The Second Law indicates that thermodynamic processes, i.e., processes that involve the transfer or conversion of heat energy, are irreversible because they all result in an increase in entropy. Nobody tries to extract energy from the waste heat, because there’s just not that much there.” The arrow of time “They burn natural gas or other gaseous fuels at very high temperature, over 2,000 degrees C, and the exhaust coming out is just a stiff, warm breeze. “The most efficient engines we build right now are large gas turbines,” said David McKee, a professor of physics at Missouri State University. Moving heat from a cold body to a hot body requires work to be done by an external energy source such as a heat pump. However, the heat will never move back the other way the difference in the temperatures of the two bodies will never spontaneously increase. When a hot and a cold body are brought into contact with each other, heat energy will flow from the hot body to the cold body until they reach thermal equilibrium, i.e., the same temperature. This is why claims for perpetual motion machines are summarily rejected by the U.S. Additionally, any device with movable parts produces friction that converts mechanical energy to heat that is generally unusable and must be removed from the system by transferring it to a heat sink. In the case of a car engine, this is done by exhausting the spent fuel and air mixture to the atmosphere. This waste heat must be discarded by transferring it to a heat sink. After the process of heating a gas to increase its pressure to drive a piston, there is always some leftover heat in the gas that cannot be used to do any additional work. One thing the Second Law explains is that it is impossible to convert heat energy to mechanical energy with 100 percent efficiency. This has often been summarized as, “You can’t unscramble an egg.” According to Wolfram, Boltzmann realized around 1876 that the reason for this is that there must be many more disordered states for a system than there are ordered states therefore random interactions will inevitably lead to greater disorder. However, the warm gas will never spontaneously separate itself into hot and cold gas, meaning that the process of mixing hot and cold gasses is irreversible. The result of this is that when hot gas and cold gas are placed together in a container, you eventually end up with warm gas. This approach also led to the conclusion that while collisions between individual molecules are completely reversible, i.e., they work the same when played forward or backward, for a large quantity of gas, the speeds of individual molecules tend over time to form a normal or Gaussian distribution, sometimes depicted as a “bell curve,” around the average speed. The process taken as a whole results in a net increase in disorder.
Crystals are more orderly than salt molecules in solution however, vaporized water is much more disorderly than liquid water. In another example, crystals can form from a salt solution as the water is evaporated. Even when order is increased in a specific location, for example by the self-assembly of molecules to form a living organism, when you take the entire system including the environment into account, there is always a net increase in entropy. Mitra explained that all processes result in an increase in entropy.
"At a very microscopic level, it simply says that if you have a system that is isolated, any natural process in that system progresses in the direction of increasing disorder, or entropy, of the system.” “There are a number of ways to state the Second Law," he said. Saibal Mitra, a professor of physics at Missouri State University, finds the Second Law to be the most interesting of the four laws of thermodynamics.