# Gas expansion and temperature

A piston is freed and the gas inside is expanded, volume will increase therefore temperature will increase too according to charles law, right? But at the same time pressure will decrease and so temperature will decrease too according to gay-lussac's law? I am very confused, which law should I consider in this case and why? Thank you..

• Charles' law relates volume and temperature at constant pressure so cannot be used as the pressure decreases unless there is an input of heat into the system. The gas will do work in expanding so its internal energy will decrease and so the temperature will decrease. – Farcher Oct 31 '17 at 15:09

As LostCause points out, these named laws aren't applicable to your case. But the ideal gas equation, $pV=nRT$, (which yields these laws as special cases) is applicable (provided that the gas density isn't too high).