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愛因斯坦和汽車電池Einstein and car batteries

A spark of genius
天才的靈光
Without the magic of relativity, a car's starter motor would not turn 
要不是相對論的魔力,汽車的發動馬達就轉不起來
ALBERT EINSTEIN never learned to drive. He thought it too complicated and in any case he preferred walking. What he did not know—indeed, what no one knew until now—is that most cars would not work without the intervention of one of his most famous discoveries, the special theory of relativity.
阿爾伯特.愛因斯坦從沒去學開車。他覺得開車太復雜,再者,他更喜歡走路。而他不知道的——也是直到現在人們才知道的是——沒有他的偉大發現之一,即狹義相對論,大多數汽車不可能發動起來。
Special relativity deals with physical extremes. It governs the behaviour of subatomic particles zipping around powerful accelerators at close to the speed of light and its equations foresaw the conversion of mass into energy in nuclear bombs. A paper in Physical Review Letters, however, reports a more prosaic application. According to the calculations of Pekka Pyykko of the University of Helsinki and his colleagues, the familiar lead-acid battery that sits under a car's bonnet and provides the oomph to get the engine turning owes its ability to do so to special relativity.
狹義相對論同物理極限相關。該理論掌握了亞原子粒子在強大的加速器的作用下可以達到接近光速的速度這一表現行為。相對論的公式也預見了核彈中質(量)能(量)轉換的現象。然而,一篇發表在物理評論快報上的文章,講述了狹義相對論更為一般的應用。根據赫爾辛基大學的Pekka Pyykko和他同事們的計算,我們所熟悉的在汽車發動機罩下,給汽車引擎發動提供能量的鉛酸電池,它之所以有這樣的能力都歸功于狹義相對論。
Relative values.

相對的價值
The lead-acid battery is one of the triumphs of 19th-century technology. It was invented in 1860 and is still going strong. Superficially, its mechanism is well understood. Indeed, it is the stuff of high-school chemistry books. But Dr Pyykko realised that there was a problem. In his view, when you dug deeply enough into the battery's physical chemistry, that chemistry did not explain how it worked.
鉛酸電池是19世紀技術發展的產物之一。它于1860年發明,迄今為止仍然具有很強的實用性。表面上,其機制為人熟知這些都是高中化學課本上的東西。但Pyykko博士發覺了哪里不對勁。在他看來,越是深入研究電池的物理化學特性,這些化學特性反而越不能解釋電池到底是怎么工作的。

A lead-acid battery is a collection of cells, each of which contains two electrodes immersed in a strong solution of sulphuric acid. One of the electrodes is composed of metallic lead, the other of porous lead dioxide. In the parlance of chemists, metallic lead is electropositive. This means that when it reacts with the acid, it tends to lose some of its electrons. Lead dioxide, on the other hand, is highly electronegative, preferring to absorb electrons in chemical reactions. If a conductive wire is run between the two, electrons released by the lead will run through it towards the lead dioxide, generating an electrical current as they do so. The bigger the difference in the electropositivity and electronegativity of the materials that make up a battery's electrodes, the bigger the voltage it can deliver. In the case of lead and lead dioxide, this potential difference is just over two volts per cell.
鉛酸電池是電池單元構成的集合,其中每個電池都有兩個電極,浸泡在硫酸溶液的電解液里。金屬鉛充當一處電極,另一處電極是多孔二氧化鉛?;瘜W家認為,金屬鉛是電正性物質。這表明,當鉛和酸發生反應時,它很可能失去一些電子。而另一方面,二氧化鉛是電負性物質,在化學反應中更喜歡吸收電子。如果把一根導電金屬絲放在金屬鉛和二氧化鉛之間,鉛釋放的電子會經金屬絲傳遞到二氧化鉛,這個過程會產生電流。組成電池兩級的物質的正負電荷差越大,他們發生化學反應時產生的電伏數越大。以鉛和二氧化鉛為例的電池,每節電池的電位差可產生2伏電壓。
        
