on
  mass.

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Thermal Contraction

• Slower particles collide less often and exert
  less force, so pressure decreases and the object
  contracts.
• This is called thermal contraction.

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Thermal Expansion

• Thermal expansion is an increase in the volume of
  a material due to a temperature increase.
• Thermal expansion occurs when particles of matter
  move farther apart as temperature increases.
• Gases expand more than liquids, and liquids
  usually expand more than solids.

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Thermal Expansion

• Thermal expansion is used in glass thermometers.
• As temperature increases, the alcohol (or
  mercury) in the tube expands and its height
  increases.

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Specific Heat-how fast or how slow an object
warms up or cools down.

• Specific heat is the amount of heat needed to
  rise the temperature of one gram of a material by
  one degree Celsius.
• High specific heat slow to warm and cool
• Low specific heat fast to warm up and cool down.

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Specific Heat

• Formula for Specific heat
• Q m x c x DT
• Q heat (J)
• m mass (g)
• c specific heat (J/goC)
• DT change in temperature final initial (oC)

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Sample Problem

• An iron skillet has a mass of 500.0g. The
  specific heat of iron is 0.449 J/goC. How much
  heat must be absorbed to raise the skillets
  temperature by 95.0oC?
• Q m x c x DT
• m 500.0 g
• c 0.449 J/goC
• DT 95.0oC
• Q (500.0g)(0.449 J/goC)(95.0oC) 21,327.5J

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Practice Problems

• How much heat is needed to raise the temperature
  of 100.0g of water by 85.0oC?
• How much heat in kJ is absorbed by a 750g iron
  skillet when its temperature rises from 25oC to
  125oC?

Q (100.0g)(4.18J/goC)(85.0oC) 35,530J
DT 125oC 25oC 100oC Q (750g)(0.449J/goC)(1
00oC) 33,675J K h d u d c m
33,675J 33.7kJ ? ? ?
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Practice Problems

• In setting up an aquarium, the heater transfers
  1200kJ of heat to 75,000g of water. What is the
  increase in the waters temperature?
• What mass of water will change its temperature by
  3.0oC when 525J of heat is added to it?

Q m x c x DT DT Q/(m x c) 1200kJ
1,200,000J DT 1,200,000J/(75,000g x 4.18J/goC)
3.8oC
Q m x c x DT m Q/(c x DT) m
525J/(4.18J/goC x 3.0oC) 41.9g
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Calorimeter

• A calorimeter is an instrument used to measure
  changes in thermal energy.
• A calorimeter uses the principle that heat flows
  from a hotter object to a colder object until
  both reach the same temperature.
• Based upon the law of conservation of energy, the
  thermal energy released by a test sample is equal
  to the thermal energy absorbed by its
  surroundings.

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Calorimeter
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Section 16.1 Section Assessment

• In what direction does heat flow on its own
  spontaneously?
• How is the temperature of an object related to
  the average kinetic energy of its particles?
• Name two variables that affect the thermal energy
  of an object.
• What causes thermal expansion of an object when
  it is heated?

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Section 16.1 Section Assessment

• How do the temperature increases of different
  materials depend on their specific heats?
• What principle explains how a calorimeter is used
  to measure the specific heat of a sample
  material?
• Why is it necessary to have regularly spaced gaps
  between sections of a concrete sidewalk?

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Section 16.1 Section Assessment

• The specific heat of copper is 0.39J/goC. How
  much heat is needed to raise the temperature of
  1000.0g of copper from 25.0oC to 45.0oC?
• A peanut burned in a calorimeter transfers
  18,200J to 100.0g of water. What is the rise in
  the waters temperature?

DT 45oC 25oC 20oC Q (1000.0g)(0.39J/goC)(2
0oC) 7800J
Q m x c x DT DT Q/(m x c) DT
18,200J/(100.0g x 4.18J/goC) 43.5oC
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Section 16.2 Heat and Thermodynamics

• Conduction is the transfer of thermal energy with
  no overall transfer of matter.
• Conduction occurs between materials that are
  touching.
• Conduction in gases is slower than in liquids and
  solids because the particles in a gas collide
  less often.

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Conduction

• In metals, conduction is faster because some
  electrons are free to move about.

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Conductors

• A thermal conductor is a material that conducts
  thermal energy well.
• Examples silver, copper, gold, aluminum, iron,
  steel, brass, bronze, mercury, graphite, dirty
  water, and concrete.

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Insulators

• A material that conducts thermal energy poorly is
  called a thermal insulator.
• Examples rubber, paper, wood, air, and pure
  water.

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Convection

• Convection is the transfer of thermal energy when
  particles of a fluid move from one place to
  another.
• A convection current occurs when a fluid
  circulates in a loop as it alternately heats up
  and cools down.

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Convection

• Convection currents are important in many natural
  cycles, such as ocean currents, weather systems,
  and movements of hot rock in Earths interior.

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• Convection is based upon density changes.
• Warm objects get less dense (expand)
• Cooling objects get denser (contract) and sink.

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Radiation

• Radiation is the transfer of energy by waves
  moving through space.
• All objects radiate energy. As an objects
  temperature increases, the rate at which it
  radiates energy increases.

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Thermodynamics

• The study of conversions between thermal energy
  and other forms of energy is called
  thermodynamics.

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1st Law of Thermodynamics

• The first law of thermodynamics states that
  energy is conserved.

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2nd Law of Thermodynamics

• The second law of thermodynamics states that
  thermal energy can flow from colder objects to
  hotter objects only if work is done on the
  system. (Disorder in the universe is always
  increasing.)

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3rd Law of Thermodynamics

• The third law of thermodynamics states that
  absolute zero cannot be reached.

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Section 16.2 Section Assessment

• Why is conduction in gases slower than conduction
  in liquids or solids?
• Give three examples of convection currents that
  occur in natural cycles.
• What happens to radiation from an object as its
  temperature increases?
• State the first law of thermodynamics.

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Section 16.2 Section Assessment

• What is the second law of thermodynamics?
• State the third law of thermodynamics.
• If you bedroom is cold, you might feel warmer
  with several thin blankets than one thick one.
  Explain why.