Why do we use external pressure to calculate work in gas cylinders?

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Discussion Overview

The discussion revolves around the reasoning for using external pressure to calculate work in gas cylinders, particularly in the context of gas expansion against a piston. Participants explore the implications of internal versus external pressure in various thermodynamic processes.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that internal forces cancel each other out, leading to the use of external pressure for work calculations.
  • Others argue that while internal pressure changes in most processes, external pressure remains constant, making it a more straightforward measure for work done by the surroundings.
  • A participant points out that the work done by the gas in expanding against the piston can be expressed mathematically, indicating a relationship between pressure, volume, and work during isothermal expansion.

Areas of Agreement / Disagreement

Participants express differing views on the role of internal pressure and its effects on piston movement, indicating that multiple competing perspectives remain without a consensus.

Contextual Notes

There are unresolved questions regarding the relationship between internal and external pressures, particularly in non-isobaric processes, and the implications for work calculations.

sodium.dioxid
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In a cylinder (with a piston) containing gas, why do we use the external pressure, instead of the pressure of the gas, to calculate work?
 
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I think it would be because all the internal forces cancel each other out.
 
Woopydalan said:
I think it would be because all the internal forces cancel each other out.

Then why does the piston move up due to internal pressure?
 
sodium.dioxid said:
In a cylinder (with a piston) containing gas, why do we use the external pressure, instead of the pressure of the gas, to calculate work?

Internal pressure changes in all processes except isobaric. External pressure does not. Also, the work done on the system by the surrounding is easier to understand in terms of external pressure.
 
Infinitum said:
Internal pressure changes in all processes except isobaric. External pressure does not. Also, the work done on the system by the surrounding is easier to understand in terms of external pressure.

And if the original question is on the work done by the system.
That is, the pressure of the gas in the cylinder expanding against the piston.
Then dW = Force x distance = pressure x Area x distance = p dV
Work = ∫ V1 to V2 pdV
And with PV = n RT
Work = nRT ln V2/V1
For isothermal expansion
 
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