A property is any characteristic
of a system that can be given a numerical value without regard
to the history of the system. Properties are classified as either
intensive or extensive.
An intensive property has a value
at a point, and its value is independent of the extent or size of
the system. The value of an intensive property may vary with time
and its position within the system. Examples of intensive properties
include temperature, velocity, mass density, specific volume, and
specific energy.
An extensive property does not have
a value at a point, and its value depends on the extent or size
of the system. The amount of an extensive property for a system
can be determined by summing the amount of the extensive property
for each subsystem within the system. Examples of extensive properties
include mass, charge, linear momentum, volume, and energy.
Empirical evidence as codified by
science has identified a class of extensive properties that can
neither be created nor destroyed. An extensive property that satisfies
this requirement is called a conserved property. When the accounting
principle is developed for a conserved property, both the generation
rate and the consumption rate are identically zero. We typically
consider five conserved properties in engineering science: mass,
energy, charge, linear momentum, and angular momentum. Another important
property, entropy, is not conserved. Nonetheless, the accounting
framework also provides a useful vehicle for analysis, since entropy
can only be produced as a consequence of the Second Law of Thermodynamics.
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