Physics of the fire piston and the fog bottle

Abstract

We analyze two irreversible adiabatic processes occurring in two thermodynamical devices: the fire piston and the “fog bottle”. In the first, an abrupt adiabatic compression of air inside a piston causes the burning of a small paper piece, since the final temperature reaches 700 K. In the second, an abrupt adiabatic expansion of a gas at high pressure leads to a temperature decrease with condensation of water vapour and fog formation. We show that, if the two processes were carried out reversibly, the final temperature would always be lower than in the irreversible processes