Introduction to entropy: Difference between revisions

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If you reversed a movie of coffee being mixed or wood being burned, you would see things that are impossible in the real world. Another way of saying that those reverse processes are impossible is to say that mixing coffee and burning wood are "irreversible". Irreversibility is described by an important law of nature known as the [[second law of thermodynamics]], which says 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 for this you 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 thateventually is contained so as towill reach an unchanging state, with no detectable flows, and is then said to be in its owna state of internal [[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.: SomeAn processesice cube in naturea areglass of warm water will not helpfullyspontaneously describedform infrom thisa wayglass of cool water. ForSome example,processes forin somenature purposesare almost reversible. For example, the orbiting of the planets around the sun may be thought of as 'practically reversible',: andA movie of the planets orbiting the sun which is scarcelyrun describedin byreverse would not appear to be thermodynamicsimpossible.

Entropy does not increase indefinitely. As time goes on, the entropy grows closer and closer to its maximum possible value.<ref>Strictly speaking, thermodynamics only deals with systems in equilibrium. The idea that entropy is continuously "changing" is actually an approximation in which the change is considered to be a number of individual steps, each step being an equilibrium state derived from the previous one.</ref> For a system which is at its maximum entropy, the entropy becomes constant and the system is said to be in [[thermodynamic equilibrium]]. In some cases, the entropy of a process changes very little. For example, when two billiard balls collide, the changes in entropy are very small and so if a movie of the collision were run backwards, it would not appear to be impossible. Such cases are referred to as almost "reversible". Perfect reversibility is impossible, but it is a useful concept in theoretical thermodynamics.

While the second law, and thermodynamics in general, is accurate in its predictions of intimate interactions of complex physical systems behave, scientists are not content with simply knowing how a system behaves, but want to know also WHY it behaves the way it does. The question of why entropy increases until equilibrium is reached was answered very successfully in 1877 by a famous scientist named [[Ludwig Boltzmann]]. The theory developed by Boltzmann and others, is known as [[statistical mechanics]]. Statistical mechanics is a physical theory which explains thermodynamics in terms of the statistical behavior of the atoms and molecules which make up the system. The theory not only explains thermodynamics, but also a host of other phenomena which are otside the scope of thermodynamics.

A body of matter and radiation that is contained so as to reach an unchanging state, with no detectable flows, is said to be in its own state of internal [[thermodynamic equilibrium]]. Thermodynamic entropy has a definite value for such a body. 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. Such processes are irreversible. Some processes in nature are not helpfully described in this way. For example, for some purposes, the orbiting of the planets around the sun may be thought of as 'reversible', and is scarcely described by thermodynamics.

While the second law, and thermodynamics in general, is accurate in its predictions of intimate interactions of complex physical systems behave, scientists are not content with simply knowing how a system behaves, but want to know also WHY it behaves the way it does. The question of why entropy increases until equilibrium is reached was answered very successfully in 1877 by a famous scientist named [[Ludwig Boltzmann]]. The theory developed by Boltzmann and others, is known as [[statistical mechanics]]. Statistical mechanics is a physical theory which explains thermodynamics in terms of the statistical behavior of the atoms and molecules which make up the system.

==Explanation==