THERMO Spoken Here! ~ J. Pohl © ~ 2019 (C5120-153)

3.16 About Entropy

Entropy is a thermodynamic property of the system (substance) and of the surroundings. "Entropy of Everything" is the subject of a physical law; the Second Law of Thermodynamics. Everything is the Universe has the parts: system and surroundings. The "system" part of the Universe is the "system" in our usual, previously applied, manner. Second Law applications address events of substances with events "Not Cyclic," and events that are: "Cyclic. Clearly entropy is an abstract idea. Also entropy and its mystique have been applied to Christian theological explanations. Here we consider entropy only in its secular, physics and engineering, aspects.

This discussion needs to be broken into several sub-discussions. A full discussion would include:

  • Entropy: a Thermodynamic Property:  How was entropy first identified and how was it made quantitative?
  • Entropy: the link to quantification.  Establishment of entropy as quantitative was the means to quantification of the other non measurable properties.
  • Entropy and the Second Law of Thermodynamics:  The Second Law starts with the universe as an isolated system. Next, the universe is divided into to subsystems, A and B which are at temperatures TA and TB. Thermal interaction between the subsystems show "Events of Isolated systems have an increase of entropy (taken as a whole).
  • Second Law Analysis (Steady or Transient Events):
  • Second Law Analysis (Thermal Engines: Cyclic Events):

Sorry! This is incomplete (and in need of revision)!

Unedited FIRST DRAFT below

  1. Makes Properties Quantitative  The pressure, volume and temperature of an ideal gas system are measurable. These properties can be made quantitative. For the same gas, internal energy, enthalpy and entropy are important properties but they are not measurable. Their quantitative values at a state must be deduced. These "deductions" begin with entropy and entropy is used, thereafter, to establish the others.
  2. Makes Predictions regarding Frictionless Adiabatic Processes
  3. Places Limitations upon Performance of Cyclic Engines

Expanded discussion of the list items follows:

Makes Properties Quantitative  A number of terms need to be clear...

Energy Change:  Energy change (disregarding kinetic and potential energies) of a substance is defined to occur by the mechanisms, work and/or heat. Work (adiabatic and without friction) was posed as the "measure" of energy change. There are different manners of work. These are called "work modes." Some names of modes are: compressible, magnetic, electric, surface... (and three more, I think).

Work Modes:  Most substances have only one manner by which their energy can be changed by work. Substances with but one work mode are termed to be a "simple substance." The single work mode common to all substances is the "simple compressible" work mode. (This mode states that the energy of a simple substance is changed when it is compressed). There are, importantly, some substances that have more than one work mode; these substances are classed as "complex substances." There are different types of force-displacement pairs. The different types are called "work modes."

Pure Substance:  The overwhelming majority of substances we study are "pure," meaning the samples studied have been carefully "cleaned or concentrated" so as to consist of only one molecular species. (Nothing is pure, of course; but samples of some species approach that ideal limit: pure).

Simple Substance:  Pure substances are said to be have Entropy is a property of all pure substances. This is to say the "numerical value of entropy" at states can be made quantitative. Most substances Our first task is to describe the idealization, "frictionless," as it applies to an ideal gas event.

can be applied to , states that the entropy of a system and its surroundings (of the Universe) increases with every event. Some processes (i.e., idealized events) are called reversible and (if also idealized as adiabatic) become constant entropy (isentropic). Process, properties of pure substances, work modes, simple and complex substances,... are just a few terms and jargon one come to understand on the path to understanding entropy and its uses.

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