# Work in free expansion

I have read that no work is done during free expansion and temperature of an ideal gas undergoing free expansion doesn't change.

Let us assume an ideal system of piston and gas which has vacuum around it. The gas will expand but we say there is no work done by the gas. Why so ? Gas applies a pressure(& hence a force) on the piston and the piston gets displaced. So acc to F.s = work , there is work done by the gas !

• The words "free" and "piston" contradict each other. Commented Oct 14, 2015 at 9:22
• @ivan I have been taught in school that if we assume a weightless piston and vacuum around, we can regard it as free expansion. Is that incorrect ? Commented Oct 14, 2015 at 9:25
• Weightless piston is another story. It is as good as no piston at all. Commented Oct 14, 2015 at 9:34
• I think what biogirl is referring to is a massless piston. If you do a force balance on the piston (using Newton's 2nd law), the force exerted by the gas on one side of the piston is equal to the force exerted by the vacuum on the other side of the piston (i.e., zero force). So the force that the gas exerts on the massless piston is zero, and the gas does no work on its surroundings. Commented Oct 14, 2015 at 17:44
• @ChesterMiller I know this is a silly doubt but can the pistin move freely inside the container? Commented Mar 28, 2018 at 7:29

I have seen some answers on this similar question they all point to same thing that pressure outside is zero so from $$W = p.\Delta V$$, work done will be zero or as said in the answer from @ssavec that there is no force acting on the piston, the point is that you cannot have a balloon in space, it will burst due to inner pressure.