A technical reference to the Second Law notes: The entropy function always increases in the presence of internal irreversibilities for an adiabatic, closed system. In the limiting case of an internally reversible, adiabatic process, the entropy will remain constant. (Wark)
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 QH from a hot reservoir and use it all to do work
W . Some amount of heat QC 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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