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.

选择题
阅读理解

A solar pump(太阳能水泵 ) was built in a desert small village. The pump used the deserts most common resource – sunlight, to increase its greatest need, water. Solar collectors were used to collect the sun’s rays. Flat collectors were used instead of concave (凹的) collectors. Flat collectors can be still and do not have moving parts, which can be broken in sandstorms. The system used the 20C temperature difference between the solar collectors and the ground water to work a steam engine which pumped water from under the ground.

Some of the social effects of the new pump were planed for. Children aged 6—15 used to meet the old men to receive the informal education there. In order to replace this, a school was also included in the project. But the project had not considered the traditional power structure of the village. As soon as the foreign experts left, the two richest men in the village took control of the pump and started selling water to everyone else. The result was that the majority of the people were poorer than before.

16. The basic function of the solar collectors is to ______

A. pump water           B. gather the sun’s rays 

C. start engine            D. raise the temperature

17. According to the passage which of the following statements could be true?

A. Solar collectors were concave collectors               

B. Flat collectors can be broken in sandstorms.

C. Concave collectors may be have moving parts.     

D. A solar pump is a solar collectors.

18. The underground water is pumped by means of ______.

A. solar collectors  B. the 20C temperature difference C. the system  D. a steam engine

19. Another plan was also included that_____.

A. some new pumps were to built ,too

B. children from aged 6—15 were required to meet old men.

C. a new school would take the place of the informal education.

D. the richest men in the village would control the pump.

20. From this passage , it can be inferred that_____.

A. the project was good , but people there disliked it.

B. only few rich men supported the project.

C. the project was successful even though it only brought good social effects to fewer villagers.

D. the project was a complete failure, because the result was against its purpose.