Presentation on theme: "The Laws of Thermodynamics and Air Conditioners"— Presentation transcript:
1 The Laws of Thermodynamics and Air Conditioners (your best friends during this summer)
2 What a Fridge and an Air Conditioner does ? move heat from a hot place to a cold placethese machines generically called a ‘heat pump’Are there rules/laws that govern this Motion of Heat ?Yes, these are called ‘laws of thermodynamics’Establish relationships betweenThermal quantities (Temperature T, Internal Energy U, Entropy)andMechanical quantities (Work)There are 4 laws
3 Zeroth Law of Thermodynamics (definition of temperature) Two objects that are each in thermal equilibrium witha third object are also in thermal equilibrium with one another.BAT=28 C28C28CThere is no way you can do a net transfer heat from houseA to B if both are in equilibrium with the environment.i.e. heat will not flow from A to B.
4 First Law of Thermodynamics: (Energy Conservation) The change in a stationary object’s internal energyis equal to the heat transferred into that object minus the workthat object did on its surroundings.Workdone WU = Q - WpistonUGasMoleculesat temp TQcandleInternal Energy U – measure of thermal energy of systemHeat in – heat added to systemWork done – work done by the system
5 Disorder vs Order – the meaning of entropy Entropy – a measure of the disorder of a systemORDER(small entropy)DISORDER(large entropy)ORDER to DISORDER: Statistically Likely to happenReverse….. Very rarely happensConverting Thermal Energy to Ordered Energy is rare
6 Second Law of Thermodynamics The entropy of a thermally isolated system of objectsnever decreases.What does this mean ?Let represent molecules of a COLD gasand represent molecules of a HOT gasWhich of the following events are likely to happen ?A.B.Natural Flow of Heat is from HOT to COLD !
7 Third Law of Thermodynamics As an object’s temperature approaches zero, its entropyapproaches zero.
8 What does a Fridge or Air conditioner do ? It does the Un-natural: Moves heat from HOT to COLDUses a circulating fluid, usually freon, to move heatKey Features of an Air conditionerEvaporatorlocated in hot roomtransfers heat from hot room air to fluidCondenserlocated outside roomTransfers heat from fluid to outside airCompressordoes work on the fluid by compressing it and creating entropy
9 A Buffet of Air Conditioners ResidentialCompressor/CondenserWall-mounted air conditioneroutside Apartments: compressor/condenserOutside hospitals, etc
10 Anatomy of an Air Conditioner OutsideBuildingor condenserLow pressureCold gasHot, highPressure gasHot, highPressure liquid1234InsideBuildingRoom TempLow Pressure gas
11 A compressor turns the room temperature freon gas intoa warm, high pressure gas.A condenser turns the hotliquid freon into a cold lowpressure gas via free expansion.
12 The Refrigeration Sequence in an Air conditioner Process 4 1: Heat is absorbed by freon gas in tubes insidea warm room. Becomes a room temperature lowpressure gas before entering compressor.Process 1 2: Room Temp low pressure freon gas enters acompressor and comes out as a high pressure hot gas.Passing thru tubes outside building, hot freon dumpsheat to outside airProcess 2 3: Warm freon gas enters a constriction and isfurther pressurized to form a liquid in the condenser.Process Freon liquid undergoes free expansion into a gasand cools. The cool gas then takes in heat from theroom, becoming a room temperature low pressure gasThe cycle is then repeated.
13 Although the airconditioner pumps heat from cold to hot regions, it doesn’t violate the 2nd law of thermodynamics. Thecompressor adds entropy, so that the total entropy of the systemactually increases.