Engineers are usually concerned with the changes in

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Unformatted text preview: in this chapter. Engineers are usually concerned with the changes in entropy. Therefore, the entropy of a substance can be assigned a zero value at some arbitrarily selected reference state, and the entropy values at other states can be determined from Eq. 7–5 by choosing state 1 to be the reference state (S 0) and state 2 to be the state at which entropy is to be determined. To perform the integration in Eq. 7–5, one needs to know the relation between Q and T during a process. This relation is often not available, and the integral in Eq. 7–5 can be performed for a few cases only. For the majority of cases we have to rely on tabulated data for entropy. Note that entropy is a property, and like all other properties, it has fixed values at fixed states. Therefore, the entropy change S between two specified states is the same no matter what path, reversible or irreversible, is followed during a process (Fig. 7–3). Also note that the integral of dQ/T will give us the value of entropy change only if the integration is carried out along an internally reversible...
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This document was uploaded on 11/28/2012.

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