A Novel”Positive” Approach/Analysis for Enhanced Understanding of the”Negative” Statement of the Second Law of Thermodynamics

Document Type

Conference Proceeding

Publication Date

6-23-2024

Department

Department of Manufacturing and Mechanical Engineering Technology

Abstract

According to the Kelvin-Planck (K-P) statement of the second law of Thermodynamics, “It is impossible to construct a device that will operate in a cycle and produce no effect other than the raising of a weight and the exchange of heat with a single reservoir.” Although it is impossible to prove this negative statement, it is however accepted because it rests on the fact that no experiment has ever contradicted it. Thus, this statement is accepted as an axiom which is then used to prove different theorems related to the efficiency of reversible heat engine and refrigerator cycles operating between two thermal reservoirs. A well-known example of such a theorem is the following important proposition regarding the efficiency of a reversible cycle: “It is impossible to construct an engine that operates between two given reservoirs and is more efficient than a reversible engine operating between the same two reservoirs.” Many engineering/engineering technology students of Thermodynamics for the first time find it very difficult to appreciate the true meaning and profundity of this apparently simple K-P statement. This is largely due to the fact that the student needs to “accept” as true this negative statement right at the outset of his/her study of the second law, without being offered any “positive” explanations or supporting reasons. This might explain why many students end up considering the fascinating course of Thermodynamics, which is deeply philosophical as well as intensely pragmatic at once, as a “difficult” subject. To alleviate this difficulty, we have taken a novel approach to enable the student to properly understand the negative statement in a more “positive” manner. We commence the analysis by constructing several thermodynamic cycles using an ideal gas as the working substance and consisting of both reversible and irreversible processes. The working substance in all these cycles interacts with only one thermal reservoir at a single temperature, as required by the K-P statement. It is then shown conclusively that not a single such cycle can be designed or constructed which will have the sole effect of doing positive work on the surroundings. We recognize that this is by no means a “proof” of the negative K-P statement of the second law. However, we believe and hope that the analysis presented in this article will offer an expedient tool for enabling the struggling student to properly understand the negative K-P statement and comfortably transition to studying the subsequent theorems, corollaries, and practical applications of the second law of Thermodynamics.

Publisher's Statement

Publisher version of record found here

https://peer.asee.org/46465

Publication Title

ASEE Annual Conference and Exposition, Conference Proceedings

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