answers-02-08-2011 - Answers to Physics 176 One-Minute...

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Unformatted text preview: Answers to Physics 176 One-Minute Questionnaires Lecture date: February 8, 2011 Is the experiment with the non-equilibrium system in the room dependent on anything except temperature difference (e.g., room size, room shape)? The answer is yes, the details of the nonequilibrium states do depend on the shape and size of the room and on some properties of the walls such as whether they conduct heat well (like metal) or do not (like wood or glass). That the patterns do depend on the rooms geometry was a surprise when first discovered experimentally (in the late 1970s and early 1980s) and some details remain poorly understood. Before the experiments were done, the belief of many scientists was that, if you could make the convection cell (room with copper floor and copper ceiling held at fixed temperatures) sufficiently wide, so that the width was much greater than the depth of the air, then the patterns in the fluid would not depend on the shape or size of the room, just as the periodic array of atoms in a crystal do not depend on the shape or size of the crystal once the crystal is sufficiently big compared to the spacings of the atoms. (Near the surfaces of crystals, say about 5-50 atoms in depth, there are deviations from perfect periodic structure but the influence of the surface dies out.) An even bigger experimental surprise was that the air could become turbulent (evolving nonperiodically in time and space) for a constant tem- perature difference between the floor and ceiling that theory indicated only time-independent convection rolls (stripes) could occur. Professor Robert Behringer at Duke was a coauthor of this unexpected and big discovery (he was a postdoc at Bell Labs at the time), and when I heard about this work as a graduate student at Princeton, I became fascinated with where did these patterns come from and dropped the research I was doing at the time (in condensed matter physics) to study these questions full time. Many details of the stripe, hexagon, and spiral patterns are now well understood theo- retically but the question of when and how chaotic patterns (ones irregular in time and space) arise is still about as mysterious now as when they were discovered about 20 years ago. You might think that understanding this pattern formation is an aca- demic questionwho worries about patterns in some highly artificial con- vection cell?but there are many practical questions related to engineering, medicine, chemistry, biology, meteorology, and geology for which progress is slow or limited because we dont have a good understanding of nonequi- 1 librium pattern formation. Two medical examples would be heart attacks (physicists would describe ventricular fibrillation as a transition of an or- dered periodic pattern to a disordered chaotic pattern), and epilepsy (which represents a transition from a chaotic brain state to one that more periodic in time)....
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This note was uploaded on 10/20/2011 for the course PHYSICS 176 taught by Professor Behringer during the Spring '08 term at Duke.

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answers-02-08-2011 - Answers to Physics 176 One-Minute...

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