How does pressure and volume affect the work done by gas in a vacuum?

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Discussion Overview

The discussion centers on the relationship between pressure, volume, and the work done by gas in a vacuum. Participants explore theoretical implications, definitions, and reasoning related to gas behavior under these conditions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants assert that work done by gas is zero in a vacuum because there is nothing for the gas to do work on and no pressure to overcome during expansion.
  • Others explain that pressure is defined as the force exerted by gas particles on container walls, which is zero in a vacuum due to the absence of a container.
  • One participant introduces the idea that the gas radiates energy until it reaches absolute zero, leading to a conclusion that pressure must also be zero, while noting that the ideal gas equation can still apply under certain conditions.
  • Another participant questions the concept of negative pressure doing work on the gas, suggesting that it is not well-defined and that pressure is caused by force, which is absent in a vacuum.
  • A later reply discusses the concept of pneumatic energy, suggesting that different combinations of pressure and volume can yield equivalent energy states, even in a vacuum scenario.

Areas of Agreement / Disagreement

Participants express differing views on the nature of pressure and work in a vacuum, with no consensus reached on the implications of negative pressure or the relationship between pressure and volume in terms of work done.

Contextual Notes

Some arguments depend on specific definitions of pressure and work, and the discussion includes unresolved assumptions about the behavior of gases in vacuum conditions.

nil1996
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I learned that work done by gas is zero when a gas expands in vacuum.
But the volume and pressure of the gas change.Then how work done by gas is zero?
 
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The point is,in a vacuum,there is nothing that the gas can do work on!
Also,there is nothing that the gas has to overcome its pressure to expand,so the work it does in expanding is zero.
For another thing,pressure of a gas is the force that its particles exert on the walls of their container divided by the area.But in a vacuum there is no container,so the pressure is zero.
For another reasoning leading to pressure being zero.In a vacuum,whatever the temperature of the gas is,its higher than the temperature of the environment,so it radiates energy until its temperature reaches 0K(classically). And using PV=nRT,we will have PV=0. But the volume of the gas can't be zero,so P=0. Using the ideal gas equation is justified because in a vacuum,the only force on the particles is the force by other ones. There is also diffusion to the directions leading to outside of the gas because the concentration decreases in those directions and I think the diffusion flux is dominant so the gas expands and it will behave like an ideal gas with a very good approximation.
 
Why can't we say that in vacuum some negative pressure is doing work on the gas?
 
nil1996 said:
Why can't we say that in vacuum some negative pressure is doing work on the gas?

1-Its not well-defined,maybe not defined at all!
2-Pressure is casued by force.If we consider the gas as one entity,then there is only one thing in our system,the gas.So there can be no force,and so,no pressure.
3-I just proved that the pressure of the gas is zero in the situation you described.
4-There is no need to,we have a satisfactory explanation!
 
thanks for that explanation
 
nil1996 said:
I learned that work done by gas is zero when a gas expands in vacuum.
But the volume and pressure of the gas change.Then how work done by gas is zero?


Pressure decreases, but we have a bigger tank of gas.

(for example when a small gas tank explodes inside a large vacuum tank)

5 liters of gas in 10 atm has as much pneumatic energy as 10 liters of gas in 5 atm
 

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