Calculating work done by gas?

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?

Woopydalan

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

sodium.dioxid

Then why does the piston move up due to internal pressure?

Infinitum

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.

morrobay

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