Experience has taught us that the behavior of a system depends upon how certain extensive propertiesspecifically, mass, momentum, charge, energy, and entropychange with time. We postulate that this change or accumulation of an extensive property occurs by one of two mechanisms: (1) transport of the extensive property across the system boundary and (2) generation (production) or consumption (destruction) of the extensive property within the system. For example, the charge within a system is related to the amount of charge transported across the boundary and the amount of charge generated (or consumed) within the system. This simple balanceaccumulation = transport + generationis referred to as the accounting principle for an extensive property. Although this principle can be applied to a system for any extensive property, it is especially useful for properties that are conserved.
For a finite-time period, the accounting principle can be written in the accumulation form:
Final amount - Initial amount = Amount entering - Amount leaving + Amount generated - Amount consumed
For a specified time interval, the amount accumulated within the system equals the net amount that enters the system plus the net amount generated within the system.
For an infinitesimal-time period, the accounting principle can be written in the rate form:
Rate of change = Transport rate in - Transport rate out + Generation rate - Consumption rate
At a specified time, the rate of change within the system equals the net transport rate into the system plus the net generation rate within the system. Four questions must be answered when writing the accounting principle for a new property: What is the extensive property? How can it be stored within the system? How can it be transported across the system boundary? How can it be generated or consumed inside the system? As shown on the next page, this leads to a consistent way of thinking about, and writing, the fundamental laws of physical systems.