Important terms

  • Thermal energy (heat)
    • The velocity of atoms and molecules in a substance, the greater the velocity, the greater the level of heat
  • Equilibrium
    • A condition whereby a system (or systems) is in balance and all variable forces or influences within it have cancelled each other out
  • Energy
    • The capacity of a system to do work
  • Entropy
  • Absolute zero
    • The lowest possible temperature in the universe equaling -273.15° Celsius, or -459.67° Fahrenheit, or 0° Kelvin, or where entropy equals zero

The Four Laws of Thermodynamics

Thermodynamics is based on four laws, which are:

  1. The zeroth law of thermodynamics*
    • If system A is in thermal equilibrium with system B, and system C is in thermal equilibrium with system B, then system A is in thermal equilibrium with system C
  2. The first law of thermodynamics
    • All energy in the universe cannot be created or destroyed, it can only change form
      • For thermodynamic systems this means that “the change in the energy of a system is the amount of energy added to [or subtracted from] the system minus the energy spent doing work [on or by the system]”. 1
  3. The second law of thermodynamics
    • Heat cannot spontaneously transfer from a colder system to a hotter system. Energy external to the two systems is required to accomplish this type of transfer.
    • In a closed system, entropy (the absence of usable energy) increases over time from usable energy to unusable energy. This includes the universe.
      • As two isolated systems not in thermal equilibrium with each other exchange energy to the point of achieving thermal equilibrium with each other, the sum of their initial entropy levels is greater or equal to the final entropy level achieved between them when they reach equilibrium with each other 2
  4.  The third law of thermodynamics
    • It is not possible for any system to reach absolute zero
    • This third law is based on the second law, which, by stating that heat cannot spontaneously transfer from a colder system to a hotter system, implies that a system in a state of absolute zero would not be able to interact with anything in the universe, because as any system approaches this temperature it will always draw heat from a warmer system 3

*Note: The reason the that the four laws of thermodynamics go from 0 to 3, instead of from 1 to 4, is that the zeroth law was so obvious that it was not initially included, and by the time its importance was understood the first three had already been established. Because of its simplicity it needed to come before the other three.

Thermodynamic Applications

(from Wikipedia) Thermodynamic applications include:


  1. Thermodynamics Overview
  2. Second Law of Thermodynamics
  3. Third Law of Thermodynamics


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