2Introductory Question If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room willbecome colderbecome hotterstay the same
3Observations about Air Conditioners They cool room air on hot daysThey emit hot air from their outside ventsThey consume lots of electric powerThey are less efficient on hotter daysSome can be reversed so that they heat houses
45 Questions about Air Conditioners Why doesn’t heat flow from cold to hot?Why does an air conditioner need electricity?How does an air conditioner cool room air?What role does the electricity play?How does an air conditioner heat outdoor air?
5Question 1 Why doesn’t heat flow from cold to hot? Does such heat flow violate the laws of motion?Does such heat flow violate some other laws?
6Laws Governing Heat Flow The four laws of thermodynamicsare the rules governing thermal energy flowand establish the relationships betweendisordered (thermal) energy and ordered energyheat and work
70th Law of Thermodynamics The law about thermal equilibrium:“If two objects are in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.”
81st Law of Thermodynamics The law about conservation of energy“Change in internal energy equals heat in minus work out”where:Internal energy: thermal + stored energiesHeat in: heat transferred into objectWork out: external work done by object
9Order versus Disorder Converting ordered energy into thermal energy involves events that are likely to occuris easy to accomplish and often happensConverting thermal energy into ordered energyinvolves events that are unlikely to occuris hard to accomplish and effectively never happensStatistically, ordered always becomes disordered
10Entropy Entropy is the measure of an object’s disorder Includes both thermal and structural disordersAn isolated system’s entropy never decreasesBut entropy can move or be transferred
112nd Law of Thermodynamics The law about disorder (entropy)“Entropy of a thermally isolated system never decreases”
123rd Law of Thermodynamics The law about entropy and temperature“An object’s entropy approaches zero as its temperature approaches absolute zero”
13More on the 2nd Law According to the 2nd Law: Entropy of a thermally isolated system can’t decreaseBut entropy can be redistributed within the systemPart of the system can become hotter while another part becomes colder!Exporting entropy is like throwing out trash!
14Natural Heat FlowOne unit of thermal energy is more disordering to a cold object than to a hot objectWhen heat flows from hot object to cold object,the hot object’s entropy decreasesand the cold object’s entropy increases,so the overall entropy of the system increasesand total energy is conservedLaws of motion and thermodynamics satisfiedDemonstration: Animation: Hot box, Cold box, Entropy block, and Entropy meter
15Unnatural Heat Flow When heat flows from cold object to hot object, the cold object’s entropy decreases,and the hot object’s entropy increasesso the overall entropy of the system decreasesalthough total energy is conservedThe 2nd law of thermodynamics is violatedTo save 2nd law, we need more entropy!Ordered energy must become disordered energy!Demonstration: Animation: Hot box, Cold box, Entropy block, and Entropy meterDemonstration: Hand-Generator-Powered Heat Pump
16Question 2Why does an air conditioner need electricity?
17Heat Machines Air conditioners use work to transfer heat from cold to hotare a type of “heat pump”
18Air conditioners (Part 1) An air conditionermoves heat from cold room air to hot outside airmoves heat against its natural flowmust convert ordered energy into disordered energyso as not to decrease the world’s total entropy!uses a “working fluid” (chlorofluorocarbons –freon-, hydrofluorocarbons)to transfer heatThis fluid absorbs heat from cool room airThis fluid releases heat to warm outside airDemonstration: Animation: Hot box, Cold box, Entropy block, and Entropy meter
19Air conditioners (Part 2) Evaporatoris located in room airtransfers heat fromroom air to working fluidCondenseris located in outside airtransfers heat from workingfluid to outside airCompressordoes work on working fluidand produces entropy
20Question 3How does an air conditioner cool room air?
21The Evaporator (Part 1) The evaporator is a long, wide metal pipe pipe is heat exchanger between air and working fluidThe working fluidarrives as a high pressure, room temperature liquidbut loses pressure passing through a constrictionand enters the evaporator as a low pressure liquidLoss of pressure destabilizes the liquid phaseThe liquid working fluid begins to evaporate!
22The Evaporator (Part 2) Working fluid evaporates in the evaporator Fluid absorbs thermal energy while evaporating,so it transforms into a cold gasHeat flows from the hot room air to the cold gasWorking fluid leaves the evaporatoras a low density gas near room temperatureand carries away some of the room’s thermal energyHeat has left the room!Demonstration: Liquid Nitrogen EvaporatingDemonstration: CO2 Cylinder Making Dry Ice
24The Compressor The compressor increases density of a gas Working fluid arrives as a low density gas near room temperature,has work done on it by the compressor,and experiences a rise in temperature as a result.Working fluid leaves the compressoras a hot, high density gasand carries away electric energy as thermal energyOrdered energy has become disordered energy!Demonstration: Pump Air into a Gallon Jug with Thermometer
25Question 5How does an air conditioner heat outdoor air?
26The Condenser (Part 1) The condenser is a long, narrow metal pipe pipe is heat exchanger between air and working fluidThe working fluidarrives as a hot, high density gasbut begins to lose heat to the cooler outdoor airLoss of heat destabilizes the gaseous phaseThe gaseous working fluid begins to condense!
27The Condenser (Part 2) Working fluid condenses in the condenser Fluid releases thermal energy while condensing,so it transforms into a hot liquidand even more heat flows from fluid into outside airWorking fluid leaves the condenseras high-pressure room-temperature liquidhaving released some of the room’s thermal energyHeat has reached the outside air!
28Air Conditioner Overview Evaporator – located in room airtransfers heat from room air to working fluidCompressor – located in outside airdoes work on fluid, so working fluid gets hotterCondenser – located in outside airtransfers heat from working fluid to outside air,including thermal energy extracted from inside airand thermal energy added by compressorDemonstration: Pump Air into a Gallon Jug with Thermometer and Pop the TopDemonstration: Air Conditioner/Refrigerator/Dehumidifier Opened and Operating
29Introductory Question (Revisited) If you operate a window air conditioner on a table in the middle of a room, the average temperature in the room willbecome colderbecome hotterstay the same
30Summary about Air Conditioners They pump heat from cold to hotThey don’t violate thermodynamicsThey convert ordered energy to thermal energy