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.

单项选择题 B型题
阅读理解

Dr.Ferguson energetically pushed the preparations for his departure,and in person directed the construction of his balloon.For a long time past he had been applying himself to the study of the Arab language and the various Mandingoe idioms,and,thanks to his talents in language,he had made rapid progress.

In the meanwhile,his friend,the sportsman,never let him out of

his sight-afraid,no doubt,that the doctor might take his departure,without saying a word to anybody.On this subject,he tried to discourage him with the most persuasive arguments,which,however,did NOT persuade Samuel Ferguson,and wasted his breath,by which the latter seemed to be but slightly moved.In fine,Dick felt that the doctor was slipping through his fingers.

The poor Scot was really to be pitied.He could not look upon the

blue sky without a wild terror: when asleep,he felt dizziness(眩晕)that made his head turn around;and every night he had visions of crashing down from immeasurable heights.

We must add that,during these nightmares,he once or twice fell out of bed. His first care then was to show Ferguson a severe bump(肿块)that he had received on the fall.“And yet,”he

would add,with warmth,“that was at the height of only three feet一not an inch more一and such a bump as this! Only think,then!”

This appeal,full of sad meaning as it was,did not seem to touch the doctor's heart.Not even the shadow of a doubt was ever suggested;and Samuel made an intolerable misuse of the first person plural:“‘Our’balloon,‘our’ car ;‘our’expedition.”

Dick trembled at them,although he was determined not to go ; he did not want to annoy his friend.Let us also disclose the fact that,without knowing exactly why himself,he had sent to Edinburgh for a certain selection of heavy clothing,and his best hunting-tools and fire-arms.

小题1:Why did Dr.Ferguson start to learn new languages?

A.To keep himself occupied.

B.To get ready for the departure.

C.To show his talent in language.

D.To Know more about balloon construction.小题2:We Call infer from the passage that Dick ____.

A.suffered from a disease during the nights

B.got hurt in a fall from the balloon in the evening

C.decided to take the adventure with the doctor at last

D.equipped himself with arms to frighten his friend小题3:Which word best describes Dr.Ferguson’s attitude towards his balloon flight?

A.Excited.

B.Optimistic.

C.Worried.

D.Determined小题4:What’s the writing pattern of the passage?

A.News.

B.Fiction.

C.Journal.

D.Biography.