Secon law of thermodynamics Direction of heat flow
This chapter introduces the second law of thermodynamics, that heat does not spontaneously flow from cold to hot. The Carnot theorem, consistent with the second law, states that the efficiency of heat engines cannot exceed that of reversible engines operating between the same reservoirs. Through the use of cycles people can treat systems as "black boxes" where their properties are eliminated in favor of what's observed external to the system. In other wordings of the Rudolf Clausius and Joule-Kelvin statements, emphasis is placed on working in a cycle. The universality of Carnot 's theorem implies that temperature can be formulated in an equally universal manner, independent of the thermometric properties of materials, the absolute temperature. The net effect is an engine that delivers heat from a cold object to a hot object with no other effect, in violation of the Clausius statement.