TPO 18-3 Lightning（自然科学--闪电）
The electrical discharge takes place when the attractive tension between a region of negatively charged particles and a region of positively charged particles becomes so great that the charged particles suddenly rush together. The coming together of the oppositely charged particles neutralizes the electrical tension and releases a tremendous amount of energy, which we see as lightning. The separation of positively and negatively charged particles takes place during the development of the storm cloud.
The separation of charged particles that forms in a storm cloud has a sandwich-like structure. Concentrations of positively charged particles develop at the top and bottom of the cloud, but the middle region becomes negatively charged. Recent measurements made in the field together with laboratory simulations offer a promising explanation of how this structure of charged particles forms. What happens is that small (millimeter-to centimeter-size) pellets of ice form in the cold upper regions of the cloud. When these ice pellets fall, some of them strike much smaller ice crystals in the center of the cloud. The temperature at the center of the cloud is about -15℃ or lower. At such temperatures, the collision between the ice pellets and the ice crystals causes electrical charges to shift so that the ice pellets acquire a negative charge and the ice crystals become positively charged. Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud. The heavier negatively charged ice pellets are left to concentrate in the center. This process explains why the top of the cloud becomes positively charged, while the center becomes negatively charged. The negatively charged region is large: several hundred meters thick and several kilometers in diameter. Below this large, cold, negatively charged region, the cloud is warmer than -15℃, and at these temperatures, collisions between ice crystals and falling ice pellets produce positively charged ice pellets that then populate a small region at the base of the cloud.
Most lightning takes place within a cloud when the charge separation within the cloud collapses. However, as the storm cloud develops, the ground beneath the cloud becomes positively charged and lightning can take place in the form of an electrical discharge between the negative charge of the cloud and the positively charged ground. Lightning that strikes the ground is the most likely to be destructive, so even though it represents only 20 percent of all lightning, it has received a lot of scientific attention.
Using high-speed photography, scientists have determined that there are two steps to the occurrence of lightning from a cloud to the ground. First, a channel, or path, is formed that connects the cloud and the ground. Then a strong current of electrons follows that path from the cloud to the ground, and it is that current that illuminates the channel as the lightning we see.
The formation of the channel is initiated when electrons surge from the cloud base toward the ground. When a stream of these negatively charged electrons comes within 100 meters of the ground it is met by a stream of positively charged particles that comes up from the ground. When the negatively and positively charged streams meet, a complete channel connecting the cloud and the ground is formed. The channel is only a few centimeters in diameter, but that is wide enough for electrons to follow the channel to the ground in the visible form of a flash of lightning. The stream of positive particles that meets the surge of electrons from the cloud often arises from a tall pointed structure such as a metal flagpole or a tower. That is why the subsequent lightning that follows the completed channel often strikes a tall structure.
Once a channel has been formed, it is usually used by several lightning discharges, each of them consisting of a stream of electrons from the cloud meeting a stream of positive particles along the established path. Sometimes, however, a stream of electrons following an established channel is met by a positive stream making a new path up from the ground. The result is a forked lightning that strikes the ground in two places.
解析：A根据tension定位到第2句，对。B根据negatively charged定位到第2句，原文没有这个比较，选B。C根据come together定位到倒数第2句，对。D根据release定位到倒数第2句，对。
解析：根据ice crystal, positively charged定位到第7句(At such temperature...)，说在这样的温度下，ice pellet和ice crystal碰撞导致电荷转移，使pellet带负电，crystal带正电，选A
解析：根据positively charged ice pellets定位到最后一句，说在这个区域下，云的温度高于-15°，crystal和pellet碰撞产生正电pellet；而前文说到-15°的地方，说在-15°之下时产生的是带负电的pellet，所以这里是因为温度高于-15°，选D
解析：A根据ice crystal, positively charged定位到第7句，对。B根据in the middle定位到倒数第4句，对。C根据温度定位到第6-7句，对。D根据at the base of the cloud定位到最后一句，但对云带负电的解释到倒数第3句就说完了，D不是带负电的原因，选D
解析：根据top of cloud, positively charged定位到倒数第4句，说风流会把轻的带正电ice crystals带到上层云，所以可以推断出带负电的ice pellets比较重不能被带到上层云。选C
解析：根据stream of charged particles from the ground定位到第2句，说来自地面100米以内的带负电电子会遇到来自地面带正电的粒子，这样会形成连接云和地面的通道，所以选B。
解析：A根据diameter, centimeter定位到第4句，不能推出widen. B根据100定位到第2句，与原文相反。C根据path（与channel同义表达）定位到最后两句，说正粒子来自高的结构比如旗杆、塔，这就是为何之后由这个通道的闪电经常袭击高的结构，选C
解析：插入句提到stream of electrons, established path说明前句应该要涉及这两方面的内容。BC前都有这两方面的内容，B前是说channel包含沿着path的stream，是这两个信息首次出现。而插入句还提到new positive-stream channel，根据句意，C前说到following an established channel，再出现new更合理，选C
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