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I wondering if is possible to build a cyclic ideal machine that converts the given work completely into heat. It would be the opposite of a thermal machine.

I am interested in this question because books on this subject says that you cannot build a perfect thermal machine but I think is also impossible to build a perfect machine that converts all the given work in heat.

Thanks in advance.

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  • $\begingroup$ The key word is completely and the answer is no. You can convert most of the work to heat, but you'll never get a 100% conversion. $\endgroup$
    – MaxW
    Commented Sep 26, 2016 at 18:04
  • $\begingroup$ Hi MaxW, then the problem that there is not a perfect machine is not related with the Thermodynamics second law as books very often say. I think it is impossible, in general, to create a cyclic thermodynamic machine that transforms completely work into heat or vice-versa, right? $\endgroup$
    – bebop
    Commented Sep 26, 2016 at 20:29

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Building a machine that turns work into heat is trivial. In fact, it's a great challenge to produce as little "waste" heat as possible. The experiment that proved that heat is a type of energy transfer was a machine that does just what you're asking. Similarly, electric stoves take energy that could power a device that does mechanical work and turns it all into heat.

If the goal is to get as much heat as possible, though, you can do even better than turning all of the work into output heat. If you arrange a heat pump you can use the energy to extract heat from an outside source, getting more heat where you want it than if you'd just turned the energy into heat directly, but more slowly.

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  • $\begingroup$ Thank you, Sean. But my question is about a cyclic process. Can you imagine a cyclic process that converts all the given work into heat? For example, a sequence of thermodynamics process in Pressure/Volume diagram that converts all the given work into heat and returns to initial point. $\endgroup$
    – bebop
    Commented Sep 26, 2016 at 17:46
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    $\begingroup$ Sure - take two Carnot engines. Setup one to extract heat from the hot reservoir, do work, and expel heat to the cold. Setup the other to run as a refrigerator that takes the work from the first engine, extracts heat from the hot reservoir, and expels it to the cold. $\endgroup$
    – Sean E. Lake
    Commented Sep 26, 2016 at 17:56
  • $\begingroup$ Sean, I din't understad. Is his coupling of Carnot engine be a cyclic machine that converts completely work in heat? I don't think so. $\endgroup$
    – bebop
    Commented Sep 26, 2016 at 20:25
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    $\begingroup$ What do you mean by perfect? If you mean can be run reversibly, then you are right, no reversible engine can just intake heat from a hot source and exhaust it to a cold source. Because were that the case, you could simply reverse it and make a Clausius 2nd law violator. The moment you add irreversible processes, though, the efficiency of the engine drops, potentially to zero. $\endgroup$
    – Sean E. Lake
    Commented Sep 26, 2016 at 21:13
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    $\begingroup$ @Sean Lake is right; the addition of an irreversible process to a cyclic engine is all that is required. Just consider a flywheel with friction on the axle; the wheel motion is cyclic (returns again and again to its initial position). $\endgroup$
    – Whit3rd
    Commented Sep 27, 2016 at 7:39

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