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有读书笔记Swirling dust shocks physicists

1 shizhao 添加于 2010-4-13 20:19 | 2292 次阅读 | 0 个评论
  •  作 者

    Brumfiel G
  •  摘 要

    Scientists have explained how lightning can occur even in the driest deserts. A new theory describes how neutral dust can gain an electrical life of its own. For centuries, researchers have known that clouds of neutral particles can sometimes gain a net charge. This can cause even the driest sand to generate lightning, and sugar refineries and coal-processing plants can experience unexpected explosions. Most researchers have ascribed such events to static build-up, but Troy Shinbrot, a physicist at Rutgers University in Piscataway, New Jersey, was unconvinced. Under normal conditions, sand and dust don't conduct electricity, he says, so how could they generate fields strong enough to spark massive lightening bolts? "These materials are insulators under very dry conditions, so where are the charges coming from?" he asks. Charge of the balloon brigade Shinbrot sat up at night for months thinking about it, and eventually he developed a theory. He began by visualizing the sand particles as party balloons. In an electric field, he thought, the balloons would polarize: In other words, each balloon would develop a positive and negative hemisphere. He then thought about what would happen if a negative hemisphere from one balloon touched the positive hemisphere of another. The touching hemispheres would neutralize, but each balloon's other hemisphere would not because they are in an independent electric field. Insulating particles become charged through a series of collisions.T. Pähtz1, H. J. Herrmann and T. Shinbrot / Nature Physics When the balloons parted ways, they would repolarize in the electrical field around them. But, as the balloons repolarize, the hemispheres that never came into contact with each other would gain an extra unit of charge. In this way, the balloons could gain very, very high charges, even though they were initially neutral. By Shinbrot's own admission, the idea of neutral particles charging through the act of neutralizing "just didn't seem right". But when he began modelling his theoretical sand particles, he found that the idea held up. Moreover, the models predicted optimal densities of particles where the effect should be most pronounced. Shinbrot and his team tested their models with an experiment. The team placed coloured glass beads into a jar and placed it under a 30-kilovolt electrical field. They then puffed air through the jar and watched what happened. Sure enough, at non-optimal densities, only a few beads became charged (see video), but at the optimal density predicted by the model, many beads took flight (see video). "This crazy idea seemed to work," he says. The team's work appears online today in Nature Physics1. A farewell to chainsaws "I think their model makes sense," says Daniel Lacks, a chemical engineer at Case Western Reserve University in Cleveland, Ohio. It may seem like esoteric research, but it will probably end up coming in handy, he adds. ADVERTISEMENT Small particles are commonplace in industrial processes, and charging is a perennial problem. For example, particles used in the production of polyethylene — the world's most common plastic, used in plastic shopping bags — often gain a static charge and cling to the walls of reaction chambers, clogging up the equipment. At the moment, the solution is decidedly low-tech. "They have to go into reactors with chainsaws and blowtorches", to cut away the accumulated mass of particles, says Lacks. The new model could help improve production techniques. But the mystery isn't entirely solved. For one thing, the team still hasn't explained the origin of the external electrical field needed to kick off the charging process. Shinbrot thinks it may be possible for moving clouds of dust or sand to self-generate fields. But Lacks suspects that unknown mechanisms may also be contributing. "Everything seems right," says Lacks, "but I don't think this is the final story."
  •  详细资料

    • 文献种类: Journal Article
    • 期刊名称: Nature
    • 期刊缩写: Nature
    • 期卷页: 2010
    • ISBN: 1476-4687
  • 学科领域 工程技术 » 环境科学

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    为什么干燥沙漠中也能产生闪电(左图)?一种新理论试图解释电中性尘埃是如何产生电荷的。

    很久以来,研究人员就知道,一片电中性粒子有时能得到净电荷,这样即使是最干的沙漠也能产生闪电,炼糖厂和煤处理工厂可能会发生出其不意的爆炸。很多研究者将这归咎于静电积聚,但是有一些研究人员对此有疑问。

    研究者说,不妨把粒子看成气球,在外部电场中,气球可能会极化,出现正电区和负电区。一个气球的正电区和另一个气球的负电区接触其电荷会互相中和,但是每个气球剩下的半球上的电荷仍会保留。碰撞结束后,粒子会在外围电场中再次极化,之后上次没有和其它粒子接触的那一半半球会得到更多的电荷。通过这种方式,尽管气球起初是电中性的,最后却会得到很高很高的电荷。

    研究者自己最开始觉得这个理论不太可能是正确的,但是当他开始为他理论中的沙粒建模后,他发现这个理论可能能够成立。而且,理论模型预言了该种效应最强处的理想粒子密度。理论似乎得到了实验验证,研究者将彩色玻璃珠放进罐子里,在罐子区域加上30千伏的电场,之后喷进空气,观察发生的现象。结果发现,粒子密度未达到理论计算的理想密度时,只有很少几个玻璃珠带电,但是粒子密度达到理论给出的密度时,很多粒子都带上了电,论文发表于《自然—物理学》。

    像聚乙烯制造这种工业处理过程中,存在很多小颗粒,颗粒带电现象很普遍,带来很多麻烦,新模型也许能改进制造工艺。

    这一理论仍不完善,背景电场从何而来?这个问题尚无法解释,而正是这个背景电场使得一切能够开始。

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