问题 问答题

如图所示,一质量为m、电量为q的带电粒子,以速度v沿平行于板面的方向射入电场中.极板间的电压为U,板长为L,板间距离为d.若不计带电粒子的重力,求带电粒子射出电场时

(1)侧向位移的大小;

(2)速度的大小与方向.

答案

(1)粒子在电场中做类平抛运动,在垂直于板面方向的分运动为匀加速运动.带电粒子受到的电场力为 F=qE ①

由牛顿第二定律得带电粒子的加速度为 a=

F
m

由于极板间为匀强电场,故 U=Ed ③

带电粒子在平行于板面的方向不受力,所以在这个方向做匀速运动,由L=vt 可求得:t=

L
v

带电粒子射出电场时,在垂直于板面方向偏移的距离为y=

1
2
at2

由①②③④⑤可得:y=

qUL2
2mdv2

(2)带电粒子射出电场时垂直于极板的分速度为 vy=at=

qU
md
L
v

则速度大小为 V=

v2+
v2y
=
v2+
q2U2L2
m2d2v2

设速度与水平方向的夹角为α,则 tanα=

vy
v
=
qUL
mdv2
,α=arctan
qUL
mdv2

答:

(1)侧向位移的大小为

qUL2
2mdv2

(2)速度的大小为

v2+
q2U2L2
m2d2v2
,方向与水平方向成arctan
qUL
mdv2
角斜右下方.

阅读理解

阅读理解。

     Mobile phone has become a problem for middle schools. Some middle schools in Australia have

 banned students from carrying mobile phones during school hours.

     Mobile phone use among children has become a problem for the school this year. Several children have

  got mobile phones as Christmas gifts, and more students will want them.

     Mary Bluett, an official, said mobile phone use is a distraction to students during school hours and it

 also gives teachers so much trouble in their classrooms. Teachers were also saying that sometimes

 students might use phone messages to cheat during exams.

     She said some schools had tried to ban mobile phones. Some parents felt unhappy because they

 couldn't get in touch with their children.

     Many teachers said students should not have mobile phones at school, but if there was a good reason,

 they could leave their phones at school office. They also said there were many reasons why the students

 should not have mobile phones at school; they were easy to lose and were a distraction from studies.

     Many people say that they understand why parents would want their children to have phones, but they

  think schools should let the students know when they can use their mobile phones.

1. Some middle schools in Australia have banned students from carrying mobile phones _____.

    A. because they are students

    B. when they are free

    C. when they are at school

    D. because they are children

2. We know from the passage that some children get mobile phones from _____.

    A. the makers and sellers

    B. the passers-by and strangers

    C. their parents and friends

    D. some mobile phone users

3. What does the underlined word "cheat" mean in the passage?

    A. 聊天

    B. 核对

    C. 查询

    D. 作弊

4. Some parents felt unhappy because they couldn't _____ during school hours.

    A. use their mobile phones

    B. leave their mobile phones at school office

    C. help the teachers with their work

    D. get in touch with their children

单项选择题