What is Second law of thermodynamics?
The '''second law of [[thermodynamics]]''' says that when energy changes from one form to another form, or matter moves freely, [[entropy]] (disorder) in a [[closed system]] increases. Differences in [[temperature]], [[pressure]], and [[density]] tend to even out after a while. Due to the force of [[gravity]], density and pressure do not even out vertically. Density and pressure on the bottom will be more than at the top. Entropy is a measure of spread of matter and energy to everywhere they have access. The most common wording for the second law of thermodynamics is essentially due to ''Rudolf Clausius'': It is impossible to construct a device that produces no other effect than transfer of heat from lower temperature body to higher temperature body In other words, everything tries to maintain the same temperature over time. There are many statements of the second law which use different terms, but all mean the same thing. Another statement by Clausius is: [[Heat]] cannot of itself pass from a colder to a hotter body. An equivalent statement by Lord [[Kelvin]] is: A transformation whose only final result is to convert heat, extracted from a source at constant temperature, into work, is impossible. The second law only applies to large systems. The second law is about the likely behavior of a system where no energy or matter gets in or out. The bigger the system is, the more likely the second law will be true. [[File:Heatpump.svg|thumb|300px|A simple stylized diagram of a heat pump's [[vapor-compression refrigeration]] cycle: 1)&nbsp;condenser, 2)&nbsp;expansion valve, 3)&nbsp;evaporator, 4)&nbsp;compressor.]] == Overview == In a general sense, the '''second law''' says that temperature differences between systems in contact with each other tend to even out and that [[thermodynamic work|work]] can be obtained from these non-equilibrium differences, but that loss of thermal energy occurs, when work is done and entropy increases.<ref>{{cite book|author=Mendoza, E. |title=Reflections on the Motive Power o