Some further discussion about what a system is, giving some examples
The boundaries of a system is defined by the scientist or engineer in
a convenient fashion to fit the needs of the problem at hand. It is
a physical space, defined in x,y,z coordinates. It is conveniently
defined to correspond to the boundaries of some material item, such as an
industrial plant or a laboratory beaker.
Examples
- A researcher has a beaker of a water solution. The researcher
defines a system as consisting of everything inside a beaker, including
the beaker itself. Thus, the boundaries of the system are the outside
surfaces of the beaker and a flat surface across the top of the beaker.
- An engineer decides that the system is the entire oil refinery.
Thus the boundaries of this system consist of the fencing an a predefined
air space above the refinery.
- A student doing calorimetry defines the system as including everything
inside the calorimeter and the calorimeter itself. Thus the boundaries
of this system is the outer surface of the calorimeter. In a
further modification the student decides to define the calorimeter itself
as a separate system. This subsystem therefore has an inner and an
outer boundary consisting of the inner and outer surfaces of the calorimeter.
These definitions seem very hard to comprehend and work with. In
practice, some boundaries are highly defined; whereas, others are not so
well defined. In the calorimeter case, these boundaries are usually
very well defined in order to obtain very precise and accurate results.
On the other hand, energy and matter flows into and out of an oil refinery
are not too sensitive on the precise definitions of the boundaries, provided
some important component of the plant is not excluded.
In the case of the beaker example, the upper boundary is often not thought
about much. It does not make much difference whether the boundary
is define at the top of the beaker or at the surface of the liquid.
This is because the amount of material in the space between the liquid surface
and the beaker's lip is very small.
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