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Changes in Entropy Using the 1st & 2nd Laws

Integral form of the definition of

entropy

1st Law, closed system, internally reversible process:

TS Diagram
(internally reversible process)

TS diagram showing a differential slice of the area under the process path from state 1 to state 2.

PV Diagram
(internally reversible process)

PV diagram showing that boundary work is the area under the process path.

Assume the system contains a pure substance and that boundary work is the only type of work interaction that occurs.

Lesson
7D Blog

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Lesson 7D: page 3 of 17

Let’s begin by considering a pure substance that undergoes an internally reversible process within a closed system.
The system doesn’t need to be closed, but it is easier to imagine a closed system in which boundary work is the only form of work interaction.
The TS and PV diagrams show the process path.
The equations on the diagrams show how areas beneath the process paths can be interpreted.
Review the previous lesson if you are not comfortable with the TS Diagram and the relationship between the area under the curve and heat transfer in an internally reversible process.
The 1st Law provides us with a link or connection between dQ and dW for our internally reversible process.
dQ – dW is equal to dU. This is just the differential form of the 1st Law.
Now, let’s put this all together.

Changes in Entropy Using the 1st & 2nd Laws

entropy

Ch 7, Lesson D, Page 3 - Changes in Entropy Using the 1st & 2nd Laws