I'm slightly confused by a common situation in thermochemistry.

Consider a container with gas inside and a movable, massless piston. The pressure of the gas inside is 6 atm and the external pressure is 5 atm. Thus the gas will expand. However since the piston is massless, the two forces on the piston must be equal by Newton's 2nd Law. But the pressure from the gas and the pressure from the outside atmosphere are clearly unequal, so what's the problem here?

  • $\begingroup$ chemistry.stackexchange.com/questions/38345/… $\endgroup$ – Mithoron Oct 27 '15 at 22:15
  • $\begingroup$ Also your reasoning makes no sense... So you should probably elaborate $\endgroup$ – Mithoron Oct 27 '15 at 22:16
  • $\begingroup$ Which part doesn't make sense? $\endgroup$ – carbenoid Oct 27 '15 at 23:08
  • $\begingroup$ What @orthocresol is saying is not correct. If the piston mass gets very small, its acceleration obviously does not become infinite. This is because the force per unit area exerted by the gas on the piston will approach the pressure exerted externally. But the gas pressure in the cylinder is not uniform, and viscous stresses are also present. For more details, see my answer in this thread:chemistry.stackexchange.com/questions/38345/… $\endgroup$ – Chet Miller Oct 28 '15 at 0:01
  • $\begingroup$ @ChesterMiller I'll just go ahead and delete it then $\endgroup$ – orthocresol Oct 28 '15 at 0:05

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