Thermodynamics and Chemistry

CHAPTER 2 SYSTEMS AND THEIR PROPERTIES

2.4 THESTATE OF THESYSTEM 45

V b b b b b b

Figure 2.7 A thermopile used to measure the difference in the temperatures of two

bodies.

Athermocoupleconsists of wires of two dissimilar metals (e.g., constantan alloy and
copper) connected in series at soldered or welded junctions. A many-junction thermocouple
is called athermopile(Fig.2.7). When adjacent junctions are placed in thermal contact with
bodies of different temperatures, an electric potential develops that is a function of the two
temperatures.
Finally, two other temperature-measuring devices are thequartz crystal thermometer,
incorporating a quartz crystal whose resonance frequency is temperature dependent, and
optical pyrometers, which are useful above about 1300 K to measure the radiant intensity
of a black body emitter.

2.4 The State of the System

The thermodynamicstateof the system is an important and subtle concept.^9 At each in-
stant of time, the system is in some definite state that we may describe with values of the
macroscopic properties we consider to be relevant for our purposes. The values of these
properties at any given instant define the state at that instant. Whenever the value of any
of these properties changes, the state has changed. If we subsequently find that each of the
relevant properties has the value it had at a certain previous instant, then the system has
returned to its previous state.

2.4.1 State functions and independent variables

The properties whose values at each instant depend only on the state of the system at that
instant, and not on the past or future history of the system, are calledstate functions(or
state variables or state parameters). There may be other system properties that we consider
to be irrelevant to the state, such as the shape of the system, and these arenotstate functions.

(^9) Do not confuse thestateof the system with the kind ofphysical stateor state of aggregation of a phase
discussed in Sec.2.2.1. Achange of staterefers to a change in the state of the system, not necessarily to a phase
transition.