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.

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 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).

absolute temperature scale

[More to come...]

The First Principle

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

Discussion

Suggested Fourth Principle: the Onsager reciprocity relations for non-equilibrium, open systems

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