力是颗粒状态转换的关键
文章来源:未知 文章作者:enread 发布时间:2013-02-18 07:12 字体: [ ]  进入论坛
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Ever wonder why sand can both run through an hourglass(沙漏) like a liquid and be solid enough to support buildings? It's because granular(颗粒的) materials -- like sand or dirt -- can change their behavior, or state. Researchers from North Carolina State University have found that the forces individual grains exert on one another are what most affect that transition. Physicists1 have explored the changing behavior of granular materials by comparing it to what happens in thermodynamic systems. In a thermodynamic system, you can change the state of a material -- like water -- from a liquid to a gas by adding energy (heat) to the system. One of the most fundamental and important observations about temperature, however, is that it has the ability to equilibrate(平衡,相称): a hot cup of tea eventually cools to match the temperature of the room.
 
Physicists thought they could use thermodynamics' underlying3 ideas to explain the changes in granular materials, but didn't know whether granular materials had properties which might equilibrate in a similar way. In other words, instead of temperature being the change agent in a granular system, it might be a property related to the amount of free space, or the forces on the particles. But no one had really tested which of the two might exhibit this property of equilibration.
 
NC State physicist2 Karen Daniels and former graduate student James Puckett devised a way to do just that. Puckett used two different types of plastic "granules" with different properties that floated atop a layer of air on a small table. Puckett and Daniels wanted to see what would bring the two types of particles into equilibrium4 with one another. In order to make their measurements, they used a plastic material that indicated a change in force by a change in brightness.
 
First, they measured compactivity, which describes the number of ways particles can arrange themselves inside a given space, or volume, by reducing the physical space around the granules, but the two types of particles failed to achieve equilibrium. When they measured the ways that the forces between the particles could rearrange, they saw the equilibrium they were looking for.
 
Their findings appear in Physical Review Letters.
 
"Physicists often have ideas that are theoretically elegant, such as the idea that there might be new temperature-like variables to be discovered, and then it's exciting to go into the lab and see how well these ideas work in practice," says Daniels. "In this case, we found it is possible to take the temperature of a granular system and find out more about what makes it change its state. The 'thermometer' for this temperature is actually the particles themselves."


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1 physicists 18316b43c980524885c1a898ed1528b1     
物理学家( physicist的名词复数 )
参考例句:
  • For many particle physicists, however, it was a year of frustration. 对于许多粒子物理学家来说,这是受挫折的一年。 来自英汉非文学 - 科技
  • Physicists seek rules or patterns to provide a framework. 物理学家寻求用法则或图式来构成一个框架。
2 physicist oNqx4     
n.物理学家,研究物理学的人
参考例句:
  • He is a physicist of the first rank.他是一流的物理学家。
  • The successful physicist never puts on airs.这位卓有成就的物理学家从不摆架子。
3 underlying 5fyz8c     
adj.在下面的,含蓄的,潜在的
参考例句:
  • The underlying theme of the novel is very serious.小说隐含的主题是十分严肃的。
  • This word has its underlying meaning.这个单词有它潜在的含义。
4 equilibrium jiazs     
n.平衡,均衡,相称,均势,平静
参考例句:
  • Change in the world around us disturbs our inner equilibrium.我们周围世界的变化扰乱了我们内心的平静。
  • This is best expressed in the form of an equilibrium constant.这最好用平衡常数的形式来表示。
TAG标签: sand state granular
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