The second law of thermodynamics is a fundamental principle in physics, stating that in any natural thermodynamic process, the total entropy of a closed system (or the combination of a system and its surroundings) always increases over time or remains constant in ideal cases. This law is often expressed in several different forms, including the Kelvin-Planck statement and the Claudius statement. One of the most common statements of the second law of thermodynamics is the Claudius statement, which asserts that it is impossible to transfer heat from a colder body to a hotter body without the input of external work.
Mathematically, the second law of thermodynamics can be expressed in various forms. One of the mathematical expressions of the second law relates to the change in entropy () of a system during a reversible process and the heat () exchanged during that process. For a reversible process:
Where:
- is the change in entropy of the system (J/K) ,
- is the heat added to or removed from the system in (J),
- is the temperature at which the heat transfer occurs, typically measured in Kelvin (K).
In SI units, entropy is measured in joules per kelvin (J/K) .