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:
1 Starter QuestionIf you touch two objects that are the same temperature, why would one feel colder than the other?
2 Starter QuestionIf 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
3 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
4 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?
7 Cooling SystemA 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
8 2nd law of thermodynamics Heat flows from hot to coldHeat flows until thermal equilibrium is reached Temp inside = Temp outsideDuring the summer…Heat flows from outside your home to inside and does not stop unless thermal equilibrium is reachedRefrigerator…Heat flows from outside the refrigerator to inside
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 23.8 Energy and Changes of Phase Applications of Phase ChangesA refrigerator’s cooling cycle uses the changes of phase of the refrigeration fluid (not water).
14 23.8 Energy and Changes of Phase Applications of Phase ChangesLiquid is pumped into the cooling unit, where it is forced through a tiny opening to evaporate.
15 23.8 Energy and Changes of Phase Applications of Phase ChangesIt draws heat from the things stored in the food compartment.
16 23.8 Energy and Changes of Phase Applications of Phase ChangesThe gas then goes to coils located outside the cooling unit.
17 23.8 Energy and Changes of Phase Applications of Phase ChangesAs the gas condenses in the coils, heat is given off.
18 23.8 Energy and Changes of Phase Applications of Phase ChangesThe liquid returns to the cooling unit, and the cycle continues.
19 Air Conditioning and Refrigeration Work is done to remove heat that is entering a house, car or refrigeratorEvaporation removes heat and condensation releases heat (evaporation inside, condensation outside)Analogy: Leaky boatWater represents heat flowBailing represents removal of thermal energyCollecting 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 surroundingsCondensation is a warming process: releases heat into surroundingsUses the property of gases cooling during expansion and warming during compressionUses 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)
23 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.
24 REVERSE HEAT ENGINE Cooler gas becomes warmer when compressed MECHANICAL ENERGY IN : THERMAL ENERGY OUT
25 HEAT ENGINETHERMAL ENERGY IN : MECHANICAL ENERGY OUT
26 Definition of a Cooling System A device that transfers energy as heat out of an object to lower its temperature.