How does heating a gas in a vessel with a piston affect the pressure inside?

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Discussion Overview

The discussion revolves around the effects of heating a gas in a cylindrical vessel with a piston on the pressure inside the vessel. Participants explore the relationship between pressure, volume, and temperature in the context of thermodynamic equilibrium, particularly focusing on whether the pressure changes as the piston moves due to heating.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions whether the pressure inside the vessel changes as the piston moves up when the gas is heated, suggesting a relationship between pressure and the movement of the piston.
  • Another participant proposes that if the pressure does not change, the piston can still rise if the volume of the heated gas increases.
  • A different participant describes the process as occurring in infinitesimal steps, where heating causes a slight increase in pressure that lifts the piston, which in turn increases the volume and reduces the pressure back to a certain equilibrium state.
  • This participant also notes that in a thermodynamic system at equilibrium, pressure and temperature can fluctuate around their equilibrium values, allowing for changes while maintaining equilibrium.
  • There is a repeated inquiry regarding whether the pressure in the work equation W = PdV refers to the external pressure or the pressure exerted by the gas, with one participant asserting they are the same in equilibrium.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between pressure and the movement of the piston, with some suggesting that pressure remains constant while others imply it changes. The discussion does not reach a consensus on these points.

Contextual Notes

Some assumptions about the nature of equilibrium and the behavior of gases under heating are not explicitly stated, and the discussion relies on the concept of infinitesimal changes without resolving the implications of these changes on pressure and volume.

Who May Find This Useful

This discussion may be of interest to students studying thermodynamics, particularly those working on problems related to gas behavior under varying conditions and those exploring concepts of equilibrium in physical systems.

AdityaDev
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If I fill a cylindrical vessel with a gas and put a piston of some mass on top of it and slowly heat the vessel, the piston will move up. But does the pressure inside the vessel change? ( vessel is insulated).if pressure doesn't change, how does the piston move up?

Pressure on piston : ##P - p## where P is atmospheric pressure and p is pressure exerted by gas.
Pressure on gas: ##mg/A + P##

Also in W = PdV, does P denote external pressure or pressure exerted by gas?
 
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If pressure doesn't change piston will go up if the volume of the heated gas increase.
 
AdityaDev said:
If I fill a cylindrical vessel with a gas and put a piston of some mass on top of it and slowly heat the vessel, the piston will move up. But does the pressure inside the vessel change? ( vessel is insulated).if pressure doesn't change, how does the piston move up?
This process proceeds by infinitesimal steps so that the system is always in equilibrium. The heat causes an infinitesimal increase in pressure, which lifts the piston infinitesimally, which increases the volume infinitesimally, which reduces the pressure so that it goes back to ##mg/A## with the piston at the new, slightly higher position and the system still in equilibrium. Then as heat continues to flow into the vessel the pressure increases again, moving the piston again... Imagine this process happening as an infinitely large number of infinitely small steps, and you'll see how the system can remain in equilibrium even as the piston moves and work is being done.
(If you've studied calculus and are familiar with the concepts behind integration, this may seem more natural).

It is also well to remember that when a thermodynamic system is in equilibrium, that doesn't mean that quantities such as pressure and temperature are fixed - they are constantly fluctuating around their equilibrium values. For any reasonably-sized system, these fluctuations are far too small to notice, but they provide a natural scale for the "small steps" that allow the system to change while remainingin equilibrium.

Also in W = PdV, does P denote external pressure or pressure exerted by gas?
They're the same, as the system is always in equilibrium.
 
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Thank you... i just solved 14 problems from Irodov's problems in general physics using that concept.
 

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