Second Law of Thermodynamics

The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy.

Second Law
statements
Heat engine
Refrigerator
Entropy
Heat transfer

The maximum efficiency which can be achieved is the Carnot efficiency.

Qualitative statements of the Second Law of Thermodynamics
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Second law concepts

Heat engine concepts
 
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Second Law: Heat Engines

Second Law of Thermodynamics: 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.

This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law.

Alternative statements: Second Law of Thermodynamics
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Second law concepts

Heat engine concepts
 
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Second Law: Refrigerator

Second Law of Thermodynamics: 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. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.

This is the "second form" or Clausius statement of the second law.

It is important to note that when it is stated that energy will not spontaneously flow from a cold object to a hot object, that statement is referring to net transfer of energy. Energy can transfer from the cold object to the hot object either by transfer of energetic particles or electromagnetic radiation, but the net transfer will be from the hot object to the cold object in any spontaneous process. Work is required to transfer net energy to the hot object.

Alternative statements: Second Law of Thermodynamics
Index

Second law concepts

Heat engine concepts
 
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Second Law: Entropy

Second Law of Thermodynamics: In any cyclic process the entropy will either increase or remain the same.

Entropy: a state variable whose change is defined for a reversible process at T where Q is the heat absorbed.
Entropy: a measure of the amount of energy which is unavailable to do work.
Entropy: a measure of the disorder of a system.
Entropy: a measure of the multiplicity of a system.

Since entropy gives information about the evolution of an isolated system with time, it is said to give us the direction of "time's arrow" . If snapshots of a system at two different times shows one state which is more disordered, then it could be implied that this state came later in time. For an isolated system, the natural course of events takes the system to a more disordered (higher entropy) state.

Alternative statements: Second Law of Thermodynamics
Biological systems are highly ordered; how does that square with entropy?
Index

Second law concepts

Heat engine concepts

Entropy concepts
 
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