1.9.1 Quasi-static Process and Work Reservoir
Work in mass body dynamics is a straightforward concept: work done on a mass
body equals mass body mechanical energy gain; work performed by a system is
equal to the system’s energy loss (see Sect. 3.4). Things become complicated in
thermodynamics and it may thus be constructive to make a brief note of preparatory
purpose here on two particular points before we divulge into details in Chap. 6.
The first point regards the “quasi-static” heat Q and work W formula,
dQ ¼ TdS
(see Eq. [83] in Chapter 6)
and
dW ¼ pdV
(see Eq. [81] in Chapter 6)
(S denotes the entropy, a state variable that will be introduced in Chap. 5). The
usual assumption that they are applicable under the condition of quasi-staticity or
quasi-equilibrium is mistaken—the argument will be laid out in Chap. 6. This is the
first point of misunderstanding.
The second point concerns the role of a work reservoir in the consideration of the
work of a thermodynamic system. Consider a case of expansion external work.
System external work is made up of (1) expansion work against a surrounding
thermal-and-pressure reservoir and (2) useful work, which is transmitted and stored
in a work reservoir. Both reservoirs are necessary parts of the whole picture—
explicitly or implicitly (as in the case of Figs. 1.5 and 1.6)—in the consideration of
system work. Consider for instance the classical Joule free expansion [Sects. 3.8
and 6.3]: as it will be discussed below in Sect. 6.3 and Problems 6.2 and 6.3, the
system work of Joule free expansion being zero is because reservoirs of both kinds
are explicitly absent. (So is in Fig. 1.6, in which the work reservoir is explicitly
absent.) The critical role of reservoirs is a central point in the irreversible universe,
which will be fully discussed in Chap. 6:
Comprehension of a system in the irreversible universe necessitates the consideration of the
system and its reservoir(s) as an interconnected whole.
That is, though the object of our study is nominally a system, it is inadequate to
study the system in itself. The thermodynamic literature, unfortunately, continues
the tradition of focusing solely on systems, their interaction with work reservoirs is
rarely discussed in explicit terms. This is the second mistake.
The two mistakes are at the bottom inseparable. The underlying root of both is
the presupposition of mechanistic scientific knowledge (see below).
20
1 Introduction: Temperature and Some Comment on Work
Work in mass body dynamics is a straightforward concept: work done on a mass
body equals mass body mechanical energy gain; work performed by a system is
equal to the system’s energy loss (see Sect. 3.4). Things become complicated in
thermodynamics and it may thus be constructive to make a brief note of preparatory
purpose here on two particular points before we divulge into details in Chap. 6.
The first point regards the “quasi-static” heat Q and work W formula,
dQ ¼ TdS
(see Eq. [83] in Chapter 6)
and
dW ¼ pdV
(see Eq. [81] in Chapter 6)
(S denotes the entropy, a state variable that will be introduced in Chap. 5). The
usual assumption that they are applicable under the condition of quasi-staticity or
quasi-equilibrium is mistaken—the argument will be laid out in Chap. 6. This is the
first point of misunderstanding.
The second point concerns the role of a work reservoir in the consideration of the
work of a thermodynamic system. Consider a case of expansion external work.
System external work is made up of (1) expansion work against a surrounding
thermal-and-pressure reservoir and (2) useful work, which is transmitted and stored
in a work reservoir. Both reservoirs are necessary parts of the whole picture—
explicitly or implicitly (as in the case of Figs. 1.5 and 1.6)—in the consideration of
system work. Consider for instance the classical Joule free expansion [Sects. 3.8
and 6.3]: as it will be discussed below in Sect. 6.3 and Problems 6.2 and 6.3, the
system work of Joule free expansion being zero is because reservoirs of both kinds
are explicitly absent. (So is in Fig. 1.6, in which the work reservoir is explicitly
absent.) The critical role of reservoirs is a central point in the irreversible universe,
which will be fully discussed in Chap. 6:
Comprehension of a system in the irreversible universe necessitates the consideration of the
system and its reservoir(s) as an interconnected whole.
That is, though the object of our study is nominally a system, it is inadequate to
study the system in itself. The thermodynamic literature, unfortunately, continues
the tradition of focusing solely on systems, their interaction with work reservoirs is
rarely discussed in explicit terms. This is the second mistake.
The two mistakes are at the bottom inseparable. The underlying root of both is
the presupposition of mechanistic scientific knowledge (see below).
20
1 Introduction: Temperature and Some Comment on Work