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Welcome to our Physics lesson on The Second Law of Thermodynamics, this is the fourth lesson of our suite of physics lessons covering the topic of Entropy and the Second Law of Thermodynamics, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
From the example in the previous paragraph, it is obvious that the change in entropy in the universe is positive for all thermodynamic processes. This means every decrease in the local entropy of a thermodynamic system brings an increase at a higher extent of the entropy in the surroundings (we often say "in the universe" instead of "in the surroundings"). This conclusion forms the basis of the Second Law of Thermodynamics, which says:
The total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible.
The total entropy of a system remains constant only in ideal cases, i.e. when the process is completely reversible.
The Second Law of Thermodynamics is expressed in various forms that apparently seem as not related to each other but that are all equivalent. Some of them include:
Entropy of an isolated system either increases or at best, it remains constant during any change in the system.
It is impossible to convert entirely the heat energy supplied to a thermodynamic system into work. This means no heat engine can provide an efficiency of 100 percent, i.e. no perfect machine can exist.
It is impossible for heat to flow by itself from a colder object to a hotter one (Clausius Statement).
You have reached the end of Physics lesson 13.10.4 The Second Law of Thermodynamics. There are 6 lessons in this physics tutorial covering Entropy and the Second Law of Thermodynamics, you can access all the lessons from this tutorial below.
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