An easy to perform but often counterintuitive demonstration of gas expansion

During their thermodynamics courses, students learn that the temperature of an ideal gas will drop during an adiabatic reversible expansion. They also usually learn that no change of temperature occurs as a result of a certai n free expansion. These results often become intuitively connected with gas expansion. However when air expands freely into an evacuated chamber from a constant pressure atmosphere, its temperature increases. This can be easi ly demonstrated using only simple equipment and makes for a memorable lesso n on the importance of identifying what exactly is the system and then simp ly applying the first law of thermodynamics. (C) 1999 American Association of Physics Teachers.