The second law of thermodynamics states that the entropy of an isolated system increases during a spontaneous process. Another commonly used definition is the Kelvin-Planck statement, which we shall introduce in the next article.
The first law of thermodynamics states that the total energy of a system and its surroundings is constant. The state functions 
and 
, though very insightful and useful, are not able to indicate whether a process can proceed spontaneously. To determine the spontaneity of a process, scientists sought another state function. Rudolf Clausius, a German scientist, analysed the Carnot cycle, which had been developed earlier by Sadi Carnot, a French engineer, and eventually conceptualised a new thermodynamic state function 
, called entropy, which predicts the spontaneity of certain processes.