This document contains comments about the article Second law of thermodynamics in Wikipedia
• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions.
This sentence is not clear. Proper science assumes that one should start with an experiment or a description of our experience is.
One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unless energy is supplied to reverse the direction of heat flow.
The same problem. What is the physical definition of hooter and colder? What means energy?

### 1. Introduction

The first law of thermodynamics provides the definition of the internal energy of a thermodynamic system, and expresses its change for a closed system in terms of work and heat.
This requires a clear defintion of the concepts work and heat
It can be linked to the law of conservation of energy.
IMO the law of conservation of energy is the most important law. THis law should be explained by means of an example.
The second law is concerned with the direction of natural processes.
I prefer to say, with the evolution of natural processes. (?)
It asserts that a natural process runs only in one sense, and is not reversible.
implying also, that they are not time reversible.
For example, when a path for conduction or radiation is made available, heat always flows spontaneously from a hotter to a colder body.
More techinical information has to be supplied.
It is more natural to claim that each system evolves to an equilibrium state. That means the temperature in water will always be every where the same. In fact that means, there is energy transfer between all induvidual water molecules and there immediate neighbours.
Such phenomena are accounted for in terms of entropy change.
The concept of entropy IMO is not helpfull in our understanding.
If an isolated system containing distinct subsystems is held initially in internal thermodynamic equilibrium by internal partitioning by impermeable walls between the subsystems, and then some operation makes the walls more permeable, then the system spontaneously evolves to reach a final new internal thermodynamic equilibrium, and its total entropy, S, increases.
In practice this means that the final temperture will be the average temperature, which will be somewhere between a local maximum value and a local minimum value. To mention S is no improvement

### 2.2 Clausius statement

The German scientist Rudolf Clausius laid the foundation for the second law of thermodynamics in 1850 by examining the relation between heat transfer and work. His formulation of the second law, which was published in German in 1854, is known as the Clausius statement:
Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.
There are two considerations:
• In a normal system the final state of the system will be an equilibrium state, defined as the average temperature.
• If the average temperature is not what we want, extra heat has to be added in the form of an heat engine, or heat has be subtracted by means of a cooling engine.

### 5.1 Account given by Clausius

In 1865, the German physicist Rudolf Clausius stated what he called the "second fundamental theorem in the mechanical theory of heat" in the following form:
Integral (delta Q / T)=-N
where Q is heat, T is temperature and N is the "equivalence-value" of all uncompensated transformations involved in a cyclical process.
Two questions:
• how are the parameters Q, T and N measured?
• What is the physical meaning of this equation?
At face value zero.
Clausius would come to define "equivalence-value" as entropy.
Hum.
On the heels of this definition, that same year, the most famous version of the second law was read in a presentation at the Philosophical Society of Zurich on April 24, in which, in the end of his presentation, Clausius concludes:
The entropy of the universe tends to a maximum.
And what does that mean?
How is entropy measured? And what is this maximum?

### 6 Statistical mechanics

Statistical mechanics gives an explanation for the second law by postulating that a material is composed of atoms and molecules which are in constant motion.
All matter is composed of atoms and molecules.
A particular set of positions and velocities for each particle in the system is called a microstate of the system and because of the constant motion, the system is constantly changing its microstate.
Except that this microstate cann't be calculated.
Statistical mechanics postulates that, in equilibrium, each microstate that the system might be in is equally likely to occur, and when this assumption is made, it leads directly to the conclusion that the second law must hold in a statistical sense.
Nothing can be said if the chance for each microstate is the same. In reality it is the question if all possible microstates will ever happen. Most atoms and molecules exist in clusters, which prevent that.
That is, the second law will hold on average, with a statistical variation on the order of 1/√N where N is the number of particles in the system.
Where N is the total number of atoms in the Universe?

### 9 Gravitational systems

As gravity is the most important force operating on cosmological scales, it may be difficult or impossible to apply the second law to the universe as a whole.
Difficult to understand.

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Created: 27 October 2022

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