A philosophical question about piston and vacuum

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Discussion Overview

The discussion centers around a philosophical question regarding the behavior of a piston in a vacuum-sealed tube, particularly focusing on the pressure dynamics as the piston is pulled out. The scope includes theoretical considerations and conceptual implications of pressure differences in a controlled environment.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • Frank proposes a scenario involving a piston in a rigid tube with a one-way valve, questioning the implications of pressure changes as the piston is pulled out.
  • One participant suggests that once the pressure difference reaches 1 bar, the force applied (F) should allow for continued movement of the piston, assuming no friction.
  • Another participant clarifies that the pressure difference does not gradually approach 1 bar but rather becomes 1 bar immediately upon the piston losing contact with the closed end of the tube.
  • A further response indicates that Frank's initial assumption included some air trapped between the piston and the closed end, which would affect the pressure dynamics.
  • It is acknowledged that in this case, the pressure would indeed approach the surrounding pressure as the piston is pulled out.

Areas of Agreement / Disagreement

Participants express varying interpretations of the pressure dynamics involved, with some agreeing on the immediate nature of the pressure change while others focus on the initial conditions assumed. The discussion remains unresolved regarding the implications of these dynamics in practical scenarios.

Contextual Notes

The discussion does not resolve the complexities of real-world applications, such as the effects of friction, seal imperfections, and the nature of vacuum conditions.

Who May Find This Useful

This discussion may be of interest to those exploring theoretical physics, fluid dynamics, and philosophical implications of physical systems.

frankja
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Hello everybody,

I have kind of a philosophical question about vacuum and piston. I know this does not work in the real world because, of imperfection in seals and lack of "real vacuum", but here it goes.

If you have a tube which is closed in one end, open in the other end, and 100% rigid. You put a piston into it with a one way valve so that air can escape out of it. You push it to the end of the tube, so that there are very little air between the piston and the end of the pipe.

If you now start to pull the piston out (perfect seal) what will happen? The pressure difference between the inside and outside of the piston will go towards 1 bar (we do this in normal pressure on earth). You pull it out far enough and the force you are using is dependent on the pressure difference. When the pressure difference has reached 1 bar, and the force is F, will you now be able to keep moving the piston outwards using the force F?

Thanks,

Frank
 
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frankja said:
When the pressure difference has reached 1 bar, and the force is F, will you now be able to keep moving the piston outwards using the force F?
Sure, why not? (Ignoring friction and all such complications.)
 
Also, the pressure difference doesn't just "go towards" 1 bar. It becomes 1 bar as soon as the piston loses contact with the closed end.
 
I think he was assuming a little bit of air still trapped between piston and closed end to start with.
 
Ah yes, in that case the pressure would indeed approach 1 bar (or whatever the surrounding pressure is).
 
Thanks, everyone. Me and a friend had a discussion about this, and we needed a closure :)

Frank
 

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