# Gas expanding against a piston

## Homework Statement

Consider a cylindrical piston in which an ideal gas is kept. The gas is currently at pressure P1, volume V1, and temperature T. The temperature T is assumed to be constant throughout the experiment. The piston is massless and frictionless. On the piston is a block of mass M and this mass is not changed throughout the experiment. The block's cross-sectional area is A. Thus, the pressure of the block on the piston is Mg/A.

Assume that initially P>Mg/A so that the gas begin to expand until its pressure equilibrates with the pressure of the block. Its final volume is V2.

1) Throughout this process, is the pressure of the gas a constant?
2) To calculate the work done by the gas, do you use the gas pressure P or the external pressure Mg/A.

W = F*d
PV = nRT

## The Attempt at a Solution

1) As the gas expands, its pressure must decrease due to Boyle's law such that the product PV is a constant.
2) My textbook says we use the external pressure to calculate work, but I can't understand this. My argument is that to calculate work done by the gas, you need to use the pressure of the gas since it is the gas which is responsible for the force applied in raising the piston.

BiP