Physics
Thermodynamics and Statistical Mechanics

Why the second law of thermodynamics is called a directional law of nature?

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February 17, 2012 11:46PM

In nature heat only moves naturally from warmer systems to cooler systems. One direction only. Never naturally from something cold into something hot.

We can pump heat out of a system by doing work on it, such as a refrigerator where the refrigerant is compressed - making it much hotter than the surroundings - then letting it give off heat to the surroundings, then expanding it across a valve where the evaporation and expansion causes it to get colder than the inside of the fridge - then allowing it to absorb heat from the inside of the fridge, then sending it back to the compressor to start all over again.