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I don't understand why the reverse Carnot Cycle works. Let's consider the isothermal expansion of a gas (body). The gas does work, but its temperature remains constant. For this to happen, the gas must receive heat from somewhere (so that the gas temperature does not decrease). The internet and textbooks say that the gas receives this heat (Q2) from the cooler (with a low temperature), but why does the gas receive heat from the cooler and not from the heater? This is very strange, because logically it should receive heat from a hotter source. Is this because during isothermal expansion, the gas is isolated from the heater and allowed to interact only with the cooler (and the same happens during isothermal compression, but interaction with the cooler is blocked)? Please explain why this happens.

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  • $\begingroup$ The image is from a poor resource; consider looking for alternate resources to avoid future misconceptions. The hot reservoir shouldn’t be labeled “heater”; it is itself heated during this process. Maybe “hotter” is intended, but that phrasing is still nonstandard. $\endgroup$ Commented yesterday

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During heating, the working medium is not in contact with the hot environment. Similarly, during heat rejection, the working medium is not in contact with the cold environment. Below follows a short, general discussion on cycles such as the reverse Carnot cycle.

A reverse Carnot cycle is basically a heat pump cycle or a refridgeration cycle, depending on its purpose.

The working medium receives heat $Q_C$ from the cold environment at a sufficiently low temperature such that heat flows from the cold environment to the working medium. Then we do work on the working medium (for example through compression) to sufficiently increase its temperature before letting it come in contact with the hot environment. Since the temperature is now sufficiently high, heat $Q_H$ will be rejected from the working medium to the hot environment.

You can read about common refrigeration cycles online. A common working medium is R-134A (1,1,1,2-tetrafluoroethane).

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  • $\begingroup$ Only the last paragraph is answering the OP. Not that is necessarily a bad thing, but the answer could be made much more concise, or if the last paragraph came first and were followed up narrative detail it might be less jarring on a first read. fwiw $\endgroup$ Commented yesterday
  • $\begingroup$ @kangermu, thank you for your comment. I have changed the structure so now the last paragraph comes first. $\endgroup$ Commented yesterday
  • $\begingroup$ that flows well $\endgroup$ Commented yesterday
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You are on the right track, and I believe that you should be concerned with which reservoir the heat is transferred from. In the Reverse Carnot Cycle, the gas only interacts with the cold reservoir during the isothermal expansion.

From Wikipedia, it clearly states that the other reservoir is thermally isolated.

"During this step (1 to 2 on Figure 1), the gas is in thermal contact with the hot temperature reservoir, and is thermally isolated from the cold temperature reservoir." Wiki

For a Reverse Carnot Cycle, I imagine the opposite to apply.

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    $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$ Commented yesterday
  • $\begingroup$ @samisafarini - fwiw, seems clear to me. $\endgroup$ Commented yesterday

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