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Intuitively, it seems like heat engines and refrigerators require a good bit of technology to make a working substance pass through different thermodynamic states and then finally return to the initial state. Usually, at the very least, some tubing and pumps are involved. Thus such devices appear to be pretty special occurrences in nature, requiring a human to make them.

On the other hand, there are all sorts of material cycles in nature, like the water cycle and the carbon cycle. But I'm not sure if those constitute thermodynamic cycles.

So the question is, are there any genuine thermodynamic cycles that are not man-made? Of course we will count approximate cycles, because even man-made cycles are only approximate. Refrigerators are constantly leaking a few molecules of refrigerant, for example. And the initial state may never be precisely reached again, although it gets close enough to be functional.

EDIT: What makes me hesitate to immediately consider the material cycles to be examples of thermodynamic cycles is the difficulty in identifying a thermodynamic system that stays together through the whole cycle. The system has to be small enough that it takes definite values of thermodynamic quantities such as temperature and pressure at a given time. But over time matter gets thoroughly mixed together, making it hard for the initial system to stay together and remain plot-able on a thermodynamic diagram. Perhaps there is a conceptual way around that issue that I haven't thought of yet.

ether
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2 Answers2

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As its name suggests, a thermodynamic cycle is a sequence of thermodynamic processes (e.g. heating, cooling, expansion, contraction, etc.) that happens, well, in a cycle. A man-made machine is one example, but thermodynamic cycles are everywhere in nature. You should be able to spot a thermodynamic cycle by looking at whether a thermodynamic quantity is involved and whether it changes in a cycle.

My favorite example is probably ocean circulation, which is partly driven by the ocean temperature (a thermodynamic quantity) and salinity that affect the water density (another thermodynamic quantity). Here is one great video explaining ocean circulation qualitatively.

Another example I can think of is the water cycle. Water on bodies of water heats up due to the sun, evaporates, condensates into clouds when it is cold enough up there, hangs out in the sky, and then comes back down as rain when it gets dense enough, completing the cycle. In this cycle, the water undergoes a thermodynamic cycle in which it undergoes phase transitions from liquid, to gas, back to liquid. You should admit that it is cooler when it is cloudy, which makes the water cycle one, big air cooler.

Why do textbooks provide examples using man-made machines? I am not a book writer, myself, so I would not know. However, I can say that man-made machines are simpler than whatever goes on in nature, which may sound more abstract when we try to spot the thermodynamic part of the process. The mathematical description is also simpler since we can describe man-made machines pretty well using simple thermodynamic cycles such as the Carnot cycle, Otto cycle, etc.

hendlim
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Growing up near Yellowstone, my mind goes to geysers. Simplified:

  1. Water fills up underground chamber
  2. Heat converts water to steam
  3. Steam pressure ejects water
  4. Repeat

As a biologist, I also think of plant photosynthesis during the day/respiration at night, plankton vertical migrations in response to sunlight, etc, but I'm not sure these would count from a strict physics perspective.

anjama
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