2. 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

3. 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?

5. Trapped air makes a good insulator

6. 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

7. 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

8. Air conditioners and Refrigerators

9. 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

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

11. The Function of an air conditioner

12. 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.

13. Applications of Phase Changes 23.8 Energy and Changes of Phase A refrigerator’s cooling cycle uses the changes of phase of the refrigeration fluid (not water).

14. 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.

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

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

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

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

19. COOLING SYSTEM IS REVERSE HEAT ENGINE MECHANICAL ENERGY IN : THERMAL ENERGY OUT With enough pressure, a gas can condense into a liquid

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)