Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev

Thermodynamics

Civil Engineering (CE) : Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev

The document Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev is a part of the Civil Engineering (CE) Course Thermodynamics.
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Relations for Enthalpy, Entropy and Internal Energy

 

One may conveniently employ the general energy relations and Maxwell equations to obtain expressions for change in enthalpy and entropy and internal energy for any process, which in turn may be used for computing the associated heat and work interactions.


Let H = H (T, P)

 

Then:    Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev

 

But:   Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev

 

Thus:  Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                    ...(5.16)

 

Using:  Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                 ...(5.17)

 

From Maxwell relations as in eqn. 5.15:  Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                ...(5.18)

 

Thus using eqns. 5.17 and 5.18 in 5.16 we get:

 

Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                ...(5.20)

 

In the same manner starting from the general function: U = U (T,V ) andS = S (T, P ) and applying appropriate Maxwell relations one may derive the following general expressions for differential changes in internal energy and entropy.

 

Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev               ...(5.21)

 

Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                ...(5.22)
 

Or, alternately: Relations for Enthalpy, Entropy and Internal Energy Civil Engineering (CE) Notes | EduRev                ...(5.23)

 

Thus, eqns. 5.20 to 5.23 provide convenient general relations for computing enthalpy, internal energy and entropy changes as function of volumetric properties and specific heats. If a fluid is described by a suitable EOS, these equations may be conveniently integrated to obtain analytical expressions for energy and entropy changes.  

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