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问题 选择题

如图所示的电路中，R1=1Ω，R2=6Ω，电源内阻r=1Ω，若开关闭合后，铭牌上标有“6V  12W”的电动机刚好正常工作，则(　　)

A．流过电动机的电流为6A

B．流过R2上的电流为1A

C．电源电动势E=12V

D．若电动机线圈电阻为0.5Ω，则电动机输出功率为7.5W

答案

BC

题目分析：闭合开关电动机正常工作，则电动机与R2两端电压为6V，流过电动机的电流

由＝2A，A错误；电动机的输入功率为12W，电动机线圈消耗的功率为P内＝，则电动机的输出功率为10W，D错误；由知流过R2的电流为1A，B正确；通过以上分析知流过电路的总电流为3A，由知，R1两端电压为3V，内阻两端电压为3V，由闭合电路欧姆定律知，电源电动势为12V，C正确。

单项选择题

设备安装工程具备无负荷联动试车条件，由发包人组织试车，并在试车（）书面通知承包人。

A．前24小时

B．前12小时

C．前72小时

D．前48小时
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单项选择题

Our ape-men forefathers had no obvious natural weapons in the struggle for (1) in the open. They had neither the powerful teeth nor the p claws of the big cats. They could not (2) with the bear, whose strength, speed and claws (3) an impressive " small fire" weaponry. They could not even defend themselves (4) running swiftly like the horses, zebras or small animals. If the ape-men had attempted to compete on those terms in the open, they would have been (5) to failure and extinction. But they were (6) with enormous concealed advantages of a kind not possessed by any of their competitors.

In the search (7) the pickings of the forest, the ape-men had (8) efficient stereoscopic vision and a sense of color that the animals of the grasslands did not (9) . The ability to see clearly at close range permitted the ape-men to study practical problems in a way that lay far (10) the reach of the original inhabitants of the grassland. Good long-distance sight was (11) another matter. Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the (12) the viewpoint, the greater the range of sight-so (13) they had had to do was climb a tree. Out in the open, however, this simple solution was not (14) . Climbing a hill would have helped, but in many places the ground was flat. The ape-men (15) the only possible solution. They reared up as high as possible on their hind limbs and began to walk (16) . This vital change of physical position brought about considerable disadvantages. It was extremely unstable and it meant that the already slow ape-men became (17) still. (18) , they persevered and their bone structure gradually became (19) to the new, unstable position that (20) them the name Homo erectus, upright man.

1（）

A.survival

B.hunting

C.security

D.inhabitance
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