问题 计算题

如图所示是用金属导线制成一矩形框架abcd,其中ab=cd=2ad=2bc=2l=2m,框架放在水平面上,磁感强度为B=1T的匀强磁场垂直于框架平面竖直向下,用同样的金属导线MN垂直于ab和cd,从ad处开始以v0=0.5m/s的速度匀速向右运动,已知该金属导线每米电阻为0.1Ω,求在MN从ad向bc运动的过程中:

(1)MN两点间最大的电势差.

(2)MN运动过程中框架消耗的最大电功率Pm

答案

(1)Um=0.3V;(2)Pm=0.625W。

题目分析:(1)由于MN切割磁感线的速度是一定的,长度一定,故产生的电动势不变,当外电路的电阻最大时,MN两点间的电势差是最大的;而当MN滑到ab的中点时,外电路的电阻是最大的。

运动到ab的中点时,MN产生的感应电动势:E=BLv=1×1×0.5V=0.5V

MN的电阻:r=0.1×1Ω=0.1Ω,外电路总电阻:R==0.15Ω;

则MN两点间的电势差:Um===0.3V;

(2)框架消耗的最大电功率就是当外电路的电阻与MN的电阻相等时所消耗的;

此时电路的总电阻为0.1Ω+0.1Ω=0.2Ω,

电路中的电流为=2.5A

则框架消耗的最大电功率Pm=(2.5A)2×0.1Ω=0.625W.

阅读理解

阅读理解。

     Few students would dream of getting a job as a shoe shiner working along the street. College students, in

their own eyes and in the eyes of the public, are expected to get at least a proper job, if not working in a high-

income or white-collar position.

     Cai Yong's decision to work as a shoe shiner shocked not only his parents, who had high hopes for him,

but also his girlfriend.

     "It is right for young graduates to be ambitious (有抱负的). But they have to start in a solid and down-to-

earth way," Cai said. Cai is not alone in making such a choice. The company has got 12 college students in all.

     "I come for the 800 yuan monthly pay," said Xiao Ji, who is a student from Nanjing University. "I have

grown up and no longer live on my parents," he said.

     "Working as shoe shiners can mean a bright future," Cai said.

     Not everyone can accept this job choice. Cai's girlfriend left him because she felt ashamed to be with a

shoe shiner, and Cai accepted the fact. "This is the price to be paid for a job," he said.

     A former shoe shiner, Zhong Haibo, from a medical college, believes it was a good training for the rest of

his life. "There is nothing I can't do after working as a shoe shiner," Zhong said, meaning that such kind of

experience in early life is helpful in a future job.

1. Cai Yong decided to work as _____. [ ]

A. a college teacher

B. a company manager

C. a shoe shiner

D. a medicine maker

2. Why did Cai's girlfriend leave him? [ ]

A. They didn't get along well with each other.

B. Her parents didn't like him.

C. She found it a shame to marry a shoe shiner.

D. Cai had got a new girlfriend.

3. Most people think Cai's choice _____. [ ]

A. unbelievable

B. nice

C. enjoyable

D. wonderful

4. From the passage we know Zhong Haibo _____.[ ]

A. doesn't like college life

B. thinks highly of working as shoe shiners

C. is a middle school student

D. is still a shoe shiner at the moment

单项选择题 A1型题