你所知的静电场都是错的

原作者：
来源What You Learned About Static Electricity Is Wrong
译者dlq

For many of us, static electricity is one of the earliest encounters we have with electromagnetism, and it’s a staple of high school physics. Typically, it’s explained as a product of electrons transferred in one direction between unlike substances, like glass and wool, or a balloon and a cotton T-shirt (depending on whether the demo is in a high school class or a kids’ party). Different substances have a tendency to pick up either positive or negative charges, we’re often told, and the process doesn’t transfer a lot of charge, but it’s enough to cause a balloon to stick to the ceiling, or to give someone a shock on a cold, dry day.

       对我们大多数人而言，静电场是我们最早学习的电磁场知识之一，是高中物理课程的重点内容。通常它被解释为电子在性质不同的物质间沿某一方向运动形成的，比如玻璃和木头之间，气球和纯棉T恤之间（选择什么物品就取决于你是在高中课堂还是在儿童乐园咯）。我们常说，不同的物质对电子的吸引能力不同，此过程并没有大量电子的转移，但是已经足以导致气球飞向天花板或者在干冷的日子给人一个电击了。

Nearly all of that is wrong, according to a paper published in today’s issue of Science. Charges can be transferred between identical materials, all materials behave roughly the same, the charges are the product of chemical reactions, and each surface becomes a patchwork of positive and negative charges, which reach levels a thousand times higher than the surfaces’ average charge.

       然而根据最近《科学》杂志的一篇论文，所有这些几乎都是错的。电荷可以在相同的材料间转移，所有材料表现都大致一样。化学反应产生电荷，物质各表面形成正负电子对，比表面平均电荷要高三个数量级。

Where to begin? The authors start about 2,500 years ago, noting that the study of static began with a Greek named Thales of Miletus, who generated it using amber and wool. But it wasn’t until last year that some of the authors of the new paper published a surprising result: contact electrification (as this phenomenon is known among its technically oriented fans) can occur between two sheets of the same substance, even when they’re simply allowed to lie flat against each other. “According to the conventional view of contact electrification,” they note, “this should not happen since the chemical potentials of the two surfaces/materials are identical and there is apparently no thermodynamic force to drive charge transfer.”

       从哪开始呢？作者从2500年前说起，最早研究静电的是古希腊人泰勒斯，他通过琥珀和羊毛摩擦而发现的。但是直到去年论文作者公布了一个令人吃惊的结论：摩擦生电（科技迷熟知的现象）可以发生在两块相同的物质之间，甚至它们只需要贴在一起。“按照常规摩擦生电的观点”他们说，“这是不可能的，因为两材料/表面化学势是相等的，没有促使电荷转移的热动力。”

One possible explanation for this is that a material’s surface, instead of being uniform from the static perspective, is a mosaic of charge-donating and charge-receiving areas. To find out, they performed contact electrification using insulators (polycarbonate and other polymers), a semiconductor (silicon), and a conductor (aluminum). The charged surfaces were then scanned at very high resolution using Kelvin force microscopy, a variant of atomic force microscopy that is able to read the amount of charge in a surface.

       不从常规的静态角度看，一种可能的解释是，材料表面有大量的电子和空穴。研究者为此用绝缘体（聚碳酸酯和其他聚合物），半导体（硅）和导体（铝）分别做摩擦生电实验，然后用高分辨率开尔文力显微镜观测带电表面，开尔文力显微镜是一种能够测量表面电荷大小的原子力显微镜。

The Kelvin force microscopy scans showed that the resulting surfaces were mosaics, with areas of positive and negative charges on the order of a micrometer or less across. All materials they tested, no matter what overall charge they had picked up, showed this mosaic pattern. The charges will dissipate over time, and the authors found that this process doesn’t seem to occur by transferring electrons between neighboring areas of different charge—instead of blurring into the surroundings, peaks and valleys of charge remain distinct, but slowly decrease in size. The authors estimate that each one of these areas contains about 500 elementary charges (that’s ±500 electrons), or about one charge for each 10nm2.

The reason that this produces a relatively weak charge isn’t because these peaks and valleys are small; the charge difference between them is on the order of 1,000 times larger than the average charge of the whole material. It’s just that the total area of sites with positive and negative charges are roughly equal (the two are typically within a fraction of a percent of each other). The distribution appears to be completely random, as the authors were able to produce similar patterns with a white noise generator that fluctuated on two length scales: 450nm and 44nm.

       导致这一弱带电的原因并不是因为这些峰谷很小，这之间的电荷差别比整块材料的平均电荷量要大三个数量级左右。只是带正负电的区域总面积是大致相当的（差别不超过千百分之一）。这个分布是完全随机的,而研究者能够用两个波段的白噪声发生器（450nm和44nm）产生相似的花样。

表面带静电（下）和不带静电（上）

So, what causes these charges to build up? It’s not, apparently, the transfer of electrons between the surfaces. Detailed spectroscopy of one of the polymers (PDMS) suggests that chemical reactions may be involved, as many oxidized derivatives of the polymer were detected. In addition, there is evidence that some material is transferred from one surface to another. Using separate pieces of fluorine- and silicon-containing polymers allowed the authors to show that signals consistent with the presence of fluorine were detected in the silicon sample after contact.

      那么是什么导致带电能力增强呢？很显然不是表面的电子转移。通过对其中一种聚合物进行详尽的光谱分析表明，可能发生了化学反应，观测到了聚合物的氧化产物。此外，有证据表明有些材料从一个表面迁移到另一个表面。通过分离含氯和硅的聚合物表明标记的氯在接触后的硅样品表面被探测到了。

The exact relationship between the charge transfer and the processes seen here—chemical reactions and the transfer of materials between the surfaces—isn’t clear at this point. But there are plausible mechanisms by which these processes could build up charges, and the authors very clearly intend to follow up on these findings.

         电荷转移和此过程（化学反应和表面之间的材料迁移）之间更准确的关系目前还不太清楚。但是研究者已经找到了此过程能增强电荷的可能机制，并且明确表示将跟进这一结果。

In the meantime, you can be duly impressed with how much charge you can shuffle around when you build up static. Each square inch is equivalent to about 6.5 x 1014 square nanometers, so based on the authors’ numbers, that’s a lot of electrons.

       同时，当你建立一个静电场时产生了多少电荷这一问题将给你留下深刻的印象。每平方英寸大约等于6.5*1014平方纳米，所以按照研究者的数据，那可是很多电子了。

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