Why is the answer to this question "No change"

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Discussion Overview

The discussion revolves around the behavior of pressure in a piston system, particularly focusing on whether the pressure inside the piston changes under certain conditions. Participants explore concepts related to equilibrium, external pressures, and temperature effects, with a mix of theoretical and conceptual considerations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that the downward force on the piston, which includes atmospheric pressure and the weight of the piston and additional weights, must equal the upward force from the air below, suggesting that air pressure must remain constant.
  • Others contend that the pressure inside the piston should change, particularly if external conditions are altered, and that the discussion of equilibrium is crucial to understanding the situation.
  • There is a suggestion that the pressure inside the piston may only stabilize after a new equilibrium is reached, indicating a potential misunderstanding of the problem's context.
  • Some participants emphasize that temperature changes do not affect the pressure inside the piston, while others note that mechanical equilibrium can be maintained even if temperature is not uniform.
  • A participant expresses confusion regarding the distinction between 'temperature equilibrium' and 'mechanical equilibrium', indicating a need for clarification on these concepts.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are multiple competing views regarding the impact of external pressure and temperature on the pressure inside the piston, leading to an unresolved discussion.

Contextual Notes

Some assumptions about external conditions, such as the presence of air outside the piston and the nature of equilibrium, are not fully clarified, which may affect the interpretations of the problem.

Syndy
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Well, look at the piston. The force downwards is: (Atmospheric pressure)*(piston area) plus (sum of the masses of the piston and the weights)*g. These are not temperature dependent. If the piston is to stay where it is, the force upwards from the air below the piston (which is (air pressure)*(piston area)) must equal the force downward. Hence: The air pressure must stay constant.
 
Ok . Thank you very much.
 
I don't think that's correct. The example shown days nothing about the outside air. The pressure of the air inside the piston should absolutely change.
 
Drakkith said:
The pressure of the air inside the piston should absolutely change.
I think they mean the pressure after a new equilibrium was found.
 
Drakkith said:
I don't think that's correct. The example shown days nothing about the outside air. The pressure of the air inside the piston should absolutely change.

Not as I read the problem... The volume will change in such as a way as to maintain a constant pressure.
 
A.T. said:
I think they mean the pressure after a new equilibrium was found.

Okay. That makes sense. I wasn't thinking about after equilibrium.
 
Pressure in the piston will change if the outside pressure changes, but temperature changes won't change anything.
Even far away from a temperature equilibrium the piston will still be in mechanical equilibrium (unless you change the temperature with an explosion...).
 
mfb said:
Pressure in the piston will change if the outside pressure changes, but temperature changes won't change anything.
Even far away from a temperature equilibrium the piston will still be in mechanical equilibrium (unless you change the temperature with an explosion...).

Can you elaborate on this?
 
  • #10
Force equilibrium, as Svein explained. If the (frictionless) piston does not move, pressure from below multiplied by the area is equal to pressure above multiplied by the area plus the weight of piston and the added weights. If we assume that pressure outside does not change, and we do not change the weights, pressure inside won't change.
 
  • #11
I think there's some confusion. I was under the impression that we weren't taking the outside air into account since the 'air' is labeled as being inside the piston.
 
  • #12
It is not given that we have a vacuum outside so air is a natural assumption - but even that does not matter because the answer "the pressure does not change" is independent of the absolute pressure.
 
  • #13
Oh oh oh, I get it now. I was confused by 'temperature equilibrium' vs 'mechanical equilibrium'.
 

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