Thermodynamics vs. Statistical Mechanics¶
Thermal Physics covers two very broad and deep topics that are related but largely independent of one another.
Thermodynamics (abbreviated “Thermo)
Statistical Mechanics (abbreviated “Statmech”)
Thermodynamics¶
Thermodynamics is the study of the extrinsic and intrinsic properties of matter as it relates to the following quantities:
Temperature
Pressure
Volume
Count of particles
Entropy
In Thermodynamics, broad statements that are readily verifiable in experiments you can do at home are made and proven. The basic laws of thermodynamics are:
Temperature exists. If the temperatures of A and B are the same, and B and C are the same, then the temperatures of A and C are the same.
Conservation of energy: The total energy of a system changes by the amount of energy flowing in or out.
Entropy. Entropy must be conserved, or increased.
There is no absolute zero.
These laws combined exist solely to make chemists and engineers miserable. There is no miraculous source of free energy. You are going to spend energy trying to use it. And you can never reach absolute zero. All of these laws combined tell us what the “division by zero” things are when it comes to the physical universe.
Statistical Mechanics¶
Statistical Mechanics is where all the excitement is. Once you assume that atoms and molecules exist (something we now have solid proof of), then you can begin reasoning about their behavior. When you consider large groups of them, you need not understand how any individual bit behaves, but how they all behave as a group. Understanding the statistics behind this behavior, along with the underlying physics (such as Newtonian Mechanics, Special and General Relativity, Electrodynamics, Quantum Mechanics and Quantum Field Theory) allows us to make predictions about Thermodynamic quantities.
There is a lot of strange if not magical areas here, including things like negative temperatures, fusion energy, and more. Understanding basic physical concepts and combining them with statistical mechanics also leads to very interesting mathematical challenges that need to be surmounted.
Which one is better?¶
I tell people who aren’t serious about physics that they should learn Thermodynamics and be done with it. They are utterly hopeless when it comes to reasoning about microscopic behavior and applying physical concepts to statistical mechanics, and they will remain hopeless as long as they remain ignorant of the underlying math and physics. It’s particularly painful watching people come to incorrect conclusions based on what seems to be sound reasoning, so it’s better just to encourage them not to try at all.
If you’re a physicist, however, you’re going to want to learn and master statistical mechanics. You won’t find too many applications for thermodynamics except as a vehicle to communicate with chemists and engineers. You’ll be more interested in examining the fundamental laws of physics and the combination through statistical mechanics to really worry too much about things like temperature, pressure and volume.
Certainly, statistical mechanics involves mathematics that is far more interesting than the math behind the other fields. At least, that is my opinion.
