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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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Presentation on theme: "Starter Question If you touch two objects that are the same temperature, why would one feel colder than the other?"— Presentation transcript:

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

3 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

4 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

5 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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7 Trapped air makes a good insulator

8 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

9 2 nd 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

10 Air conditioners and Refrigerators

11 FUNCTION OF A REFRIGERATOR Remove unwanted Thermal Energy from inside Deposit unwanted Thermal Energy on the outside By evaporation of Refrigerant By condensation of refrigerant

12 The Function of an air conditioner

13 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.

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

15 Applications of Phase Changes 23.8 Energy and Changes of Phase Liquid is pumped into the cooling unit, where it is forced through a tiny opening to evaporate.

16 Applications of Phase Changes 23.8 Energy and Changes of Phase It draws heat from the things stored in the food compartment.

17 Applications of Phase Changes 23.8 Energy and Changes of Phase The gas then goes to coils located outside the cooling unit.

18 Applications of Phase Changes 23.8 Energy and Changes of Phase As the gas condenses in the coils, heat is given off.

19 Applications of Phase Changes 23.8 Energy and Changes of Phase The liquid returns to the cooling unit, and the cycle continues.

20 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)

21 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)

22 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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24 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.

25 REVERSE HEAT ENGINE MECHANICAL ENERGY IN : THERMAL ENERGY OUT Cooler gas becomes warmer when compressed

26 HEAT ENGINE THERMAL ENERGY IN : MECHANICAL ENERGY OUT

27 Definition of a Cooling System A device that transfers energy as heat out of an object to lower its temperature.

28 CHANGES OF STATES OF MATTER

29 Heat can be made to flow the other way only if work is done to the system. External effort! Example: air conditioner or refrigerator


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