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

[sodium.dioxid]

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?

 

[member 392791]

I think it would be because all the internal forces cancel each other out.

 

[sodium.dioxid]

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?

 

[Infinitum]

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.

 

[Point Conception]

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