That much has been known since the lead-acid battery was invented. However, although the properties of these basic chemical reactions have been measured and understood to the nth degree, no one has been able to show from first principles exactly why lead and lead dioxide tend to be so electropositive and electronegative. This is a particular mystery because tin, which shares many of the features of lead, makes lousy batteries. Metallic tin is not electropositive enough compared with the electronegativity of its oxide to deliver a useful potential difference.
自鉛酸電池發明以來,上述的理論就已為人熟知。然而,盡管很大程度上我們都掌握和了解這些基礎化學反應,卻沒有人能夠真正說明最根本的原理——為什么鉛和二氧化鉛帶有這般的電正性和電負性呢?這一點顯得頗為神秘,因為和鉛特性差不大多的錫,無法用來做電池。比起鉛來,金屬錫的電正性沒有二氧化錫的電負性強,所以無法產生可用的電位差。
This is partly explained because the bigger an atom is, the more weakly its outer electrons are bound to it (and hence the further those electrons are from the nucleus). In all groups of chemically similar elements the heaviest are the most electropositive. However, this is not enough to account for the difference between lead and tin. To put it bluntly, classical chemical theory predicts that cars should not start in the morning.
原子越大,其外層電子受原子束縛力越弱,這是解釋鉛和錫兩者差別的一部分原因。在化學性質相似的同族元素中,質量越重帶的正電越強。然而這依然不能充分說明鉛和錫的差別。直截了當地說, 古典化學理論預言了早上(要離家上班)汽車是發不動的。
Which is where Einstein comes in. For, according to Dr Pyykko's calculations, relativity explains why tin batteries do not work, but lead ones do.
那愛因斯坦怎么被扯進來了,根據Pyykko博士的計算,相對論解釋了為什么鉛可以用來做電池,而錫不可以。
His chain of reasoning goes like this. Lead, being heavier than tin, has more protons in its nucleus (82, against tin's 50). That means its nucleus has a stronger positive charge and that, in turn, means the electrons orbiting the nucleus are more attracted to it and travel faster, at roughly 60% of the speed of light, compared with 35% for the electrons orbiting a tin atom. As the one Einsteinian equation everybody can quote, E=mc2, predicts, the kinetic energy of this extra velocity (ie, a higher E) makes lead's electrons more massive than tin's (increasing m)—and heavy electrons tend to fall in and circle the nucleus in more tightly bound orbitals.
他一連串的理由是這樣解釋的。鉛比錫重,核子里的質子數更多(82比50)。這表明原子核的正電更強,同理表明更容易吸引繞著原子核的電子,電子傳播的速度也更快,其速度是光速的60%,相比之下,繞著錫原子的電子速度只能達到光速的35%。人人都會引用的愛因斯坦相對論公式:E=mc2,公式表明這一額外速度(即更高的能量)產生的動能使得鉛的電子比錫的更重(不斷增加的質量)——而重的電子往往會下落,圍著結合更緊密的原子核軌道繞行。
That has the effect of making metallic lead less electropositive (ie, more electronegative) than classical theory indicates it should be—which would tend to make the battery worse. But this tendency is more than counterbalanced by an increase in the electronegativity of lead dioxide. In this compound, the tightly bound orbitals act like wells into which free electrons can fall, allowing the material to capture them more easily. That makes lead dioxide much more electronegative than classical theory would predict.
產生的結果是,金屬鉛的電正性沒有古典化學理論認為的那么強(或者說,更為電負性)——看起來似乎鉛不適合用來做電池。但是, 二氧化鉛電負性的增加不但全部抵消了這個趨勢還有剩余。在這個混合物里,結合緊密的軌道就像一口井,自由電子落入其中,使得物質更容易捕獲電子。二氧化鉛的電負性其實比古典化學理論認為的要更強。
And so it turned out. Dr Pyykko and his colleagues made two versions of a computer model of how lead-acid batteries work. One incorporated their newly hypothesised relativistic effects while the other did not. The relativistic simulations predicted the voltages measured in real lead-acid batteries with great precision. When relativity was excluded, roughly 80% of that voltage disappeared.
然后他們得出了結論。Pyykko博士和他的同事們作了兩個版本的電腦模型,來觀察鉛酸電池是怎么工作的。其中一個模型結合使用了相對論效應的新假設,另一個沒有用。相對論模擬模型預測的鉛酸電池產生的電壓相當精確。而那個不用相對論的模型大約80%的電伏都沒有計算到。
That is an extraordinary finding, and it prompts the question of whether previously unsuspected battery materials might be lurking at the heavier end of the periodic table. Ironically, today's most fashionable battery material, lithium, is the third-lightest element in that table—and therefore one for which no such relativistic effects can be expected. And lead is about as heavy as it gets before elements become routinely radioactive and thus inappropriate for all but specialised applications. Still, the search for better batteries is an endless one, and Dr Pyykko's discovery might prompt some new thinking about what is possible in this and other areas of heavy-element chemistry.
這是個非同凡響的發現,這個發現也提出了一個問題。是否還有以前沒想到的,潛伏在元素周期表末端的電池材料?諷刺的是,現在最時髦的電池材料,鋰,是周期表中第三輕的元素——如果用相對論效應是料不到它可以用來做電池。鉛是周期表中放射性元素之前最重的元素,因此除了用于專門應用外不適用于他處。而有關更好的電池材料的研究是沒有止境的,Pyykko博士的發現也許給我們提供了一些新的思考方向——化學重金屬在電池和其他地方還有什么作為?

 
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