The second law of thermodynamics can be found in basic physics textbooks presented in two different ways. Firstly, related to the concept of thermal machines in which the principles of Clausius and Kelvin are postulated. Subsequently, this law is introduced using the concept of entropy and its non negative variation in a thermodynamical transformation (also known as principle of maximum entropy). It is common to find in textbooks the relation between the principles of Clausius and Kelvin of the law, but not so evident the relation between these principles and that one related to variation of the entropy. In this paper we present an incursion into the second law of thermodynamics as seen in these three different ways, specially considering the connection between the second law for entropy variation and as dictated by Clausius and Kelvin.
Keywords:
thermodynamics; second law; equivalence between statements; entropy
