The Second Law of Thermodynamics: Fundamental Concepts and Sample Problem

Abstract

The material presented in this lecture is adapted from Chapter 4 of T&M. As we saw in Lectures 3 and 4, the First Law of Thermodynamics is simply a restatement of the “Principle of Energy Conservation” in a given process, where heat, Q, and work, W, can be converted into Energy, E. Recall that if the system is open, then, enthalpic contributions need to be included in the formulation of the First Law of Thermodynamics (see Lectures 5 and 6). In this lecture, first, we will show that the First Law of Thermodynamics does not provide any information about the directionality of the process! Second, we will stress that nevertheless, from experience, there are processes which always occur spontaneously in a unique, well-defined direction and are referred to as natural processes. Third, to rationalize the directionality of natural processes, which are inherently irreversible, we will need to introduce a new thermodynamic function of state called entropy, which will provide a mathematical criterion in the context of what is known as the Second Law of Thermodynamics to ascertain if a process can occur in a particular direction. Fourth, we will introduce a heat engine, define its efficiency, and discuss under what conditions a process is reversible. Finally, we will solve Sample Problem 7-1, which will help us crystallize the material taught.