Maxwell's Demon

CORE DEFINITION

Maxwell's demon is a thought experiment that attempts to refute the second law of thermodynamics, conceived by British physicist James Clerk Maxwell in 1867. It imagines a demon capable of detecting and controlling the motion of individual molecules. At the time, Maxwell realized that there exist energy control mechanisms in nature that counteract the increase of entropy, but he could not clearly explain such mechanisms. He could only humorously postulate a 'demon' that could distribute randomly moving particles into certain phase cells according to a certain order and rules. Maxwell's demon is a prototype of dissipative structures. It can be simply described as follows: an adiabatic container is divided into two equal compartments, with a small 'door' controlled by the demon. Air molecules in the container collide with the door during their random thermal motion. The door can selectively allow faster molecules into one compartment and slower molecules into the other, thus making one compartment hotter than the other. This temperature difference can be used to drive a heat engine to do work. This is an example of a perpetual motion machine of the second kind.