answers-01-25-2011

# answers-01-25-2011 - Answers to Physics 176 One-Minute...

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Unformatted text preview: Answers to Physics 176 One-Minute Questionnaires Lecture date: January 25, 2011 Will all quizzes and exams be as long as today’s relative to the time allotted? This seems like it would penalize students who write more slowly. I try to design the quizzes so that 15-20 minutes should be enough time to answer all the questions provided that you have been actively and critically thinking about the course material and homework problems. Designing a quiz or an exam is an imperfect process and I take that into account by scaling the grades. But I also purposely want the quizzes to challenge the class, at least a little bit, so as to encourage you to improve your problem solving abilities and your understanding of the material through frequent feedback. If you find that you do write slowly, one way to help with that is to practice solving more problems before a quiz or exam. Try skimming the other thermal physics books on reserve or in the library and try to solve representative problems that other authors have identified as important, also try solving some extra problems in Schroeder. What is the difference between the T/F question on relaxation time on the quiz (one in vacuum, on in air)? I will be posting detailed answers to all quizzes a few days after each quiz. But briefly, the rod of length L and radius r L that is sitting in vacuum has a relaxation time of L 2 /κ since information can only travel along the length of the rod for equilibration to occur. For the rod sitting in air of constant temperature T , it is the equilibration of the rod with the air, not with itself, that now matters since the final equilibrium temperature will be T . Since no part of the rod is further away from the air than a distance r , the relaxation time is now r 2 /κ . On the 2009 quiz 1, the solutions said that the mechanical relax- ation time was τ mech = d/v sound for the balloon problem. Why does this not vary like L 2 ? I was not correct in explaining that solution. In 2009, I had explained to the class that imbalances in mechanical forces typically propagate quickly 1 through a system compared to diffusion of heat or concentration, typically at the speed of sound in the material. Thus when you pop a balloon with a pin, the time it takes the balloon to disintegrate is roughly the circumference of the balloon divided by the speed of sound in the skin of the balloon. But this is just the time for one part of a system to know about some other part of the system having a pressure difference, it is not the time for equilibrium which indeed occurs on a slower diffusive time scale as we discussed in class. Are there other commonly used coordinate systems besides rect- angular, cylindrical, and spherical?...
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answers-01-25-2011 - Answers to Physics 176 One-Minute...

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