问题 问答题

如图所示,绝缘细线一端固定于O点,另一端连接一带电荷量为q,质量为m的带正电小球,该装置处于水平向右的匀强电场中,小球静止时细线与竖直方向成α角,求细线的拉力F及匀强电场的电场强度E.

答案

小球受力如图所示:

小球处于平衡状态所以有:

绳子拉力:T=

mg
cosα

根据:Eq=mgtanα,得:E=

mgtanα
q

故绳子上拉力为:

mg
cosα
,电场强度为:
mgtanα
q
,方向水平向右.

阅读理解

Dear friends,

     My name is Tony. I am English. I am twelve years old this year. I am tall, and I am good at playing

football. I am in Grade Seven. There are thirty boys and twenty girls in my class. We are learning

Chinese in our school. Our Chinese teacher is Mr. Wu. He is a little short, but he is very strong. He is

very nice to us. I have some good friends in my class. Tom is a tall boy and has short black hair. He is

good at playing football, too. Alice is short and she loves wearing red dresses. She is good at English.

Shelly has long hair. She can speak Japanese and she likes swimming. We all study hard.

     What about your school and your friend?  Please write soon.

                                                                                                                                                Yours,

                                                                                                                                                 Tony

1. Which country is Tony from?

A. America.    

B. England.    

C. China.      

D. Japan.

2. Who is Mr. Wu?  

A. He is Tony's English teacher.    

B. He is Tony's father.            

C. He is Tony's friend.        

D. He is Tony's Chinese teacher.

3. How many students are there in Tony's class?

A. 50.        

B. 30.          

C. 20.        

D. 52.

4. Who likes swimming?

A. Tom.        

B. Alice        

C. Shelly.   

D. Mr. Wu.

5. Who writes this letter?

A. Tony      

B. Tom.    

C. Shelly.    

D. We don't know.

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