Modern Thermodynamics
- Chapter 1
3
1.1 Thermodynamic Systems
Thermodynamic description of natural processes usually begins by dividing the world into a "system" and
its "exterior", which is the rest of the world. This cannot be done, of course, when one is considering the
thermodynamic nature of the entire universe indeed thermodynamics can be applied to the entire
universe, though there is no "exterior". The definition of a thermodynamic system depends on the
existence of "boundaries", boundaries that separate the system from its exterior and restrict the way the
system interacts with its exterior. In understanding the thermodynamic behavior of a system, the manner
in which it exchanges energy and matter with its exterior, is important. Therefore, thermodynamic
systems are classified into three types: isolated, closed and open systems according to the way they
interact with the exterior.
Isolated systems do not exchange energy or matter with the exterior. Such systems are generally
considered for pedagogical reasons, while systems with extremely slow exchange of energy and mater
can be realized in a laboratory. Except the universe as a whole, truly isolated systems do not exist in
nature.
Closed systems exchange energy, but not matter, with its exterior. It is obvious that such systems can
easily be realized in a laboratory: A closed flask of reacting chemicals which is maintained at a fixed
temperature is a closed system. The Earth, on a time scale of years, during which it exchanges negligible
amount matter with its exterior, may be considered a closed system; it only absorbs radiation from the sun
and emits it back into space.
Open systems exchange both energy and matter with their exterior. All living and ecological systems are
open systems. Their complex organization in open systems is a result of exchange of matter and energy
and the entropy generating irreversible processes that occur within.
In thermodynamics, the state of a system is specified in terms of macroscopic state variables
such as volume V, pressure p, temperature T, moles, Nk, of chemical constituent k, which are self-evident.
These variables are adequate for the description of equilibrium systems. When a system is not in
thermodynamic equilibrium more variables, such as rate of convective flow or of metabolism, may be
needed to describe a systems. The two laws of thermodynamics are founded on the concepts of energy U,
and entropy, S, which, as we shall see, are functions of state variables.
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