There are two main fields of Thermodynamics Classical Thermodynamics and Statistical Thermodynamics.
Thermodynamics was developed as a result of the study of steam engines and was based entirely on empirical measurement and it lead to the four laws of Thermodynamics. In their simplest forms Laws of Thermodynamics can be stated as follows:
- 1st Law of Thermodynamics: Energy can be neither created nor destroyed.
- 2nd Law of Thermodynamics: The amount of unusable energy in a closed system increases.
- 3rd Law of Thermodynamics: As the temperature of a substance approaches absolute zero it’s entropy of approaches zero.
- 0th Law of Thermodynamics: Two objects in thermal equilibrium with a third object are also in equilibrium with each other.
Statistical Thermodynamics works at microscopic as well as macroscopic level. It explains why the four laws of Thermodynamics work as the do as well as explaining things that Classical Thermodynamics can not.
In conclusion the principles of Thermodynamics are the best tested scientific principles known. They are also so general that they apply to everything including the universe itself. Thermodynamics is path independent so it often does not matter how one get from state “A” to state “B” which is why it applies to everything.