Second law of thermodynamics

The second law of thermodynamics is a law of thermodynamics that states that all work tends towards the production of greater entropy over time. Another way of saying this (known as the Clausius formulation) is that it is impossible to construct a perfect refrigerator. (This is why refrigerators always require an external power source.) An equivalent statement, known as the Kelvin-Planck formulation, is that "It is impossible for any cyclic process to occur whose sole effect is the extraction of heat from a reservoir and the perfromance of an equivalent amount of work." (If the reader is aware of the concept of heat engines, please understand that heat engines work by allowing two heat reservoirs of different temperatures to come in equilibrium with each other, and as such they do not violate the 2nd law.)

General description

The first law of thermodynamics states that one form of energy (such as kinetic, potential, electrical, thermal, etc,) can be converted into another without loss. The second law states that thermal energy (heat) is special: all forms of energy can be converted into heat, but it is not possible to convert the heat back fully in its original form. In other words, heat is a form of energy of lower quality.

Related Topics:
First law of thermodynamics - Kinetic - Potential - Electrical - Thermal - Heat

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What makes heat so special? According to the kinetic theory, heat is due to the random movement of atoms and molecules, so it looks much like kinetic energy. The difference is that those movements of atoms and molecules that produce heat cannot be observed or predicted, while all the other forms of energy are the result of some orderly movement of particles. The second law says that the amount of random movement (entropy) can only increase in a closed system; in other words, the randomness cannot be ordered without some external influence. (Some systems, such as living cells, spontaneously become structured when they receive energy from the outside. See dissipative structures.)

Related Topics:
Kinetic theory - Atom - Molecule - Cell - Dissipative structure

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The following example illustrates the law:

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:When a stone falls on earth, its kinetic energy is converted into heat (it becomes random movements of particles). The second law says that this random movement will never become ordered again. For example, the random movement will never become synchronized to throw the stone back in the air: the heat energy cannot revert to the original kinetic energy.

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There is one thing predictable about heat: it flows from hot to cold bodies. This can be used to convert some heat into mechanical energy, using a Carnot heat engine. The cycle stops when both bodies reach the same temperature (equilibrium); it can be shown that the amount of random movements has not decreased in the process.

Related Topics:
Carnot heat engine - Equilibrium

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The second law of thermodynamics is important in engineering because it provides a way to determine the quality and amount of degradation of energy during a process. It is also used to determine the theoretical upper limits for the performance of many commonly-used engineering systems, such as refrigerators, internal combustion engines, and reactors.

Related Topics:
Engineering - Refrigerator - Internal combustion engine - Reactor

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Many claimed perpetual motion machines would have to violate the second law of thermodynamics to function. In general, such machines seem to generate energy from "nowhere." In all cases, when examined, the machine has some hidden mechanism for drawing energy in from the outside. One example of this would be a device that can do work such as pumping water, simply by taking energy from the air. Such claims are labeled "perpetual motion machines of the second kind."

Related Topics:
Perpetual motion machine - Pumping water

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