问题 选择题

如图所示,a、b、c、d是某匀强电场中的四个点,它们正好是一个矩形的四个顶点,ab=cd=L,ad=bc=2L,电场线与矩形所在平面平行.已知点a电势为20V,b点电势为24V,d点电势为12V.一个质子从b点以v0的速度射入此电场,入射方向与bc成450,一段时间后经过c点.不计质子的重力.下列判断正确的是(  )

A.c点电势高于a点电势

B.强场的方向由b指向d

C.质子从b运动到c所用的时间为

2
L
v0

D.质子从b运动到c,电场力做功为4eV

答案

A、B连接bd,bd两点的电势差为24V-12V=12V;ad两点的电势差为20V-12V=8V;因

8
12
=
2
3
,故将bd分成3份,每一份表示4V,如图所示,e点的电势一定为20V,连接ae即为等势线,f的电势为16V,连接fc,由几何知识得知,fcae,则知c的电势是16V,故a点电势高于c点的电势,故A错误;

C、由于

ab
ad
=
be
ed
,则根据几何知识得知,∠bed=45°,因为电场线与等势线ae垂直,所以场强的方向与ab的夹角为45°,质子从b运动到c时做类平抛运动,沿初速度v0方向做匀速直线运动,则运动时间为t=
2L?cos45°
v0
=
2
L
v0
.故C正确;

D、若质子由b到c,因bc间的电势差为24V-16V=8V,则电场力做功W=Ue=8ev;故D错误.

故选C

单项选择题
阅读理解

阅读理解。

Dear Michelle,

    My mother is not a good example for me and my sister. She smokes, stays out late, and she curses (咒骂).

I love her but I am ashamed of her. I do not want to bring my friends home because she does not act like any

of the other mothers. She says I am a loser because I sit at home and study. I wish I could live in another

family that was normal, but it will be 5 years before I am 18. What am I supposed to do until then?

                                                                                                                     Prisoner of a Crazy House

Dear 7th Grader,

    I am glad that you love your mother because the rest of your feelings about her may change as you get

older and mature.

    While it is true that your mother's smoking, partying and cursing bring you shame, the fact that you want

to stay separate from her doings is notable and very smart. It shows that you are a winner, not a loser. I am

guessing, therefore, that you do not bring shame to yourself, and that is a good thing. So, keep it that way

and keep away from your mother's habits. 

    At the same time, try to get the best grades and join clubs and school activities. Offer social service on

weekends and visit your friends at their homes.

    Fortunately, you are not alone. Many teens live in homes with immature, misguided, selfish parents. But

they develop good friendships with nice kids, do well in school, and often find a teacher or guidance advisor

who makes all the difference!

    Do it, Miss Winning 13 years old, and keep looking forward to your own future. Maybe by then your

mother may even see the light and realize how good you really are.

                                                                                                                                          Michelle

1. Michelle doesn't address her letter receiver as "Prisoner of a Crazy House" possibly because she ______. [ ]

A. tries to be polite

B. doesn't write to the kid alone

C. knows that's a girl in Grade 7

D. wants to cheer the kid up

2. According to the passage, Michelle advises the girl to ______.[ ]

A. try talking to her mother for her future

B. stick to her attitude towards her mother's some habits

C. understand her mother's habits and get used to them

D. try to keep away from her mother

3. What does Michelle mainly express by writing the last four paragraphs? [ ]

A. Don't worry too much about the problem.

B. Many parents act terribly.

C. Parents' manners make little difference.

D. The girl is not speaking for herself.

4. What does Miss Winning 13 years old refer to in the last paragraph but one? [ ]

A. the girl who turns to Michelle for help

B. Michelle

C. the girl's mother

D. a famous person whom we all admire