The Demon demonstrates that the Second Law of Thermodynamics (heat flows from hot to cold bodies and work requires to be done to achieve the reverse) is essentially a statistical law ie the motions of individual molecules cannot be directed or controlled by any human agency. Suppose a vessel containing a gas is divided into two compartments by a partition which contains a small trap door. The Maxwell Demon stands guard at the door which he can open and close. The Demon lets fast moving molecules collect in the right hand compartment by letting through fast moving molecules going from the left hand compartment to the right compartment but not vice versa. Equally the Demon collects slow moving molecules in the left hand compartment. The right hand compartment therefore becomes hot and the left hand compartment cool in violation of the Second Law. The Second Law is valid because there are no Maxwell Demons. The laws of probability apply. If the trap door is left open (unattended by any Demon) there is a minuscule probability of all the fast molecules ending in the right hand compartment and vice versa.
Last modified 24 March 2006