问题 多选题

如图所示,光滑绝缘水平桌面上有A、B两个带电小球(可以看成点电荷),A球带电量为+2q,B球带电量为-q,将它们同时由静止开始释放,A球加速度的大小为B球的2倍.现在AB中点固定一个带电小球C(也可看作点电荷),再同时由静止释放A、B两球,释放瞬间两球加速度大小相等.则C球带电量可能为(  )

A.

1
10
q

B.

1
2
q

C.q

D.4q

答案

由静止开始释放,A球加速度的大小为B球的2倍.根据牛顿第二定律可知,A、B两个带电小球的质量之比为1:2;

当在AB中点固定一个带电小球C,由静止释放A、B两球,释放瞬间两球加速度大小相等,则有C球带正电,

根据库仑定律与牛顿第二定律,且有:对A来说,K

2q•q
(2r)2
-K
QC•2q
r2
=ma  

                                 对B来说,K

2q•q
(2r)2
+K
QCq
r2
=2ma

综上解得,QC=

q
10

根据库仑定律与牛顿第二定律,且有:对A来说,K

QC•q
r2
-K
2q•q
(2r)2
=ma

                                 对B来说,K

QCq
r2
-K
2q•q
(2r)2
=2ma

综上解得,QC=

q
2
,故AB正确,CD错误;

故选:AB

阅读理解

A little girl thought she was not as beautiful as other girls, and nobody liked her. So she was always unhappy and didn’t like to talk to others. However, one day, her mother gave her a beautiful hair clip(发夹). When she wore it, she looked much more beautiful than before. She decided to wear it to school.

On her way to school she found that everyone who saw her smiled at her. Most of her schoolmates said “Hello” to her, but this never happened before. She thought that the beautiful hair clip had brought her them all. She was so happy about all of the wonderful things. Although she didn’t tell her classmates about her beautiful hair clip, they all wanted to know what had happened to her.

When she went back home after school, her mother asked her: “Did you know you dropped your hair clip? I found it by the door this morning.”

She understood that she hadn’t worn the hair clip to school at all.

小题1:Which of the following sentence is true?

A.The girl is not as beautiful as other girls.

B.Nobody liked the girl.

C.The girl’s classmates thought she was more beautiful than before with the hair clip.

D.The girl wanted to be more beautiful, so she decided to wear the hair clip.小题2:Why was the girl so happy?

A.She found that everyone who saw her smiled at her.

B.She heard most of her schoolmates said “Hello” to her.

C.She thought what she experienced today never happened before.

D.Both A, B, and C小题3: From this passage we know that ____________________

A.the girl was really ugly without the hair clip

B.the girl was as beautiful as her classmates.

C.the girl was not self-confident enough at first.

D.the girl was happy all the time.小题4:Her classmates wanted to know what had happened to the girl because__________________

A.she didn’t tell her classmates about her beautiful hair clip.

B.she was always unhappy but that day she was so happy.

C.she looked more beautiful wearing the hair clip.

D.she wanted to talk to others.小题5:We can learn from this passage that_______________________

A.A friend is easier lost than found.

B.Make your enemy your friend.

C.A friend in need is a friend indeed.

D.The most important is how we think about ourselves.

单项选择题