Starter Question

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Starter Question

• If you touch two objects that are the same
  temperature, why would one feel colder than the
  other?

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Starter Question

• If you touch two objects that are the same
  temperature, why would one feel colder than the
  other?
• One is a better conductor of thermal energy than
  the other.
• Example sauce pan

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Insulators vs. Conductors

• Conductors transfer thermal energy rapidly
  (metals)
• Insulators reduce the transfer of thermal energy
  (wood, foam)
• Insulation reduces unwanted transfer of thermal
  energy

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R-value of insulating materials
Which materials are the best insulators? What is
insulating glass and how does it differ from flat
glass? How does changing the thickness of
building materials affect insulating
effectiveness?
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Trapped air makes a good insulator
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Cooling System

• A device that transfers energy as heat out of an
  object to lower its temperature.
• Work must be done against heat flow to transfer
  heat from inside air to outside air

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2nd law of thermodynamics

• Heat flows from hot to cold
• Heat flows until thermal equilibrium is reached
  Temp inside Temp outside
• During the summer
• Heat flows from outside your home to inside and
  does not stop unless thermal equilibrium is
  reached
• Refrigerator
• Heat flows from outside the refrigerator to inside

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Air conditioners and Refrigerators
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FUNCTION OF A REFRIGERATOR
Deposit unwanted Thermal Energy on the outside
Remove unwanted Thermal Energy from inside
By condensation of refrigerant
By evaporation of Refrigerant
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The Function of an air conditioner
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HOW DO AIR CONDITIONERS AND REFRIGERATORS WORK?

• These appliances produce a thermal energy flow by
  evaporation and condensation.
• Evaporation removes thermal energy.
• Condensation releases thermal energy.

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23.8 Energy and Changes of Phase

• Applications of Phase Changes

A refrigerators cooling cycle uses the changes
of phase of the refrigeration fluid (not water).
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23.8 Energy and Changes of Phase

• Applications of Phase Changes

• Liquid is pumped into the cooling unit, where it
  is forced through a tiny opening to evaporate.

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23.8 Energy and Changes of Phase

• Applications of Phase Changes

• It draws heat from the things stored in the food
  compartment.

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23.8 Energy and Changes of Phase

• Applications of Phase Changes

• The gas then goes to coils located outside the
  cooling unit.

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23.8 Energy and Changes of Phase

• Applications of Phase Changes

• As the gas condenses in the coils, heat is given
  off.

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23.8 Energy and Changes of Phase

• Applications of Phase Changes

• The liquid returns to the cooling unit, and the
  cycle continues.

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Air Conditioning and Refrigeration

• Work is done to remove heat that is entering a
  house, car or refrigerator
• Evaporation removes heat and condensation
  releases heat (evaporation inside, condensation
  outside)
• Analogy Leaky boat
• Water represents heat flow
• Bailing represents removal of thermal energy
• Collecting water in bucket (evaporation of
  refrigerant)
• Dumping water outside boat (condensation of
  refrigerant)

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Evaporation and Condensation

• Evaporation is a cooling process takes heat from
  surroundings
• Condensation is a warming process releases heat
  into surroundings
• Uses the property of gases cooling during
  expansion and warming during compression
• Uses the concept of reverse heat engine to
  compress a gas (mechanical energy in thermal
  energy out)

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The evaporation-condensation process
Blue is evaporation of refrigerant (draws heat
from inside) Red is condensation of refrigerant
(releases heat to the outside)
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Nature of Gases

• Gases cool as they expand.
• Gases warm as they are compressed.
• Why?
• Gas particles have to do work to expand (farther
  to travel). This reduces the average KE and
  therefore the gas cools.
• Gases heat up when they are compressed because
  the work done to compress the gas is transferred
  to the gas particles, increasing the average KE.

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REVERSE HEAT ENGINE
Cooler gas becomes warmer when compressed
MECHANICAL ENERGY IN THERMAL ENERGY OUT
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HEAT ENGINE
THERMAL ENERGY IN MECHANICAL ENERGY OUT
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Definition of a Cooling System

• A device that transfers energy as heat out of an
  object to lower its temperature.

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CHANGES OF STATES OF MATTER
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• Heat can be made to flow the other way only if
  work is done to the system. External effort!
• Example air conditioner or refrigerator