First Law for an Isobaric Process

Ch 4, Lesson E, Page 3 - First Law for an Isobaric Process

• Let’s consider an isobaric, quasi-equilibrium process.  We specify a quasi-equilibrium process because we need this assumption to help us evaluate work.
• We will consider a closed system, of course, because that is the subject of this chapter.
• Here is the 1st Law as it was introduced in Lesson C.
• In MOST, but not all, of the processes that we will consider in this course, changes in kinetic and potential energy are negligible.  So, let’s eliminate those two terms from the 1st Law.
• We can break the total work into two parts: moving boundary work and OTHER.
• Other work includes shaft work, electrical work and other types of work that we will not cover in this course.
• We do this because we already know how to evaluate boundary work.  We learned how to do this in Lesson A.
• Boundary work is just the integral of P with respect to volume from V1 to V2.
• This is where we take advantage of the fact that the process is isobaric.  Since P is constant, we can pull it out of the integral and the integral is easy to solve.
• So, the boundary work in an isobaric process is just P ΔV.
• We can substitute this back into the 1st Law, as shown here.
• But this doesn’t look any simpler than it did when we started.
• Trust me though.  It will get better.  Flip the page and see.