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Discussion in 'General' started by fruitality, Jan 19, 2014.
i just farted so loud people across the street laughed at me
The idea of "irreversibility" is central to the understanding of entropy. Everyone has an intuitive understanding of irreversibility - if one watches a movie of everyday life running forward and one of it running in reverse, it is easy to distinguish between the two. The movie running in reverse shows impossible things happening - water jumping out of a glass into a pitcher above it, smoke going down a chimney, water "unmelting" to form ice in a warm room, crashed cars reassembling themselves, and so on. The intuitive meaning of expressions such as "don't cry over spilled milk" or "you can't take the cream out of the coffee" is that these are irreversible processes. There is a direction in time by which spilled milk does not go back into the glass.
In thermodynamics, one says that the "forward" processes - pouring water from a pitcher, smoke going up a chimney, etc. are "irreversible" - they cannot happen in reverse, even though, on a microscopic level, no laws of physics are being violated. All real physical processes involving systems in everyday life, with many atoms or molecules, are irreversible. For an irreversible process in an isolated system, the thermodynamic state variable known as entropy is always increasing.The reason that the movie in reverse is so easily recognized is because it shows processes for which entropy is decreasing, which is physically impossible (or, more correctly, statistically improbable)[SIZE=9.333333015441895px] [/SIZE]In everyday life, there may be processes in which the increase of entropy is practically unobservable, almost zero. In these cases, a movie of the process run in reverse will not seem unlikely. For example, in a 1-second video of the collision of two billiard balls, it will be hard to distinguish the forward and the backward case, because the increase of entropy during that time is relatively small. In thermodynamics, one says that this process is practically "reversible", with an entropy increase that is practically zero. The statement of the fact that entropy never decreases is found in the second law of thermodynamics.
In a physical system, entropy provides a measure of the amount of thermal energy that cannot be used to do work. In some other definitions of entropy, it is a measure of how evenly energy (or some analogous property) is distributed in a system. Work and heat are determined by a process that a system undergoes, and only occur at the boundary of a system. Entropy is a function of the state of a system, and has a value determined by the state variables of the system.
The concept of entropy is central to the second law of thermodynamics. The second law determines which physical processes can occur. For example, it predicts that the flow of heat from a region of high temperature to a region of low temperature is a spontaneous process â€“ it can proceed along by itself without needing any extra external energy. When this process occurs, the hot region becomes cooler and the cold region becomes warmer. Heat is distributed more evenly throughout the system and the system's ability to do work has decreased because the temperature difference between the hot region and the cold region has decreased. Referring back to our definition of entropy, we can see that the entropy of this system has increased. Thus, the second law of thermodynamics can be stated to say that the entropy of an isolated system always increases, and such processes which increase entropy can occur spontaneously. Since entropy increases as uniformity increases, the second law says qualitatively that uniformity increases.
First post is the only important part guys.
There is a great short story about the reversal of entropy. Can't remember the name but google should pull it up i love it.
the last question by isaac asimiv