(10.1.1)--5.5Principleofentropyincrease.pdf
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1、Principle of entropy increase The purpose of introducing the state function entropy is to establish the mathematical expression of the second law of thermodynamics so that we can easily determine whether the process is reversible or irreversible.The calculation of the change in entropy in an irrever
2、sible processExample:a container is divided into two parts of equal volume by a clapboard.The left half is filled with mole of ideal gas,and the right half is vacuum.What is the change in entropy of the system after the clapboard is removed and the ideal gas goes through free expansion?Solve:The ide
3、al gas in free expansion Q=0,W=0,U=0,so the temperature does not change.dbbaaQSST可逆 If you plug Q=0 in the formula of entropy you get the wrong conclusion that entropy goes to zero in free expansion.This is because free expansion is an irreversible process,so the formula cannot be used to calculate
4、the entropy change directly.Instead,a reversible process connecting the same initial and final states should be found to calculate the entropy change.dbbaaQSST可逆2222111dddln2VVQpVSSVRRTTVSo we can see that in the irreversible adiabatic process of free expansion S 0.pVOV2V mole gas can be assumed to
5、undergo a reversible isothermal expansion.Example:In an adiabatic vacuum vessel,there are two identical isolated objects A and B whose temperature are respectively .The isobaric heat capacity is Cp which is constant.Now the two objects are brought into contact and the thermal equilibrium is reached.
6、Try to find the total entropy change in this process.Solve:This is the heat transfer process under constant pressure.Let the heat balance temperature be T,then0dd21TTpTTpTCTC0)()(21TTCTTCpp)(2121TTT Since this is an irreversible process,a reversible process connecting the same initial and final stat
7、es should be considered when calculating the change in entropy.For example,it can be assumed that object A reaches thermal equilibrium by contacting many heat sources whose temperature gradually decreases from T1 to T in turn,making its temperature drop from T1 to T quasi-statically.Suppose that obj
8、ect B reaches thermal equilibrium by contacting a number of heat sources whose temperature rises gradually from T2 to T in turn,so that its temperature rises quasi-statically from T2 to T.)(,2121TTTT Let the entropy of the initial state and the entropy of the final state of these two objects be S10
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