Answer to Question #164396 in Molecular Physics | Thermodynamics for anna francisco

Question

Imagine a special air filter placed in a window of a house. The tiny holes of the filter allow only air molecules moving faster than a certain speed to exit the house, and allow only air molecules moving slower than that speed to enter the house from the outside. Explain why such an air filter would cool the house, and why the second law of thermodynamics makes building such an air filter an impossible task.

Answer 1

Answer

Now let us talk about physics concepts

Such a device is not impossible, it's merely impossible to operate it for an infinite amount of time at a constant temperature difference. A battery operated AC system would be a practical implementation of a device that would cool the air on the way in and expel hot air out. At some point, because you require an isolated house, the batteries would go empty, though. If you want to argue with the second law, I wouldn't use the entropic formulation but the Clausius statement, which explicitly rejects the batteries in my AC exampleexample.

Now some facts are also here

If the house is an isolated system(no heat exchange between the house and the environment), then by the second law of thermodynamics, the entropy of the house cannot decrease. But in this process, the average velocity of the molecules decreaces, thus the entropy decreases.therefore this is not not impossible.