m A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics. A basic illustration of Maxwell's machine is provided in Fig. 15.1. As Maxwell describes it, the machine consists of a gas container or "vessel," which is separated by a wall into two chambers, A and B. The gas absolute temperature T is uniform, 2 2 To recall, the absolute temperature is expressed in units of kelvin (K). It has a one-to-one correspondence with the Celsius (C) temperature scale, with absolute zero being OK = -273 °C and the melting-ice temperature being 0 °C = +273 K. 15.1 Maxwell's demon and Landauer's principle and is given by the formula E = (3/2)k T, where E is the average kinetic energy of the gas molecules (k = Boltzmann's constant). Both temperature and energy represent macroscopic measures, as individual molecules don't have a "temperature" and have different velocities of their own, some moving slower and some moving faster. As the figure shows, an imaginary, witty but not malicious creature (later dubbed "Maxwell's demon") guards a trapdoor, which allows individual molecules to pass from A to B or the reverse, without friction or inertia. The demon checks out the molecules' velocities and allows only the faster ones (E' > E) to pass from A to B, and only the slower ones (E' < E) to pass from B to A. After a while, most of the fast molecules are found in B, while most of the slow ones are in A. As a result, the gas temperature in A (T') is lower than that in B (T"), with T' < T < T". The simultaneous heating of part B and cooling of part A, which is accomplished by the demon without deploying energy or work, is in contradiction to the second law of thermodynamic. This second law stipulates, in its simplest formulation, that: "Heat does not flow spontaneously from a cold to a hot body of matter." Since heat is a measure of the average kinetic energy of the gas, it has a natural tendency 