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 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

3. 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. B A
T=28 C
28C
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.

4. 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
that object did on its surroundings.
Work
done W
U = Q - W
piston U
Gas
Molecules
at temp T Q
candle
Internal Energy U – measure of thermal energy of system
Heat in – heat added to system
Work done – work done by the system

5. 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

6. 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 !

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

8. 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
Evaporator
located in hot room
transfers heat from hot room air to fluid
Condenser
located outside room
Transfers heat from fluid to outside air
Compressor
does work on the fluid by compressing it and creating entropy

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

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

11. 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.

12. 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 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.

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