If I fill a cylindrical vessel with a gas and put a piston of some mass on top of it and slowly heat the vessel, the piston will move up. But does the pressure inside the vessel change? ( vessel is insulated).if pressure doesn't change, how does the piston move up? Pressure on piston : ##P - p## where P is atmospheric pressure and p is pressure exerted by gas. Pressure on gas: ##mg/A + P## Also in W = PdV, does P denote external pressure or pressure exerted by gas?