Section E.1 Physical Systems

Subsection E.1.1 Physical Systems

Definition E.1.1.

A physical system is a collection of interacting components whose behavior can be described by a set of measurable macroscopic variables.

Context E.1.1.1. Thermodynamic systems

In thermodynamics, a system is a collection of matter and/or energy that conceptually isolated for the purpose of observation, measurement, or analysis.

Claim E.1.2.

Among all accessible macrostates, physical systems overwhelmingly tend to be observed in those corresponding to the largest number of compatible microstates.

Subsection E.1.2 Macrostates and Microstates

Macrostates.

Definition E.1.3.

A macrostate is the complete specification of the measurable macroscopic properties that characterize a physical system at a given instant.

The collection of all state variables defines the macrostate of a physical system.

Example E.1.4. Macrostate examples.

temperature
pressure
volume
particle number
charge
electric potential
magnetization

Remark E.1.5.

A macrostate is typically a compressed description of a much larger collection of microscopic information. Remark E.1.7 illustrates that macroscopic descriptions necessarily discard microscopic detail.

Microstates.

Definition E.1.6.

A microstate is a complete microscopic description of a physical system that specifies the state of every constituent degree of freedom.

Remark E.1.7.

Multiple different microstates may give rise to the same macrostate.

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