问题 问答题

如图所示,平行正对金属板相距为d,板长为L,板间电压为U,C是宽为d的挡板,其上下两端点与A和B板水平相齐,且C离金属板与屏S的距离均为

L
2
,C能吸收射到它表面的所有粒子.现让电荷量为q的带电粒子沿A、B两板中线入射,带电粒子的质量、速率均不相同,不计重力.求:

(1)带电粒子到达屏S上的宽度;

(2)初动能多大的粒子能到达屏上.

答案

(1)带电粒子进入电场后做类平抛运动,从右侧飞出电场后做匀速直线运动,这时速度方向的反向延长线与粒子射入方向恰相交于两极板的正中央O1点,如图所示.

带电粒子到达屏S上的宽度为A´B´,由图可知:

A´B´=O´B´-O´A´

由几何关系:

O´B´
d
2
=
3L
2
L
2
    得O´B´=
3
2
d

O´A´
d
2
=
3L
2
L
    得O´A´=
3
4
d

故带电粒子到达屏S上的宽度为

3
4
d.

所以A´B´=

3
2
d-
3
4
d=
3d
4
,即带电粒子到达屏S上的宽度为
3d
4

(2)设从偏转电场边缘射出的粒子偏向角为θ1,由几何关系有:

tgθ1=

O′B
O1O
=
3
2
d
3
2
L
=
d
L
,cosθ1=
L
L2+d2

则该粒子射出电场的速度υt1=

v0
cosθ1
=
υ0
L2+d2
L

由动能定理得:

qU
2
=
1
2
t2-
1
2
02=
1
2
0 2 (
L2+d2
L2
-1

所以此粒子的动能为

1
2
0 2=
qUL2
2d2

设从挡板边缘射出的粒子偏向角为θ2,由几何关系有:

tgθ2=

O′A′
O1O
=
3
4
d
3
2
L
=d
d
2L
,cosθ2=
2L
4L2+d2

则该粒子射出电场的速度υt2=

v0
cosθ2
=
υ0
4L2+d2
2L

由动能定理得:

qU
4
=
1
2
t22-
1
2
02=
1
2
0 2 (
4L2+d2
4L2
-1

所以此粒子的动能为 

1
2
0 2=
qUL2
d2

所以,初动能满足

qUL2
2d2
<EK
qUL2
d2
的粒子能到达屏上.

填空题

课文回放:根据课文内容将下面短文补充完整,单词的首字母已经为你写出 (注意:要写出所填单词

的首字母及其它字母的正确形式,否则不给分)。

☆A (Unit 1 Book 4 Women of Achievements) ★

     Jane Goodall, who had intended to work with animals in their own environment since her (1) c________,

went to (2) A________ and devoted herself to (3) ________ (study) the behavior of chimps. It was tough but

she considered it as a (4) w________ career. Through her study, we learned much more about chimps. She

argues that the life of these animals should be (5) r________ and that they should be left in the wild and not

used for (6) e________ or advertisements. Besides, she was very (7) c________ of these animals. Now, she

has achieved everything she wanted to do, which inspired those who want to cheer the achievements of

women.

☆B (Unit 2 Book 4 Working the Land) ★

     Yuan Longping, a man who graduated from Southwest Agricultural College in 1953 and now works the

land, does his research and grows hybrid rice. He is an agricultural (8) p________ to have a high output of

rice without (9) e________ the area of fields. He devotes his whole life to finding ways to increase the rice

output, to (10) f________ the Chinese and the world people. He is (11) c________ his knowledge so that

more farmers can be free from (12) h_______ and satisfied with their lives. Yuan cares little about his money

and fame, devoting a great deal of time and money helping others doing agricultural research. He wishes for

more cheap rice for all the people. Yuan Longping is a great (13) p_________.

☆C (Unit 3 Book 4 A Taste of English Humour) ★

      Chaplin's parents were poor music hall performers. It was (14) a_________ that Chaplin was brought

up successfully when he was very young. Unfortunately, his parents died, leaving the family even worse off.

Chaplin could act the fool doing ordinary everyday tasks. Nobody was ever bored watching him, for he made

everything entertaining. He became known (15) t_________ the world later. He wrote, directed and produced

the films he starred in. In 1972, he was given a special Oscar for his outstanding work in films. He spent his

last years in Switzerland, where he was buried in 1977.

判断题