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The Laws of Thermodynamics and Air Conditioners (your best friends during this summer)

What a Fridge and an Air Conditioner does ? • move heat from a hot place to a cold place • these 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 between Thermal quantities (Temperature T, Internal Energy U, Entropy) and Mechanical quantities (Work) • There are 4 laws

Zeroth Law of Thermodynamics (definition of temperature) Two objects that are each in thermal equilibrium with a third object are also in thermal equilibrium with one another. A 28 C T=28 C B 28 C There is no way you can do a net transfer heat from house A to B if both are in equilibrium with the environment. i. e. heat will not flow from A to B.

First Law of Thermodynamics: (Energy Conservation) The change in a stationary object’s internal energy is equal to the heat transferred into that object minus the work Work that object did on its surroundings. U = Q - W done W piston U Gas Molecules at temp T candle Q Internal Energy U – measure of thermal energy of system Heat in – heat added to system Work done – work done by the system

Disorder vs Order – the meaning of entropy Entropy – a measure of the disorder of a system ORDER (small entropy) DISORDER (large entropy) ORDER to DISORDER: Statistically Likely to happen Reverse…. . Very rarely happens Converting Thermal Energy to Ordered Energy is rare

Second Law of Thermodynamics The entropy of a thermally isolated system of objects never decreases. What does this mean ? Let represent molecules of a COLD gas and represent molecules of a HOT gas Which of the following events are likely to happen ? A. B. Natural Flow of Heat is from HOT to COLD !

Third Law of Thermodynamics As an object’s temperature approaches zero, its entropy approaches zero.

What does a Fridge or Air conditioner do ? • It does the Un-natural: Moves heat from HOT to COLD • Uses a circulating fluid, usually freon, to move heat Key Features of an Air conditioner 1. Evaporator • located in hot room • transfers heat from hot room air to fluid 2. Condenser • located outside room • Transfers heat from fluid to outside air 3. Compressor • located outside room • does work on the fluid by compressing it and creating entropy

A Buffet of Air Conditioners Residential Compressor/ Condenser Wall-mounted air conditioner outside Apartments: compressor/condenser Outside hospitals, etc

Anatomy of an Air Conditioner or condenser Low pressure Cold gas Inside Building 3 4 Outside Building Hot, high Pressure liquid Hot, high Pressure gas 2 1 Room Temp Low Pressure gas

A compressor turns the room temperature freon gas into a warm, high pressure gas. A condenser turns the hot liquid freon into a cold low pressure gas via free expansion.

The Refrigeration Sequence in an Air conditioner Process 4 1: Heat is absorbed by freon gas in tubes inside a warm room. Becomes a room temperature low pressure gas before entering compressor. Process 1 2: Room Temp low pressure freon gas enters a compressor and comes out as a high pressure hot gas. Passing thru tubes outside building, hot freon dumps heat to outside air Process 2 3: Warm freon gas enters a constriction and is further pressurized to form a liquid in the condenser. Process 3 4. Freon liquid undergoes free expansion into a gas and cools. The cool gas then takes in heat from the room, becoming a room temperature low pressure gas The cycle is then repeated.

Although the airconditioner pumps heat from cold to hot regions, it doesn’t violate the 2 nd law of thermodynamics. The compressor adds entropy, so that the total entropy of the system actually increases.