Caption: The 2nd law of thermodynamics illustrated.

Features:

1. Thermodynamics is the science of heat energy and temperature to give the short definition.

2. Now in thermodynamics there are many thermodynamic variables for matter, some familiar from everyday life and some NOT: e.g.,
   density, entropy, heat energy, heat capacity, latent heat, matter phase, pressure, specific heat, temperature, volume.

   Thermodynamics studies how the thermodynamic variables evolve with time---which is why thermodynamics is a form of dynamics.

3. Physical laws relating the thermodynamic variables determine the evolution.

4. The 1st law of thermodynamics is just the law of conservation of energy in the context of thermodynamics.

   ???? something missing ??? The quasistatic, irreversible free expansion of a gas sample from one compartment into another through a pair of holes.

   The expansion is free expansion since the gas exerts no force on the surroundings to expand.

   It is a quasistatic process since its limiting form can be infinitely slow and an irreversible process since the physical system will NOT return to its original macroscopic state without the expenditure of energy from some external source---which means, among other things, the physical system will NOT spontaneously return to its original state.

   Now neither Newtonian physics nor quantum mechanics forbid a macroscopic sample of gas from staying in one compartment (nor if initially spread between the two compartments from spontaneously all gathering in one compartment), but the requirement is that microscopic initial conditions be fine-tuned well beyond anything that is ever observed for a macroscopic sample.

   Neither nature nor humans can create such fine-tuned microscopic initial conditions. The microscopic initial conditions in practice, are random and unfine-tuned and invariably lead to the free expansion.

   In the free expansion the system is understood as becoming more disordered because more microscopic states are energetically possible after the expansion than before the expansion starts by opening the holes.

   Disorder is measured by the thermodynamic variable entropy.

   For a closed macroscopic system, entropy always increases due to random processes (which can be random initial conditions) until it reaaches maximum entropy---which is a macroscopically unchanging state called the thermodynamic equilibrium state.

   This rule is the 2nd law of thermodynamics.

   Thermodynamic equilibrium is a timeless, dead state, in fact---but many physical systems are in it (just as many macroscopic structures are in mechanical equilibrium) and it is relatively easy to analyze, and thus it is of great importance in thermodynamics.

   One can often analyze non-thermodynamic equilibrium states as a time sequence of thermodynamic equilibrium states.

   The 2nd law of thermodynamics seems so basic to the nature of reality that it is difficult to imagine any hypothetical universe---beyond extremely simple ones---in which it is NOT obeyed.

   The 2nd law of thermodynamics is an emergent principle---it arises in sufficiently complex physical systems.

   For example, it arises qualitatively in your living room.

   Subject to random processes, your living room gets more disordered until total squalor prevails.

   Why pick something up if you can just step over it?

   Credit/Permission: © Guy Vandergrift, 2013 / Creative Commons CC BY-SA 3.0.
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