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问题 问答题

在如图所示的电路中，R3=4Ω，A的示数为0.75A，V的示数为2V．突然有一个电阻被烧断，使得A的示数变为0.80A，V的示数变为3.2V（图中电表均视为理想电表）．

（1）哪一个电阻被烧断？

（2）求电阻R1和R2的阻值；

（3）求电源的电动势和内电阻．

答案

（1）R2断路．断路前后，R1均有电流，排除R1．断路后，电压表有读数，排除R3，则必定是R2发生断路．

（2）电阻R2被烧断后，V的示数就是电源的路端电压，也就是电阻R1两端的电压.R1=

U
I
=
3.2
0.8
Ω=4Ω

电阻R2烧断前，通过R2的电流为I23=

I1•R1-U1
R3
=
0.75×4-2
4
A=0.25A

电阻R2的阻值为R2=

U1
I23
=
2
0.25
Ω=8.0Ω．

（3）由闭合电路欧姆定律得E=I1R1+（I1+I23）r，E=I2R1+I2r

联立上面两式，解得E=4V，r=1Ω

答：（1）必定是R2发生断路．

（2）电阻R1和R2的阻值分别是4Ω和8Ω；

（3）电源的电动势和内电阻分别是4V，1Ω．

单项选择题

氯气是一种____，有强烈的刺激气味。

A.黄绿色的剧毒气体 

B.红色的气体 

C.绿色的气体
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单项选择题

The effect of the baby boom on the schools helped to make possible a shift in thinking about the role of public education in the 1920’s. In the 1920’s, but especially (1) the Depression of the 1930’s, the United States experienced a (2) birth rate. Then with the prosperity (3) on by the Second World War and the economic boom that followed it, young people married and (4) households earlier and began to (5) larger families than had their (6) during the Depression. Birth rates rose to 102 per thousand in 1946, 106.2 in 1950, and 118 in 1955. (7) economics was probably the most important (8) , it is not the only explanation for the baby boom. The increased value placed (9) the idea of the family also helps to (10) this rise in birth rates. The baby boomers began streaming (11) the first grade by the mid-1940’s and became a (12) by 1950. The public school system suddenly found itself (13) The wartime economy meant that few new schools were buih between 1940 and 1945. (14) , large numbers of teachers left their profession during that period for better-paying jobs elsewhere.

(15) , in the 1950’s, the baby boom hit an antiquated and inadequate school system. Consequently, the custodial rhetoric of the 1930’s no longer made (16) ; keeping youths ages sixteen and older out of the labor market by keeping them in school could no longer be a high (17) for an institution unable to find space and staff to teach younger children. With the baby boom, the focus of educators (18) turned toward the lower grades and back to basic academic skills and (19) . The system no longer had much (20) in offering nontraditional, new, and extra services to older youths.

3（）

A.took

B.produced

C.brought

D.carried
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