Perpetuum mobile?

Imagine I have two containers set one upon the other. The upper one contains a fuel cell that unites Hydrogene and Oxygene to water, producing energy. The lower one contains an electrolytic device that uses energy in order to split up water into hydrogene and oxygene.
First, I need a certain amount of energy, say x joule, which I put into the elecrolytic. I get hydrogene, which is less heavy than air and thus raises upwards to my fuel cell. There it is rechanged into water, which brings me back my invested x joule. The water, heavier than air, then sinks down while powering a generator which produces, say, y joule. Then the water is back in the electrolytic again, a closed circle. First, I have to invest x joule, which I get back in the fuel cell, but then the falling water produces y joule energy, so I have gained y joule out of nothing. But then it would be a perpetuum mobile, so there must be a trick in it destroying that dream! Where?

Simple. First, your X to X’ isn’t perfect, so you’d lose a bit every time there. And X will always be > Y.

Both of your devices produce heat, which will ultimately cause the system to break down due to the laws of thermodynamics. Here’s a link to an explanation of how a fuel cell works.

What you’re missing is that the energy gained from the generator comes out of the water. You’ld have to make this up in the electrolytic device. IOW, you input X+Y energy at the bottom, not X.

Oh, and here’s a link to an explanation of the 2nd Law of Thermodynamics.

Assuming that there is a heat transfer device to cool down the newly-formed water, this wouldn’t be a problem. Although, as I’ve pointed out, you don’t need to go past the energy balance.

Even if the whole process was perfect, and it was a perpetual motion machine - so what? It is a perpetual motion machine of the third kind, and therefore produces no useful work whatsoever.

FTR, perpetual motion machines come in three models this year:

First Kind - this kind creates work from nothing, or creates mass and/or energy, and violates the first law of thermodynamics.

Second Kind - this kind violates the second law of thermodynamics, in that entropy never increases, and in fact may decrease.

Third Kind - this kind has no friction and can therefore operate forever, but produces no work external to itself.

But the one kind of nergy is chemical, coming out of the molecular union between H and O, while the other sort is merely physical height energy.

Interesting. Popular Science did a better job of that, but you probably can’t find it online. From the page…

Um…? Reacting with oxygen, as the hydrogen in a fuel cell does, IS combustion and that IS chemistry.