B Generating Energy from Molecular Motion

[painter]

This question really troubles me.
Since we all know that the molecules are in perpetual random motion, why can't we make a perpetual motion machine by using molecular's always-moving energy??

I‘m a Grade 9 student and I'm unsure about this.

 

[PeterDonis]

Moderator's note: Thread moved to Classical Physics forum.

 

[PeterDonis]

Hi @painter and welcome to PF!

You seem to be aware that perpetual motion machines are impossible, which is good. You just need to recognize that the word "perpetual" is the key. In the case of random motion of molecules, any such collection of molecules only has a finite amount of energy in it due to random motion. You could conceivably convert that finite amount of energy into some other form to do useful work (though even then there are other limitations involved), but that wouldn't be "perpetual" motion because once that finite amount of energy was used up, you would be done: there would be no more energy left and whatever you were doing with the energy would stop. The reason the random motion of molecules is "perpetual" is that the energy in that random motion isn't being converted into anything else; it's not doing any useful work at all.

 

[PeterDonis]

it MUST have lost some of its energy when being converted into something else (is that?)

That's right: by conservation of energy, if you use some of the energy contained in the random motion of molecules for something else, that energy is no longer there in the random motion of molecules.

 

[painter]

Moderator's note: Thread moved to Classical Physics forum.

And thank you for moving the thread to a right place-----I'm new here and I really have no idea where to put it.

 

[painter]

That's right: by conservation of energy, if you use some of the energy contained in the random motion of molecules for something else, that energy is no longer there in the random motion of molecules.

I got it !! Thank you veeeeery much

 

[PeterDonis]

I got it !! Thank you veeeeery much

You're welcome!

 

[Bystander]

https://www.physicsforums.com/threads/feynmans-brownian-ratchet-analysis.1061778/

 

[Anachronist]

Since we all know that the molecules are in perpetual random motion, why can't we make a perpetual motion machine by using molecular's always-moving energy??

We can already make machines that take advantage of a "molecule's always-moving energy". That's thermal energy, and machines that convert heat into other forms of work already exist. A stirling cycle engine does converts heat into mechanical motion, and thermionic generators can convert heat into electricity (at its most basic level, so does a thermocouple).

None of this is perpetual motion, however. When you extract heat energy from a material, the motion of its molecules decreases.

 

[Vanadium 50]

While I like Feynman's treatment, I would not suggest it to a 9th grader.