Second Law of Thermodynamics (DP IB Physics: HL): Revision Note

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