问题 问答题

如图所示,在MN左侧QP上方有匀强电场.在MN右侧存在垂直于纸面的矩形匀强磁场(图中未画出),其左边界和下边界分别与MN、AA′重合.现有一带电粒子以初速度v0自O点沿水平方向射入,并恰好从P点射出,又经过在矩形有界磁场中的偏转,最终垂直于MN从A点向左水平射出.已知PA距离为d,O′P=

d
2
,OO′距离L=
3
3
d.不计带电粒子重力.求:

(1)粒子从下极板边缘射出时的速度;

(2)粒子在从O到A经历的时间

(3)矩形有界磁场的最小面积.

答案

(1)带电粒子在电场中平行极板方向匀速运动:

3
3
d=v0t

竖直方向从静止开始做匀加速运动:

1
2
d=
1
2
vyt,

解得vy=

3
v0

则粒子从下极板边缘射出时的速度为v=

v20
+
v2y
=2v0,且与竖直方向成300角.

(2)带电粒子在电场中运动的时间t1=

3
d
3v0
,由几何关系可得r=
d
3

离开电场后先做匀速运动,匀速运动的时间t2=

3
2v0
r=
3
6v0
d

然后进入磁场,在磁场中偏转1200到达A,t3=

T
3
=
2πr
6v0
=
πd
9v0

所以带电粒子从O运动至A所用的总时间为

t=t1+t2+t3=(

3
2
+
π
9
)
d
v0

(3)由轨迹示意图可知,磁场区域宽等于轨迹半径r,高等于

3r
2
,而r=
d
3

所以矩形有界磁场的最小面积为S=r×

3
2
r=
d2
6

答:(1)粒子从下极板边缘射出时的速度为2v0,方向与竖直方向成300角;

(2)粒子从O运动到A经历的时间时间为(

3
2
+
π
9
)
d
v0

(3)矩形有界磁场的最小面积为

d2
6

选择题
阅读理解

阅读表达

阅读下面的短文,并根据短文后的要求答题.

     [1]Perhaps you are an average student with average intelligence.However,you can receive better

grades if you want.Yes,even students with average intelligence can be top students without additional

work.Here's how:

    [2]Plan your time carefully.Make a list of your weekly tasks.Then make a schedule of your time.Be sure to set aside enough time to complete your normal reading and work assignments.Of course,studying

shouldn't occupy all of the free time on the schedule.

    [3]Make good use of your time in class.Listening to what the teacher says in class means less work l

ater.Take notes to help you remember what the teacher says.

    [4]Skim before you read.Look over a passage quickly before you begin to read it more carefully.As

you preview the material,you get some idea of the content and how it is organized.Later when you begin to read you will recognize less important material and you may skip some of these portions.Skimming helps

double your reading speed and improve you comprehension as well.

    [5]_____,Go over your notes as soon as you can after class.Review important points mentioned in class as well as points you remain confused.If you know what the teacher will discuss the next day,skim and

read that material,too.If you review your notes and textbook regularly,the material will become more

meaningful and you will remember it longer.Regular review leads to improved performance on tests.

    [6]Develop a good attitude towards tests.The purpose of a test is to show what you have learned about a subject.The word won't end if you don't pass a test,so don't worry excessively about a single test.

    [7]There are other techniques that might help you with your studying.Improving your studying habits will

improve your grades.

1.What's the main idea of the passage?(no more than 8 words)

____________________________________________________________________

2.What should you do if you fail a test?(no more than 4 words)

____________________________________________________________________

3.Please fill in the blank in the fifth paragraph with proper words.(no more than 4 words)

____________________________________________________________________

4.According to the passage,how can an average student improve his/her grades?(no more than 6 words)

____________________________________________________________________

5.What does the word "it"(line 4,paragraph 5)probably refer to?(no more than 3 words)

____________________________________________________________________