ABSTRACT
A macrosystem, or system for our present purposes, is a collection of microsystems which can exchange energy between each other on a time scale that is short compared with the time taken to make a measurement. An example is a gas, in which molecules (the microsystems) are continually colliding and exchanging energy. Suppose that we have two such macrosystems, 1 and 2, initially isolated from each other, each in a well-dehned macrostate. The macrostates of the two systems have energy U1 and U2 and multiplicity 1 and 2, respectively. If the energy of a system is changed, so is its multiplicity, and there exist functional relationships 1 = 1(U1), 2 = 2(U2). For the moment, we need not calculate these functions, but merely recognize that they exist, and we assume that they are analytic.
Figure 4.1. Two macrosystems in thermal contact with each other but isolated from their surroundings. An amount of energy dU is transferred from one to the other. The thick line represents an insulating wall and the thin line a diathermal wall.
