# PlanetPhysics/Principles of Thermodynamics

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```This is a contributed entry on Thermodynamics principles and/or laws; the latter are defined as those primary propositions that are fundamental to the logical and mathematical development of Thermodynamics in accord with all experimental findings in classical molecular physics. Thus, thermodynamics has its historical roots in studies of heat and Molecular Physics.
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In particular, such thermodynamic laws impose essential constrains on the equations of state and state functions that are employed to describe all closed thermodynamic systems. However, the thermodynamic treatment of open systems is not yet a `closed book'. One also notes that such thermodynamic laws that hold for all closed systems may still be further derived from statistical mechanics.

## Principles of Thermodynamics

### Basic Concepts

{Thermodynamic systems}: Closed vs. Open systems.

Thermodynamic Processes: Reversible vs. Irreversible := equilibrium vs. Non-equilibrium

### The Zeroth Principle

#### Thermal Equilibrium definition. Temperature and Molecular Motions

Temperature is a measure of the degree of molecular motion: the higher the average magnitude of velocities in a system measured at equilibrium with the system, the higher the temperature is (the hotter the system is).

[More to come...]

Whereas absolute temperatures of molecular systems can only take on positive values, spin temperature--or spin-lattice temperature-- for example, may take on `negative' values as a result of spin population inversion through Polarization, or cross-polarization.

Such cross-polarization processes might be thus utilized in designing and operating quantum `computers' or quantum nano-robots.

### The First Principle

Total \htmladdnormallink{energy {http://planetphysics.us/encyclopedia/CosmologicalConstant.html} Conservation}

### The Second Principle

During any thermodynamic process the entropy of a closed system always increases if the closed system is not at equilibrium (when the latter becomes constant), [or, equivalently, that perpetual motion machines are impossible].

### The Third Principle

The entropy of any crystalline system tends to zero in the limit of absolute zero temperature.

### Discussion

Suggested Fourth Principle: the Onsager reciprocity \htmladdnormallink{relations {http://planetphysics.us/encyclopedia/Bijective.html} for non-equilibrium, open systems}

Remark: Commonly, the four principles of reversible thermodynamics are also known as "the four thermodynamic laws ".