Insulators vs. Conductors

1. 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

2. 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?

4. Trapped air makes a good insulator

5. 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

6. 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

7. Air conditioners and Refrigerators

8. 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

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

10. The Function of an air conditioner

11. 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.

12. 23.8Energy and Changes of Phase Applications of Phase Changes A refrigerator’s cooling cycle uses the changes of phase of the refrigeration fluid (not water).

13. 23.8Energy 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.

14. 23.8Energy and Changes of Phase Applications of Phase Changes • It draws heat from the things stored in the food compartment.

15. 23.8Energy and Changes of Phase Applications of Phase Changes • The gas then goes to coils located outside the cooling unit.

16. 23.8Energy and Changes of Phase Applications of Phase Changes • As the gas condenses in the coils, heat is given off.

17. 23.8Energy and Changes of Phase Applications of Phase Changes • The liquid returns to the cooling unit, and the cycle continues.

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

19. 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)

20. 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)

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