Heat Engines, Entropy, & the Second Law

Absolute Temperature In discussing the Carnot engine or the Carnot cycle, we found that the ratio of the heat expended to the heat absorbed, Q

_{c}/Q_{h}, is also equal to the ratio of thecoldtemperature to thehottemperature, T_{c}/T_{h}. That is,T _{c}/T_{h}= Q_{c}/Q_{h}We can use this to define a temperature. Let one of the temperature reservoirs be at, say, the triple point of water. That is,

T _{h}= T_{3}= 273.16 KThen,

T = (273.16 K) Q/Q _{3}That is, if we operate a Carnot engine between an unknown temperature T and the triple point of water, T

_{3}= 273.16 K, we find that heat Q is absorbed at the this unknown temperature T and heat Q_{3}is expelled at the triple point heat reservoir. This unknown temperature T is then given by this equation.

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(c) Doug Davis, 2002; all rights reserved