Why Is There an Absolute Temperature Scale

How should we define a practical measure of temperature Three scales are popular Kelvin, Celsius, and Fahrenheit. The Kelvin scale is the most fundamental because it dehnes absolute temperature T. The idea of an absolute temperature scale derives from Equation (7.51), which relates the efficiency of conv erting heat to work to a ratio of temperatures, Tc/Th- An absolute zero of the temperature scale, Tc = 0, is the point at which heat is converted to work with 100% efficiency, q = 1. This defines the zero of the Kelvin scale. 

Other scales of temperature T are shifted so that their zero points coincide, for example, with the freezing point of water (Celsius scale) or with a point of coexistence of salt, water, and ice (Fahrenheit), 

Our statement of the Second Law is that isolated systems tend toward their states of maximum entropy. For example, heat does not spontaneously flow from cold to hot objects because that would decrease the entropy. The earliest statements of the Second Law of thermodynamics were based on such examples. One such statement was that heat will not flow from a colder body to a hotter one without the action of some external agent. We can express this quantitatively by modifying the system that we examined in Example 7.2. For a system in which two objects are brought into thermal contact, and are also allowed to exchange some other extensive quantity Y, in place of Equation (7.18), the direction toward equilibrium is given by 

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