General definitions of work and heat in thermodynamic processes

Abstract

There are difficulties with the concept of work (1-9), particularly for irreversible processes. A thoughtful writer on that topic has said (9) "For mechanically irreversible volume changes, we usually cannot calculate the work from thermodynamic considerations." We believe that conclusion is incorrect. In this paper we give definitions of work and heat that overcome earlier difficulties. Both quantities are defined in terms of experimental quantities. The definitions are simple and general. They apply equally well to reversible and irreversible processes. Using these definitions, we are able to resolve several important problems as well as to obtain fresh insights into the First Law. Some of the results are surprising. They force a reevaluation of one's thinking about work and heat.

Cementing the foundations of thermodynamics: Comparison of system-based and surroundings-based definitions of work and heat

“…As noted in the Introduction, authors fall into two groups when defining work and heat. One group [3,4,[13][14][15]18,20,22,27,[29][30][31][32][33]38,[40][41][42][43]45] uses surroundings-based (''surr-based'') definitions for w and q. Thus, work is defined as the negative of the energy change in the surroundings that is potentially convertible into lifting or lowering a mass in the earthÕs gravitational field.…”

“…More generally, w and q are determined by making measurements in the surroundings before, during, and after the process. The present authors [13] have given a systematic method for determining work and heat in this way.…”

“…For a given process and a particular set of rules for determining q, the equality q(sys) = Àq(surr) may or may not be true. Several authors [4,13,22,27,46,47] have argued against the universality of this equality. A summary was given in [13].…”

“…Thus, it is surprising to find that the definitions of work w, particularly pressure-volume work, and heat q remain unsettled and contentious topics in thermodynamics. Many papers have been written on this subject [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], including two by the present authors [13,18]. Examination of these many papers and a representative group of textbooks shows two fundamentally different approaches.…”

“…The second procedure uses measurements of properties of the surroundings in a similar way. The present authors 0021 gave a systematic presentation of the surroundingsbased definitions of w and q in an earlier paper [13]. Either procedure can be used to develop thermodynamics.…”