问题 问答题

某课外活动小组需要对两个定值电阻R1、R2和一只滑动变阻器R3的电阻进行测量,将这些元件连成如图所示的电路。闭合S1,断开S2,将滑动变阻器的阻值调到最大时,电流表的读数为0.2A,电压表的示数为2V;若将S1、S2都闭合,将滑动变阻器的阻值调到0时,电压表的示数为6V,电流表的示数为0.9A。求:

滑动变阻器的最大阻值R3是多少?

答案

参考答案:

若将S1、S2都闭合,将滑动变阻器的阻值调到零时,电压表的示数是6V,此时电压表示数等于电源电压,由电路图可见闭合S1,断开S2时,滑动变阻器与定值电阻R1是串联的,所以滑动变阻器两端的电压为6v-2v=4v,根据欧姆定律的变形公式可知滑动变阻器的最大阻值为

阅读理解

阅读理解。

     On October 8th, when I cooked breakfast for my children, my house started shaking (摇动). It

was an earthquake (地震).

     My children were with me. Ali, who was 5 years old, came to me and asked for breakfast. He ate

a slice of bread when the earthquake came. When we tried to run out, something struck (撞击) Ali.

Humna and I ran out of the house safely.

     Soon Ali's father came and helped us, but we couldn't find Ali and my other daughter, Ayesha. We,

with other people, looked for them everywhere. Finally we found Ayesha's dead (死的) body from

her room. We found Ali six hours later. We could hear him cry at that time. The people said to him,

"We have a bike for you. Don't cry." He was very happy after hearing it because he wanted a bike

very much. He asked for his bike when he came out, but we had nothing for him except (除……之外)

the dead body of his sister Ayesha.

1. What did Ali do when the earthquake came?

A. He ran into the house.    

B. He played in the yard.

C. He ate a slice of bread.  

D. He helped his mum cook.

2. How many children were there in the family before the earthquake?

A. Two.    

B. Three.    

C. Four.  

D. Five.

3. They spent more than _____ hours looking for Ali.

A. 3    

B. 4  

C. 5  

D. 6

4. Which of the following is NOT true ac-cording to the passage?

A. Ali was 5 years old.

B. Ayesha was dead.

C. The writer bought a bike for Ali.

D. The earthquake came in the morning.

5. The passage is about ________.

A. a bad trip                

B. a clever child

C. a great mother            

D. a terrible earthquake  

单项选择题