The Second Law of Thermodynamics
In plain English the Second Law states that entropy (that is, disorder) always increases or remains constant in a closed system. (As a practical matter, for any non-trivial system entropy tends to increase due to irreversible processes.) The entropy of an entire closed system can never decrease within that system. Since the universe can be modeled as a closed system the universe is considered to be entropic - that is, running down.
The change in entropy (delta S) is equal to the heat transfer (delta Q) divided by the temperature (T).
delta S = (delta Q) / T
Second Law of Thermodynamics(heat engine): It is impossible to extract an amount of heat Q H from a hot reservoir and use it all to do work W . Some amount of heat Q C must be exhausted to a cold reservoir. This precludes a perfect heat engine.
Second Law of Thermodynamics(refrigerator): It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator
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