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Second Law of Thermodynamics

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Second Law of Thermodynamics:

It is impossible for a process to have as its sole result the transfer of heat from a cooler body to a hotter one.
The second law of thermodynamics is formulated in many ways, as will be addressed shortly, but is basically a law which - unlike most other laws in physics - deals not with how to do something, but rather deals entirely with placing a restriction on what can be done.

It is a law that says nature constrains us from getting certain kinds of outcomes without putting a lot of work into it, and as such is also closely tied to the concept of the conservation of energy, much as the first law of thermodynamics is.

In practical applications, this law means that any heat engine or similar device based upon the principles of thermodynamics cannot, even in theory, be 100% efficient.

This principle was first illuminated by the French physicist and engineer Sadi Carnot, as he developed his Carnot cycle engine in 1824, and was later formalized as a law of thermodynamics by German physicist Rudolf Clausius.

Entropy and the Second Law of Thermodynamics

The second law of thermodynamics is perhaps the most popular outside of the realm of physics, because it is closely related to the concept of entropy, or the disorder created during a thermodynamic process. Reformulated as a statement regarding entropy, the second law reads:
In any closed system, the entropy of the system will either remain constant or increase.
In other words, each time a system goes through a thermodynamic process, the system can never completely return to precisely the same state it was in before. This is one definition used for the arrow of time, since entropy of the universe will always increase over time according to the second law of thermodynamics.

Other Second Law Formulations

A cyclic transformation whose only final result is to transform heat extracted from a source which is at the same temperature throughout into work is impossible. - Scottish physicist William Thompson (Lord Kelvin)

A cyclic transformation whose only final result is to transfer heat from a body at a given temperature to a body at a higher temperature is impossible. - German physicist Rudolf Clausius

All the above formulations of the Second Law of Thermodynamics are equivalent statements of the same fundamental principle.

Laws of Thermodynamics
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