问题 问答题

如图所示,在矩形ABCD区域内,对角线BD以上的区域存在有平行于AD向下的匀强电场,对角线BD以下的区域存在有垂直于纸面的匀强磁场(图中未标出),矩形AD边长为L,AB边长为2L.一个质量为m、电荷量为+q的带电粒子(重力不计)以初速度v0从A点沿AB方向进入电场,在对角线BD的中点P处进入磁场,并从DC边上以垂直于DC边的速度离开磁场(图中未画出),求:

(1)电场强度E的大小和带电粒子经过P点时速度v的大小和方向;

(2)磁场的磁感应强度B的大小和方向.

答案

(1)带电粒子在电场中做类平抛运动,则水平方向:L=v0t

           竖直方向:

1
2
L=
1
2
qE
m
t

              解得:E=

m
v20
qL

        在竖直方向粒子做匀变速运动竖直分速度为vy,则有

           2

qE
m
L
2
=
v2y

      代入得:vy=v0

P点的速度为v=

2
v0

速度与水平方向的夹角为θ

 

tanθ=

vy
v0
=1

     所以:θ=450

 (2)由几何关系可知:粒子在磁场中转过的圆心角为45°   

    

sin45°=

L
2
r

       得:r=

2
2
L

粒子在磁场中做匀速圆周运动

qvB=m

v
r

得:B=

2mv0
qL

磁场方向垂直纸面向外.

完形填空

完形填空(共20小题,每小题1.5分,满分30分)

阅读下面短文,掌握其大意,然后从1—20题各题所给的四个选项(A、B、C和D)中,选出可以填入空白处的最佳选项。

Once upon a time the colors of the world started to quarrel. Green said, "Clearly I am the most important. I am the sign of life and hope. I was chosen for grass, trees and leaves. Without me, all animals would ___1__."

Blue interrupted, "You only think about the __2_, but consider the sky and the sea.__3__ the water that is the basis of life and drawn up by the clouds from the deep sea.Without my peace, you would all be _4__."

Yellow chuckled (笑道), "You are all so serious. I bring laughter, fun, and _5__ into the world."

Orange started next to blow her trumpet, "I am the color of health and strength. I may be _6__, but I am precious for I serve the needs of human life. When I fill the sky __7__, my beauty is so striking that no one gives another _8__ to any of you."

Red could stand it _9__ and he shouted out, "I am the ruler of all of you. I am the color of danger and of bravery. I am willing to _10__ truth. I am also the color of passion and of love."

Then came Purple and Indigo (深蓝). …

The colors went on boasting, each convinced of his or her own __11__. Their quarreling became louder and louder. Suddenly there was a startling flash of bright lightening thunder. Rain started to pour down _12__. The colors crouched (蜷缩) down __13__, drawing close to one another for comfort.

In the midst of the clamor (叫嚷), rain began to speak, "You foolish colors, fighting _14_ yourselves, each trying to dominate _15__. Don't you know that you were each made for a special purpose, __16__? Join hands with __17__ and come to me."

Doing as they were told, the colors __18__ and joined hands. They formed a colorful rainbow. From then on, whenever a good rain ___19__ the world, a rainbow appears in the sky. They remember to __20__ one another.

1.A.stay                        B.leave                     C.go out                   D.die

2.A.earth               B.moon                    C.star                      D.sun

3.A.That is                    B.I am                            C.It is                      D.This is

4.A.anything                 B.nothing                 C.something             D.everything

5.A.warmth                  B.sadness                 C.depression           D.anxiety

6.A.usual               B.normal                  C.common             D.scarce

7.A.at midnight               B.at noon or at night    C.at sunrise or sunset        D.during the day

8.A.gift                         B.honor                    C.thought                 D.respect

9.A.for more                 B.any more               C.very much            D.no longer

10.A.turn to                  B.fight for                C.struggle with         D.bend over

11.A.superiority             B.disadvantages        C.inferiority            D.weakness

12.A.gently                   B.quietly                   C.violently                D.peacefully

13.A.with care                     B.in fear                   C.by chance           D.on purpose

14.A.amongst                B.by                        C.for                       D.against

15.A.others                   B.themselves            C.the other               D.the rest

16.A.equal and simple      B.ordinary and similar  C.more or less    D.unique and different

17.A.each other             B.me                        C.one another           D.them

18.A.combined              B.separated               C.united                   D.divided

19.A.cleans                   B.washes                 C.brightens              D.dampens

20.A.appreciate             B.quarrel with           C.ignore                   D.praise

单项选择题