Engineering Blog Post: Thermodynamics and Thermal Motion

How is most of our electrical power generated? Why is all of the energy on Earth derived from sunlight? And what is the mysterious state of "Absolute Zero"? All of these questions can be answered with today's topic, Thermodynamics, as well as it's associated product, the "Laws of Thermodynamics" poster! 

Thermodynamics is the study of the relationship between heat, work and energy. This relationship also includes temperature which is simply the average kinetic energy of the molecules in a given substance. There are generally considered to be 3 Laws of Thermodynamics, however there is a secret "0th Law" that is equally as important as the other three. The following laws govern the branch of thermodynamics ...

• The Law of Thermal Equlibrium (0th Law): If two separate systems are in thermal equilibrium with a 3rd system, they MUST be in thermal equilibrium with each other!
  • Caveat #1: This law inherently assumes that a system can reach thermal equilibrium which may not always be the case. However, it sets a consistent basis for temperature measuring devices.
• The Law of Energy Conservation (1st Law): Energy cannot be created or destroyed, only changed in form!
  • Caveat #1: The system does not care what form of energy intiates a thermodynamic process. All energy on Earth is derived from sunlight.
  • Caveat #2: No process of changing energy is 100% efficient.
  • Caveat #3: The Carnot efficiency is the maximum theoretical thermal efficiency a heat engine can produce, generally by converting energy. It is a function of the hot and cold resevoirs in the system.
• The Law of Entropy (2nd Law): Entropy, S, of a system will always increase regardless of the process due to irreversibilities (efficiency losses, heat/mechanical losses, etc), dS > 0! 
  • Caveat #1: This law also implies heat must move from a warmer temperature source to a colder temperature source.
  • Caveat #2: An ideal (reversible process) permits entropy to remain constant. If entropy remains constant, the system can return EXACTLY to it's initial state.
• The Law of Absolute Zero (3rd Law): As temperature of a system reaches absolute zero, entropy becomes constant.
  • Caveat #1: All CLASSICAL motion ceases, but every substance and it's molecules has a quantum ground state where they have a "Zero-Point Energy".
  • Caveat #2: Absolute Zero is the COLDEST possible temperature, cooling cannot occur below this value (0K, -273.15C, -459.67F). 
    

Notice that all of the thermodynamic laws, while fundamental, have caveats to them. This is because intrinsically, thermodynamic systems are in a constant state of disorder both in the classical and quantum sense. Entropy by definition is a measurement of disorder or randomness in a system. The universe itself is a system constantly trending towards more entropy which is why the "Heat Death" theory for the universe is a very popular way to describe the end of the universe. The "Heat Death" theory suggests the universe will eventually reach maximum entropy, spreading its heat and energy as evenly as possible. Therefore the universe would become a cold uniform void of nothingness where no work can occur due to a lack of temperature gradients available.

So, why should the average person care about Thermodynamics? Well our thermometers call on the 0th law to use reference thermal systems to measure the temperature of an ambient fluid/substance. Electricity and power is important to everyone. We rely on the 1st law of thermodynamics to use various sources of energy, everything from solar to fossil fuels to power our homes and our daily lives. The 2nd law allows us to see that any real process will never be 100% efficient and that all heat movement relies on a tempreature gradient. Finally, the 3rd law allows scientists to discover weird material properties of substances as they approach absolute zero leading to cutting edge tech and cooling systems. Thermodynamics is a hot topic, but is also pretty cool!