问题 问答题

在如图所示的直角坐标中,x轴的上方有与x轴正方向成45°角的匀强电场,场强的大小为E=

2
×104V/m.x轴的下方有垂直于xOy面的匀强磁场,磁感应强度的大小为 B=2×10-2T,方向垂直于纸面向外.把一个比荷为
q
m
=2×108
C/kg的正电荷从y轴上坐标为(0,1)的A点处由静止释放.电荷所受的重力忽略不计,求:

①电荷从释放到第一次进入磁场时所用的时间t;

②电荷在磁场中的偏转半径;

③电荷第二次到达x轴上的位置.

答案

(1)带电粒子从A点出发,至第一次到x轴为第一过程,在这个过程中,带电粒子做匀加速直线运动,位移大小由图中的直角三角形可以解出x=

2
m

由匀变速直线运动的位移时间关系式:x=

1
2
at2

整理得:t=

2x
a
=
2mx
qE
=10-6s

(2)粒子到达磁场时的速度:v=at=

qE
m
t=2
2
×106m/s

根据洛伦兹力提供向心力:qvB=

mv2
R

解得:R=

2
2
m

(3)带电粒子在磁场中的运动轨迹如图所示:

在直角三角形OAB中:AB=Rsin45°=

1
2
m

所以粒子再次到达x轴时距离A点左方2AB即1m处,所以电荷第二次到达x轴上的位置为(0,0).

故答案为:

①电荷从释放到第一次进入磁场时所用的时间为10-6s;

②电荷在磁场中的偏转半径

2
2
m;

③电荷第二次到达x轴上的位置(0,0)

阅读理解

阅读理解。

     It's Charlie's birthday today, but he isn't feeling happy. No one wishes him a happy birthday. Mother

says nothing about his birthday. She just tells Charlie to play outside. At lunch time, his mother calls,

"Come in for lunch, Charlie!" Charlie slowly walks into his house. But the living room looks different.

Charlie sees a big "Happy Birthday" banner (横幅). Then, he sees a birthday cake, balloons, presents,

and all of his friends. "Happy birthday!" everyone shouts. Charlie's face lights up. He is happy and

surprised (吃惊). No one forgets his birthday after all!

1. What is this story about?

A. Eating cake.                    

B. Celebrating a special day.

C. Forgetting a birthday.            

D. Having fun at a friend's party

2.Why is Charlie not happy?

A. He doesn't like his party.        

B. He doesn't like his presents.

C. He wants to eat cake.            

D. He thinks people forgets his birthday.

3. Why does the living room look different?

A. There are many decorations (装饰).

B. The lights are on.

C. There is a birthday cake.        

D. There are many presents.

4. What does the underlined (划线的) word "light up" properly mean (意思是) here?

A. 点燃        

B. 容光焕发     

C. 抬起来       

D. 照亮

5. Which of the following (下面的) is not right?

A. There are many lovely things in the living room.

B. Charlie's mother plays a trick on Charlie.

C. Charlie is happy all day.    

D. No one forgets Charlie's birthday.

单项选择题