Introduction to entropy: Difference between revisions

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In [[thermodynamics]], entropy is a numerical quantity that shows that many physical processes can go in only one direction in time. For example, cream and coffee can be mixed together, but cannot be "unmixed"; a piece of wood can be burned, but cannot be "unburned". The word 'entropy' has entered popular usage to refer a lack of order or predictability, or of a gradual decline into disorder.<ref name="lexico">{{cite web |title=Definition of entropy in English |url=https://www.lexico.com/en/definition/entropy |archive-url=https://web.archive.org/web/20190711005908/https://www.lexico.com/en/definition/entropy |url-status=dead |archive-date=July 11, 2019 |website=Lexico Powered By Oxford |access-date=18 November 2020}}</ref> A more physical interpretation of thermodynamic entropy refers to spread of energy or matter, or to extent and diversity of microscopic motion.

If a movie that shows coffee being mixed or wood being burned is played in reverse, it would depict processes impossiblehighly improbable in reality. Mixing coffee and burning wood are "irreversible". Irreversibility is described by a law of nature known as the [[second law of thermodynamics]], which states that in an isolated system (a system not connected to any other system) which is undergoing change, entropy increases over time.<ref>Theoretically, coffee can be "unmixed" and wood can be "unburned", but this would need a "machine" that would generate more entropy than was lost in the original process. This is why the second law only holds for isolated system which means they cannot be connected to some external "machine".</ref>

Entropy does not increase indefinitely. A body of matter and radiation eventually will reach an unchanging state, with no detectable flows, and is then said to be in a state of [[thermodynamic equilibrium]]. Thermodynamic entropy has a definite value for such a body and is at its maximum value. When bodies of matter or radiation, initially in their own states of internal thermodynamic equilibrium, are brought together so as to intimately interact and reach a new joint equilibrium, then their total entropy increases. For example, a glass of warm water with an ice cube in it will have a lower entropy than that same system some time later when the ice has melted leaving a glass of cool water. Such processes are irreversible: A glass of cool water will not [[Spontaneous process|spontaneously]] turn into a glass of warm water with an ice cube in it. Some processes in nature are almost reversible. For example, the orbiting of the planets around the Sun may be thought of as practically reversible: A movie of the planets orbiting the Sun which is run in reverse would not appear to be impossible.