Second Law of Thermodynamics
- The second law of thermodynamics states
In every process, the total entropy of an isolated system always increases
- In the case of a non-isolated system, it can be generalised to say that the entropy of the Universe must increase
- There are two alternative forms of this law which have slightly more practical applications
- The Clausius form of the second law states:
Thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature
- The use of the word 'spontaneously' is the key here, as heat pumps are an example of a way that heat can be transferred from a colder region to a hotter one by doing work
- This leads to the Kelvin form of the second law, which states
When extracting energy from a heat reservoir, it is impossible to convert it all into work
- The different forms of the second law have significant implications for real-world applications, such as heat engines
Two of these engines cannot exist as they violate the Clausius and Kelvin forms of the second law of thermodynamics
