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

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

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!

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

Mini-split / Air Conditioning Mini-Split Technology https://www.youtube.com/watch?v=14MmsNPtn6U Geothermal Technology: https://www.youtube.com/watch?v=mCRDf7QxjDk https://www.youtube.com/watch?v=y_ZGBhy48YI

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!

Air Conditioners

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

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!

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

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

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

Air Condition video: https://www.youtube.com/watch?v=E1vAdvsa7bE