问题 问答题

某同学要测量电阻Rx(阻值约为80Ω)的阻值,备选器材如下:

A、电流表A(量程为50mA,内阻约为50Ω)

B、电压表V(量程为3V,内阻约为3KΩ)

C、滑动变阻器R1(最大阻值为10Ω,额定电流1A)

D、滑动变阻器R2(阻值范围0~2kΩ,允许通过的最大电流0.5A)

E、低压直流电源E(电动势约4.5V,内阻忽略)

F、电键S,导线若干

(1)为提高实验结果的准确程度,滑动变阻器应选用______.(填器材代号)

(2)为达到尽可能减小实验误差的目的,请在虚线框内画出正确的实验电路原理图,并标明所用器材的代号.

答案

由题意可知,电路应采用分压接法,故滑动变阻器应选用较小的电阻,故滑动变阻器选用R1

②因题目要求所测量的值的变化范围尽可能大一些,故可知应采用滑动变阻器分压接法;

由于电流表量程只有0~50mA,若只接待测电阻,电路中电流过大损坏电流表,故可将定值电阻接入与待测电阻串联;

由于

RV
RX
=
3000
50+80
≈23;而
RX
RA
=
80
50
≈1.6
,故电压表分流较小,故应采用电流表外接法;故电路如图所示:

故答案为:R1;如上图所示.

阅读理解

阅读理解。

     I found out that sometimes doing a favor for someone could get you into a lot of trouble. I was in the

eighth grade at the time, and we were having a final test. During the test, the girl sitting next to me said

something to me, but I didn't understand. So I got closer to her and found out that she was trying to ask

me if I had an extra pen. She showed me that hers could not write. I happened to have an extra one, so I

took it out of my pocket and put it on her desk.

     Later, after the test papers had been turned in, the teacher asked me to stay in the classroom when all

the other students were out. Then she began to talk to me about honesty and emphasized (重点强调)

the fact that when people do something dishonest, they are really cheating themselves. She made me

promise that I would think seriously about all the things she had said, and then she told me I could leave.

I walked out of the room wondering why she had talked to me about all those things.

     The next day, I realized that the teacher thought I had cheated on the test. When she saw me talk to

the girl next to me, it looked as if I was copying answers from the girl's test paper. I tried to explain about

the pen, but she said it seemed very strange to her that I hadn't talked of anything about the pen when she

talked to me after the test. She continued to believe that I had cheated on the test.

1. The story took place exactly ________.

A. in the teacher's office  

B. in the classroom

C. in the playground        

D. in the library

2. Why did the boy get close to his classmate during the test?

A. Because he wanted to borrow a pen from his classmate.

B. Because he wanted to copy his classmate's answers.

C. Because his classmate wanted to borrow a pen.

D. Because his classmate wanted to copy his answers.

3. When the teacher saw all this, she asked the boy ________.

A. to go on writing his paper        

B. to stop taking the exam  

C. to leave the room immediately  

D. to stay behind the classroom

4. The underlined word "cheated" means "________" in Chinese.

A. 迟到      

B. 睡觉

C. 玩耍    

D. 作弊

5. The boy realized the teacher made a mistake ________.

A. on the day of the test  

B. when he was walking out of the room  

C. when the teacher talked to him

D. the next day

单项选择题