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Experience has taught us that
the behavior of a system depends upon how certain extensive properties—specifically,
mass, momentum, charge, energy, and entropy—change 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 balance—accumulation = transport + generation—is
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.
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