题干

某同学利用以下器材测量电源电动势、内阻和定值电阻的阻值.

待测电源(电动势约3V,内阻约1Ω)

一个阻值未知的电阻    电压表两块(内阻很大,量程3V)

电流表(内阻约为5Ω,量程0.6A)    滑动变阻器A(0~30Ω,3A)

滑动变阻器B(0~200Ω,0.2A)电键一个,导线若干

该同学设计了如图甲的电路,用U1、U2、I分别表示电表V1、V2、A的读数.将滑动变阻器的滑片移到不同位置时,得到下表所示数据:

I(A)

0.10

0.20

0.25

0.30

0.40

0.45

U1(V)

2.91

2.80

2.76

2.69

2.58

2.55

U2(V)

2.40

1.81

1.49

1.21

0.60

0.29

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同类题3

阅读下列短文,根据短文内容,从短文后各题所给的A、B、C、D四个选项中,选择最佳选项。

Some people think that they have an answer to the problems of automobile crowding and pollution in large cities. Their answer is the bicycle, or bike.

In a great many cities, hundreds of people ride bicycles to work every day. In New York City, some bike riders have even formed a group called “Bike for a Better City”. They claim that if more people rode bicycles to work there would be fewer automobiles in the downtown section of the city and therefore less dirty air from car engines.

For several years this group has been trying to get city government to help bicycle riders. For example, they want the city to paint special lanes—for bicycles only—on some of the main streets, because when bicycle riders must use the same lanes as cars, there may be accidents. “Bike for a Better City” feels that if there were special lanes, more people would use bikes.

    But no bicycle lanes have been painted yet. Not everyone thinks it is a good idea. Taxi drivers don't like the idea—they say it will slow traffic. Some store owners on the main streets don't like the idea—they say that if there is less traffic, they will have less business. And most people live too far from downtown to travel by bike.

The city government has not yet decided what to do. It wants to keep everyone happy. On weekends, Central Park—the largest open place in New York—is closed to cars, and the roads may be used by bicycles only. But “Bike for a Better City” says that this is not enough and keeps fighting to get bicycle lanes downtown. Until that happens, the safest place to bicycle may be in the park.