. P w./ & New England Associa+ion of C h e m B c h e r s *%TRY
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M. V. Sussman Tufts University Medford, Massachusetts
Maxwell's Demon Demonstrator
Thermodynamics, for all of its fascination, does not lend itself to many physical demonstrations. Thinking that some teachers and their students might welcome an addition to the limited list, I should like to present the "Maxwell's Demon Demonstrator." The basic demonstration is a simple variation of an old method for illustrating the concept of irreversibility. This first came to my attention in a "Larousse Srientitique"' which used examples similar to Figures 1, 2, and 3 to show the nature of an irreversible process. Figure 1 shows a sealed glass vessel having an elongated neck, held in an inverted position. Ten small spheres (five black and five white) have been sealed inside the vessel and rest in the neck in a definite order. I n the illustration all of t,he black spheres are surmounted by the white ones. One might think of them as a layer of hot gas molecules lying above a layer of cold gas molecules. If the flask is tilted so that the spheres run out of the nerk and mix in the body of the flask (Figure 2) an "irrerersihle" process has been performed. On reversing the t,ilting process, there spheres will run hack into the neck of the flask but in an order which will probably not be the organized initial array. The arrangement will be randon, and might. have the appearance shown in Figure 3. This is the demonstration suggested by the Larousse reference. As it stands, it is excellent because it illustrates an irreversible process and also points out its statistical nature. It is not impossible that the initial array will be restored by reversing the tilting process. It simply is unlikely. Using five black and five white
' URBAIN,G., ASD BOLL,M., "La Science. Sea Progrbs, Ses Applications." Lihrarie Lmousse, Paris, 1934, p. 372-3.
Figure 1. An 0rgonired array of particier.
Figure 2.
balls, the odds against reproducing thc initial array are 252 to 1. If one thinks in terms of 5 cm3 of hot gas and 5 cm3 of cooler gas, rather than five black and five white spheres, consideration is extended to a greatly increased number of particles (molecules). On writing the probability expression for achieving an event wherein all the low temperature gas molecules precede all of the high temperature gas molecules into the neck of the flask, the truly formidable odds against this event are appreciated and insight into the nature of irreversibility may be gained. Phase one of the demonstration is now completed. Repeated inversion of the irreversibility bottle will produce varying arrays, some distressingly close to the starting pattern. On a bad day, one might even achieve the original array. However, if the particular day is not a bad one, one now becomes conscious of a serious shortcoming in the demonstration. Because the bottle is sealed, it requires an inordinate amount of time to reconstruct the original array and ready the bottle for the next showing! Here is a real situation that would benefit by the existence of Maxwell's Demon, and from the pedagogic point of view, an excellent opportunity to introduce the Demon concept. The concept of a minute, lightning-eyed intelligence who can effortlessly separate a random distribution of fast (i.e., hot) and slow (i.e., cold) molecules (or white and black spheres). Standing a t an advantageous spot in the vessel he can pile up the molecules in carefully selected order. By destroying their random pattern as they dash wildly about in the flask he successfully violates the second law! To emphmize the difficulty of the Demon's task one should now grasp the flask at the base of the neck, and,
. . subjected to on "irreversible" procesl.
. ..
Figure 3. cannot uwolly b e restored by reversing the mixing process.
Figure 4.
The "Demon" at wort.
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Figure 5.
The Demon X-rayed.
holding it in a neck-up position so that the spheres remain in the flask body, rotate the flask briskly to impart a rapid swirling motion to the spheres. If the flask is now turned to the neck down position while continuing the rotation, the spheres will continue swirling wildly in the flask body, held there by centrifugal force. At this point in the discussion one should remind the class, that no one, unfortunately, has ever succeeded in synthesizing or trapping iNaxwellls Demon. The rotation can then be stopped. When the motion of the flask ceases, the spheres whirling about in the flask body begin to lose momentum and follow an ever tightening spiral path into the neck. If this maneuver is performed with the proper flair, the behavior of the spheres as they enter the neck
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of the bottle may cause some observers to wonder about the true elusiveness of Maxwell's Demon. As the audience watches, the black balls spiral down into the neck of the flask followed closely by the white balls. As shorn in Figure 4, the initial array is restored! Class reaction reaches a maximum as the last white ball tumbles into place in its proper order. There usually follows a series of unsolicited explanations of this obvious bit of deception. "It's done with magnets!" "The balls are weighted!" Neither of the foregoing explanations is quite correct. The "demonability" resides in the fact that the black spheres are constructed so as to lose momentum very rapidly. They are hollow and contain within themselves some momentum absorbing means. A construction that has worked very successfully is shown in Figure 5. The materials of construction for the Demon Demonstrator consists of ten 3/4-in. diameter cork or plastic spheres, and a 2000 ml volumetric flask (Pyrex). Five spheres have the internal construction shown in Figure 5, and are painted a color which contrasts sharply with the five unaltered spheres. The neck of the flask should he about 8 in. long after sealing to accommodate the total number of spheres within its length.
