Thermodynamics: Second Law of Thermodynamics

## Second Law of Thermodynamics

Up to this point all of the topics have been based of the first law of thermodynamics (the conservation of energy). However, just because something follows the first law doesn't mean that it can happen in nature. An example would be a hot cup of coffee left on your desk. Eventually that cup of coffee is going to cool down. However, by the conservation of energy equation then energy lost into the environment should be able to be placed back into that cup of coffee so that it reaches the temperature that it was originally at. Of course you know that this is impossible, but what I just said wouldn't violate the first law. Instead to heat this cup of coffee again you need to either put it in an environment that is at a higher temperature, or you need convert work to heat. Both of these conditions cause a loss of energy in the form of heat that cannot be reversed. This means that real life processes have a certain direction that energy can flow, and that energy has certain quality or efficiency to the quantity of energy that is transferred. This is essentially what the second law of thermodynamics describes.

The Kelvin Planck Statement can be used as a more formal definition of the second law. "No heat engine can have a thermal efficiency of 100%, or for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace."