Example Problem with Complete Solution

1C-1 : Identifying Open and Closed Sysytems 4 pts
 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. Read: Not much to say here. Given: Find: a.) Human being c.) A refrigerator b.) Bicycle tire d.) Planet Earth Assumptions: Assumptions are part of the answers in this problem. Equations / Data / Solve: None. Answers: a.) Human Being 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. b.) Bicycle Tire 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. c.) Refrigerator There are a few subtleties here. 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. 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. 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. d.) Earth This is a tricky one. The general answer is that the Earth can be described as a closed system. 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.