问题 问答题

如图所示,两块竖直放置的导体板间存在水平向左的匀强电场,板间距离为d.有一带电量为+q、质量为m的小球(可视为质点)以水平速度从A孔进入匀强电场,且恰好没有与右板相碰,小球最后从B孔离开匀强电场,若A、B两孔的距离为4d,重力加速度为g,求:

(1)两板间的场强大小;

(2)小球从A孔进入电场时的速度;

(3)从小球进入电场到其速度达到最小值,小球电势能的变化量为多少?

答案

(1)由题意可知,小球在水平方向先减速到零,然后反向加速.设小球进入A孔的速度为v0,减速到右板的时间为t,则有:

水平方向:d=v0t-

1
2
qE
m
t20=v0-
qE
m
t

竖直方向:4d=

1
2
g(2t)2

联立解得E=

mg
q

(2)在水平方向上根据牛顿第二定律有qE=max

根据运动学公式有0-

v20
=2(-ax)d

联立解得v0=

2gd

(3)小球进入电场后,在水平方向上做减速运动,即vx=v0-

qE
m
t′=v0-gt′

在竖直方向上做加速运动,即vy=gt'

小球在电场中的速度大小为v=

v2x
+
v2y

联立由数学知识可得t′=

v0
2g
时小球速度达到最小,最小值为
2
2
v0

此时粒子在水平方向的位移为:x=v0t′-

1
2
qE
m
t2

在此过程中电场力做功为W=-qEx   

而W=-△Ep

联立解得Ep=

3
4
mgd,即粒子的电势能增加
3
4
mgd

答:(1)两板间的场强大小为

mg
q

(2)小球从A孔进入电场时的速度为

2gd

(3)从小球进入电场到其速度达到最小值为

2
2
v0,小球电势能的增加量为
3
4
mgd

阅读理解

The new book Harry Potter and the Half-Blood is warmly welcomed by the children in China. It sells well across China. You can see and hear the advertisements about the book here and there.

Why is the book so attractive (吸引人的) to children? After reading it, I was attracted by the magic world. I can not help asking: where is our own Harry Potter?

China has a long history and colorful culture. With a large market of teenagers, China published (出版) many popular books among children. However, why are the present books not as good as those foreign ones?

Firstly, quiet a number of children’s are of strong sense of teaching, and they have little interest. Reading them, children often have a feeling of being “educated”. Certainly, they don’t like them. Secondly, fewer children like the same stories, but most Chinese books are usually much the same. One knows the ending as early as in the beginning. Finally, the writers seldom think about children’s needs.

First-class Reading for children should be interesting, and written in their language.

小题1:About the present Chinese children’s books, which is NOT right?

A.They have little interest.

B.Our writers seldom know children’s needs.

C.Most of them are the same.

D.They are better than foreign ones.小题2:Harry Potter is so popular, because __________.

A.it’s the same as Chinese books

B.it can lead children into a wonderful world

C.it’s not written in children’s language

D.it’s advertised more often on TV or posters小题3:The Chinese meaning of the underlined phrase “First-class Reading” is _______.

A.第一批读物

B.一节课的阅读

C.一流的读物

D.基础的阅读小题4:“Where is our own Harry Potter?” means __________.

A.the writer can’t find any books for children

B.Chinese presents should buy Harry Potter for their children

C.our children should learn from Harry Potter

D.Chinese writers should write better books for children小题5: Which is the best title (标题) of the passage?

A.Ways to read books.

B.A magic boy, Harry Potter.

C.Books and advertisements.

D.Our children need better books.

单项选择题 A1/A2型题