PV = nRT When a gas in a cylinder expands, the gas pushes the piston upward against a constant opposing external atmospheric pressure P. The work done by the gas on the surroundings is w= -P[itex]\Delta[/itex]V [itex]\Delta[/itex]V is the change in volume My question is: Assuming the Temperature stays constant (as my book does), then as the gas expands (the volume increases) then the Pressure must also decrease, right? bc PV = nRT and everything else is constant.... But pressure is a force, and according to Newton, every force has an equal and opposing force. So the pressure that the gas exerts must equal the atmospheric pressure, right? But how can this be true if the pressure of the gas is decreasing as the volume is increasing?