State whether each of the
following systems is an open system or a closed system. Explain the reasoning behind your answer. a.) human being, b.) a bicycle tire, c.) your refrigerator, d.) the planet Earth. |
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Read: |
Not much to say here. |
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Given: |
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Find: |
a.) |
Human being |
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c.) |
A refrigerator |
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b.) |
Bicycle tire |
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d.) |
Planet Earth |
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Assumptions: |
Assumptions are part of
the answers in this problem. |
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Equations
/ Data / Solve: |
None. |
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Answers: |
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a.) |
Human Being |
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If we assume the
person is alive, then, under most circumstances she or he should be treated
as an open system because air and other gases move in and out of the person's
lungs when they breathe in and out. For a short period of time, a person
might hold their breathe and you could make a case that a closed system would
be a reasonable choice. Over a longer period of time, perhaps days or years,
a person would be best described as an open system due to the consumption of
food and water and the elimination of waste. |
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b.) |
Bicycle Tire |
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If the tire has no
leaks and is not being filled over the time period of interest, the tire can
be reasonably modeled as a closed system. If the tire has a leak or the
filling process is being studied, then air does cross the system boundary and
the tire is an open system. |
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c.) |
Refrigerator |
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There are a few
subtleties here. |
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Some refrigerators
have ice makers and water chillers. Water flows into the system in this case.
But water only flows into the system when ice is being made or the water tap
is being used to obtain a glass of cold water. When water is flowing in and
out of the system or ice is flowing out of the system, then the refrigerator
can best be described as an open system. |
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When a person is
putting groceries into the refrigerator or taking food out of the
refrigerator, mass crosses the system boundary and the refrigerator is again
best described as an open system. |
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Most of the hours in a
given day, however, there is no mass crossing the system boundary and the
refrigerator can be considered to be a closed system. |
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d.) |
Earth |
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This is a tricky one. |
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The general answer is
that the Earth can be described as a closed system. |
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But this is not
entirely true since meteors are routinely bombarding our atmosphere, if not
the surface of the Earth. It is also true that molecules do leave our
atmosphere and move off into space. Although these mass flow rates are very
modest compared to the mass of the Earth and its atmosphere, under some
circumstances, these factors are important and Earth must be considered to be
an open system. |
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