Why is atom not a perpetuum mobile? Why is perpetual motion and interaction of elementary particles (not) considered a perpetuum mobile? In the broadest possible way of thinking about it.
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1$\begingroup$ Where is there "perpetual moton of elementary particles"? Are you thinking in the (outdated) Bohr model that conceives of classical point particles actually revolving around a nucleus? $\endgroup$– ACuriousMind ♦Commented Nov 3, 2016 at 14:51
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$\begingroup$ Even quantum mechanically, the states with angular momentum about the nucleus can be thought of as 'in motion' can they not? $\endgroup$– MasonCommented Nov 3, 2016 at 15:00
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$\begingroup$ Because you can't get net work out of it forever... $\endgroup$– Jon CusterCommented Nov 3, 2016 at 15:03
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1$\begingroup$ I'm aware that you can't get net work out of it forever, but it's still in 'perpetual motion'. $\endgroup$– MasonCommented Nov 3, 2016 at 15:09
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2$\begingroup$ @Mason. But the definition of a perpetuum mobile is not one that just moves forever. A perpetuum mobile of the first kind is one that violates the conservation of energy, i.e. you have to put in at least as much energy as the work you get out. A perpetuum mobile of the second kind violates the second law of thermodynamics, which means that you have to put in more energy than the work you get out. This term perpetuum mobile doesn't have anything to do with things just moving forever, they have to do with things moving forever while getting useful work out of them at the same time. $\endgroup$– marchCommented Nov 3, 2016 at 16:15
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1 Answer
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Physically, there's nothing wrong with being perpetually in motion. It is consistent to have an electron with angular momentum about the nucleus for an infinite amount of time and I would call such a thing in perpetual motion.
Likewise, if nothing were in the way such as other planets (or if the sun did not go supernova) or whatever, the earth would revolve around the sun forever . It is in perpetual motion about the sun.
One more example: A cannonball shot off into space at above the escape velocity of the earth sun and milk way will almost certainly be in perpetual motion unless something gets in it's way.
However, in all three of these examples, you are not free to harvest infinite free energy from these things for reasons that should be intuitive to you in the case of the earth and the cannonball. If you were to harvest energy from the earth's orbit you would shrink it's orbit, eventually causing it to fall into the sun. There is a finite amount of energy stored in it's orbit. Likewise, there is a finite amount of energy stored in a cannonball at some speed and once you harvest it all, it will be stationary in your reference frame. These arguments gain some nuance quantum mechanically, but ultimately are the same. There is a finite amount of energy stored in an atomic orbital.
Edit:
As was pointed out in the comments, I was ignorant of the actual definition of a perpetual mobile. So in the end, no the atom is not a perpetual mobile, even if it is perpetually in motion.