The Second Law of Thermodynamics.Rudolf Julius Emmanuel Clausius said it best in 1850; "heat always shows a tendency to equalize temperature differences and therefore pass from hotter to colder bodies". It has also been said that with this rather obvious and casual statement, Clausius introduced the world to
The Second Law of Thermodynamics,
and probably, according to Gibbs, "the science of thermodynamics came into existence."
At the time heat was thought to behave like a liquid. Water at the top of a hill flows down to the bottom of a hill and heat flows from hotter ('higher levels') to colder ('lower levels'). Water, and heat, never spontaneously flow 'up hill', that is, from 'lower levels' to 'higher levels'. Heat never moves from a cold object and into a hot object. Things always cool down.
But how far does it move? Water at the top of a 100 meter hill can flow a maximum of 100 meters down that hill until it reaches sea level and can flow no further. This is the maximum or total amount of distance it can fall and thus the total amount of energy available to do work (assuming it is used to turn something like a water wheel). But, if the water only flows 60 meters down the hill before it is trapped in a lake, then it will only be able to do the equivalent of 60 meters worth of work.
This '60-meters' is the amount of available energy to do work, the other '40-meters' worth is not available and is called entropy.
The two laws of thermodynamics, taken together, give us two limits to the use of energy. The first law (the law of conservation of energy) says that there is a total amount of energy present in a system and that you cannot get more out of that system than this total. While the second law says that you cannot even get at all the energy in a system, only the available energy.
The second law also tells us something else about energy on the move. As heat moves from a hot region to a cold region the hot regions cools down and the cold region heats up. As a result the difference in temperature becomes less and less. Since the available energy (to do work) depends on the magnitude of this 'difference', as the difference gets smaller, the amount of available energy gets smaller and smaller.
Of course, the total amount of energy has to stay the same (first law), so as the available energy decreases the 'unavailable' energy must increase. In any natural system (called a 'closed' system by the physicists) therefore, over time, the amount of available energy must decrease and the amount of unavailable energy must increase.
As the science of thermodynamics expanded beyond a study of heat to include all different forms of energy, it became clear that the second law of thermodynamics was a universal law that applied to all closed systems. A more modern way of stating this law, therefore could be: