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Thermal energy being the movements of particles, could we have a system that could use gravity to reduce the thermal energy of particles?

For example, if we imagine:

  • A box containing Argon in its gaseous phase.

  • Tubes starting at the top of the box and going higher, with the same orientation than the gravitational force and only large enough to let pass one atom of Argon.

Would the atoms going into the tubes be slowed down?

If applicable, would that mean that the thermal energy inside the box (and tubes) would be reduced and where would this energy go?

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  • $\begingroup$ Wouldn't there we other atoms going up the tubes, since the kinetic energy per atom in the gas is much higher than the gravitational potential?(rue that the higher one goes the cooler the air, and that is due to gravity ( in various ways) $\endgroup$
    – anna v
    Commented Jul 24, 2022 at 3:56
  • $\begingroup$ As a cloud rises, it moves into regions of lower pressure. It expands and cools. $\endgroup$
    – mmesser314
    Commented Jul 24, 2022 at 4:00

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Yes this happens all the time. This is precisely why air in the mountains is cooler - it loses kinetic energy by doing work against gravity.

There is, of course, no way to get free energy by building some contraption with a tube - you will get less energy out of the gradient you create than the energy it took to create the pressure differential in the tube to begin with.

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  • $\begingroup$ Air in the mountains is cooler because it has done work expanding due to lower pressure. This is separate from losing energy directly to gravitational potential. $\endgroup$
    – BowlOfRed
    Commented Jul 24, 2022 at 5:27
  • $\begingroup$ @BowlOfRed that's two ways of describing the same thing. Gravity is the reason the pressure is lower as you go higher ($P(z) \sim e^{(-\alpha z)}$). The work they are doing to lower pressure is traveling to a higher gravitational potential $\endgroup$
    – Señor O
    Commented Jul 25, 2022 at 1:57
  • $\begingroup$ @BowlOfRed If there were no gravity there would be no expansion. But I agree that @ Señor O could have phrased it more precisely. $\endgroup$
    – Themis
    Commented Jul 27, 2022 at 15:27
  • $\begingroup$ @Themis not really. It's actually much less precise to say that an individual air molecule (as in the premise of the question) loses kinetic energy by doing work against "pressure", which is only a macroscopic abstraction. $\endgroup$
    – Señor O
    Commented Jul 28, 2022 at 23:17
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Would the atoms going into the tubes be slowed down?

Yes. If you could have a perfect insulator, the molecules would lose energy as they climb against the gradient. However the interior of the tube has a temperature and it will be practically impossible for a single molecule not to exchange thermal energy with such a more massive object.

If applicable, would that mean that the thermal energy inside the box (and tubes) would be reduced and where would this energy go?

If you had some perfectly elastic balls bouncing in a box, you could make some shelves at the top. The balls would be going slower when they bounced up there. Balls in the shelf (although still bouncing) would be going slower. The KE of balls is lower. In the molecular case the same thing happens, which means the temperature would be lower.

The energy in both cases goes into the gravitational potential energy of the system. You have molecules further from earth and it took some energy to put them there.

In practice, this small amount of energy would be swamped by interacting with the thermal energy of the container unless you had some sort of very special container.

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