R

E

A

D

T

H

I

N

K

C

R

E

A

T

E

For example, hot coffee breaks, in a mug that it might give up some internal heat to begin with the spontaneous rotation. A glass of cool water may spontaneously transformed into a block of ice in a cup of warm water. Even if such a thing is absolutely impossible things, we generally see these in the opposite direction to leave. ... Against the second law of thermodynamics in this direction, in this process occurred. We often say that the law gives the second session of the preferred orientation, and the "Arrow of time," tells us that the natural evolution of the system, with the passage of time, and in one direction, rather than in the other.           The Entropy Principle     We can imagine many processes that never happen, even though they do not violate the law of conservation of energy. For instance, hot coffee resting in a mug might give up some internal thermal energy and spontaneously begin to rotate. A glass of cool water might spontaneously change into an ice cube in a glass of warmer water. Even though such things never happen, we commonly see them happening in the reverse direction. The second law of thermodynamics... deals with the direction in which processes occur. It is often said that the second law gives a preferred direction to the "arrow of time," telling us that systems naturally evolve with time and in one direction but not in the other.   ... There is a property of things that happen naturally in the world around us that is strange and beyond belief. Yet we are so used to it that we hardly ever think about it. It is this:   All naturally occurring processes proceed in one direction only. They never, of their own accord, proceed in the opposite direction.   Consider the following examples:   Example 1: If you drop a stone, it falls to the ground. A stone resting on the ground never, of its own accord, leaps up into the air.   Example 2: A cup of hot coffee left on your desk gradually cools down. It never gets hotter all by itself.   Example 3: If you put a drop of ink in a glass of water, the molecules of ink eventually spread uniformly throughout the volume of the water. They never, of their own accord, regroup into a drop-shaped clump.   If you saw any of these processes happen in reverse, you would probably suspect that you had been tricked.   Such spontaneous one-way processes are irreversible, which means that once they have started they keep on going. More precisely, you cannot make them go backward by making any small change in their environment. Essentially, all naturally occurring processes are irreversible.   Although the "wrong-way" events we have described above do not occur, none of them would violate the law of conservation of energy. Consider these examples again:   Example 1: The ground could spontaneously cool a little, giving up some of its internal thermal energy to the resting stone as kinetic energy, allowing it to leap up. But it does not happen.   Example 2: Here we are dealing only with the direction of energy transfer, not with changes in its amount. Energy might flow from the surrounding air into the coffee, instead of the other way around. But it does not.   Example 3: Here no energy transfers are involved. All that is needed is for the ink molecules, each of which is free to move throughout the water, all to return simultaneously to somewhere near their original locations. That will never happen.   It is not the energy of the system that controls the directions of irreversible processes; it is another property... —the entropy (symbol S) of the system. ... [I]t is just as much a property of the state of a system as are temperature, pressure, volume, and internal energy. ... [L]et us state... the entropy principle:   If an irreversible process occurs in a closed system, the entropy of that system always increases; it never decreases.   Entropy is different from energy in that it does not obey a conservation law. No matter what changes occur within a closed system, the energy of that system remains constant. Its entropy, however, always increases for irreversible processes.   [Here] we are concerned with changes in entropy—that is, with ΔS rather than S. If a process occurs irreversibly in a closed system, the entropy principle tells us that ΔS > 0. The "backward" processes that we have described—if they occurred—would have ΔS < 0 and would violate the entropy principle.   … [W]e can now extend the statement we made [above] about entropy changes to include both reversible and irreversible processes. The extended statement, which we call the 'second law of thermodynamics' is:   When changes occur within a closed system its entropy either increases (for irreversible processes) or remains constant (for reversible processes). It never decreases.   In the equation form this statement becomes   ΔS ≥ 0.   The "greater than" sign applies to irreversible processes and the "equals" sign to reversible processes. No exceptions to the second law of thermodynamics have ever been found.         — David Halliday, Robert Resnick, Kenneth S. Krane, Physics, Volume 1, Chapter 24 – Entropy and the Second Law of Thermodynamics                     Indexes/14