1. Topic #3 – Air Conditioning and Thermodynamic Processes Mr. Jean
IB Physics
Topic #3 – Air Conditioning and Thermodynamic Processes
Mr. Jean

2. What is happening to the Universe?
EXAMPLE
What is happening to the Universe?
The universe is slowly coming to an end. When the entire universe is at the same temperature, then no work will be possible, and no life and no change … billions and billions and billions of years from now … Heat Death

3. EXAMPLE
Consider a hypothetical device that takes 1000 J of heat from a hot reservoir at 300K, ejects 200 J of heat to a cold reservoir at 100K, and produces 800 J of work. Is this possible?
The maximum efficiency is emax = 1 – TL/TH = 67%, but the proposed efficiency is eprop = W/QH = 80%. This violates the 2nd law – do not buy shares in the company designing this engine!

4. EXAMPLE
Consider a hypothetical refrigerator that takes 1000 J of heat from a cold reservoir at 100K and ejects 1200 J of heat to a hot reservoir at 300K. Is this possible?
The entropy of the cold reservoir decreases by SC = 1000 J / 100 K = 10 J/K
The entropy of the heat reservoir increases by SH = 1200 J / 300 K = 4 J/K
There would be a net decrease in entropy which would violate the 2nd Law, so this refrigerator is not possible
What is the minimum work needed?
 2000 J, so that SH becomes at least 10 J/K

5. Mini-split / Air Conditioning
Mini-Split Technology
Geothermal Technology:

6. Air Conditioners
Uses a “working fluid” (freon or other nicer gas) to carry heat from cool room to hot surroundings – same as a refrigerator, moving Q from inside fridge to your kitchen, which you must then air condition!

7. Air Conditioners

8. Air Conditioners
Evaporator located in room air transfers heat from room air to fluid
Compressor located in outside air does work on fluid and heats it further
Condenser located in outside air transfers heat from fluid to outside air
Then the fluid reenters room for next cycle

9. Evaporator Heat exchanger made from a long metal pipe
Fluid nears evaporator as a high pressure liquid near room temperature
A constriction reduces the fluid pressure
Fluid enters evaporator as a low pressure liquid near room temperature
Working fluid evaporates in the evaporator – requires energy LV to separate molecules, so fluid cools & Q flows from room to fluid
Fluid leaves evaporator as a low pressure gas near room temperature, taking thermal energy with it, leaving the room cooler!

10. Compressor
Working fluid enters compressor as a low pressure gas near room temperature
Gas is compressed (PV work) so gas T rises (1st Law, T  U & U ↑ when PV work is done)
Compressing gas forces Q out of it into surroundings (open air)
Demonstration: Pump Air into a Gallon Jug with Thermometer
Fluid leaves compressor as hot, high pressure gas

11. Condenser
Fluid enters condenser (heat exchanger made from long metal pipe) as a hot, high pressure gas Q flows from fluid to outside air
Gas releases energy across heat exchanger to air and condenses forming bonds releases energy LV – thermal energy & fluid becomes hotter liquid so even more heat flows from fluid into outside air
Fluid leaves condenser as high pressure
liquid near room temperature to repeat the cycle

12. Summary
Evaporator – in room transfers heat from room air to working fluid
Compressor – outside does work on fluid, so fluid gets hotter
Condenser – in outside air transfers heat from fluid to outside air, including thermal energy extracted from inside air and thermal energy added by compressor
Demonstration: Pump Air into a Gallon Jug with Thermometer and Pop the Top
Demonstration: Air Conditioner/Refrigerator/Dehumidifier Opened and Operating
Entropy of room has decreased but entropy of outside has increased by more than enough to compensate – order to disorder

13. Air Condition video: