10+ Year Member
Jun 4, 2008
Hi everyone,

I know that the temperature decreases in the adiabatic expansion of an ideal gas due to the work on the surroundings. However, one of the EK 1001s seems to say that when a gas expands at constant pressure, the temperature increases because PV = nRT (V increases, so T has to increase). Are these two both true (can they both be true)? They seem to contradict each other.



10+ Year Member
Mar 6, 2008
Resident [Any Field]
they don't contradict each other. they are talking about two different processes.

an adiabatic process involves no transfer of heat. The pressure and volume, however, do change. Adiabatic expansion definitely results in cooling. An example of this is opening a cylinder of pressurized gas without a regulator. It is an adiabatic process, but the volume changes (it's expanding) and the pressure changes (it drops since the pressure is high in the cylinder).

the other process you're talking about is an isobaric process, which involves no pressure change. Any process in open air is isobaric. Temperature increases as volume increases.