问题 计算题

如图所示,无重力空间中有一恒定的匀强磁场,磁感应强度的方向垂直于xOy平面向外,大小为B,沿x 轴放置一个垂直于xOy平面的较大的荧光屏,P点位于荧光屏上,在y轴上的A点放置一放射源,可以不断地沿平面内的不同方向以大小不等的速度放射出质量为m、电荷量为+q的同种粒子,这些粒子打到荧光屏上能在屏上形成一条亮线,P点处在亮线上,已知OA=OP=l,求:

(1)若能打到P点,则粒子速度的最小值为多少?

(2)若能打到P点,则粒子在磁场中运动的最长时间为多少?

答案

解:(1)粒子在磁场中运动,洛伦兹力提供向心力,设粒子的速度大小为v时,其在磁场中的运动半径为R,则F=qBv

由牛顿运动定律有:

若粒子以最小的速度到达P点时,其轨迹一定是以AP为直径的圆(圆心为图中O1

由几何关系知

则粒子的最小速度V=

(2)粒子在磁场中的运动周期

设粒子在磁场中运动时其轨迹所对应的圆心角为θ,则粒子在磁场中的运动时间为

由图可知,在磁场中运动时间最长的粒子的运动轨迹如图中圆O2所示,此时粒子的初速度方向竖直向上

则由几何关系有:

则粒子在磁场中运动的最长时间:

阅读理解

阅读理解

     What separates me from everyone else? The difference is not what clothes I wear or the music I listen

too, but what I feel inside.

     Ever since I was young, I have loved professional wrestling. I woke up every Saturday to watch my

favorite "Superstars." As I grew older, I got a lot of flak for watching this "fake" sport. My peers would

laugh at me for following what was called a "man's soap opera." So, I put my love for wrestling on the

shelf. Like everyone else, I wanted to be associated with the cool clique. I yearned to be invited to the

parties of the in-crowd and hang out with the popular kids. I became pretty successful. Although my

Friday evenings were busy with parties, I would still wake up early Saturdays to watch wrestling. It

wasn't until freshman year that I realized I wasn't being myself.

     That year, I tried many new things and activities and made new friends. In my town, football was the

sport, so I decided to play football, thinking it might give me a head start in popularity. The team started

with 48 athletes. At the end, there were 14 of us left. I stuck it out not because I liked it, but because I

am not a quitter. That long season taught me a lesson: I wasn't a football player. More importantly, it

taught me to be myself.

     After that season, I went back to being a wrestling fan. I watched it religiously, no matter what insults

were thrown my way. I came across a quote: "Don't Dream It, Be It." When I read this, my friend Dan

had the same idea I had.

     "What if we build a wrestling ring?" we asked. We acquired the necessary wood and equipment for

its construction. The following weekend, we met at his house. We saw our dream in a pile in his

backyard. We worked from dawn to dusk to build our great establishment. By Sunday night, our

mission was complete.  Our hard work (combined with a little creativity) had paid off. We had a real

ring. We decided to hold an "event." We practiced for hours, trying to improve every aspect of our

wrestling ability. The date was May 24th. Our show had a start time of 9: 00 p.m. To our surprise, about

one hundred family, friends and fans showed up to support us. It was the most important night of my life

and a complete success. Since that time, we have held five shows with as many as two hundred and fifty

people turning out. We continue to live this dream. We accomplished what we set out to do. We are

now well known throughout school. When  I walk down the halls, I am respected by my peers. Some are

the same peers who ridiculed me for watching wrestling when I was younger. When they approach me,

they often say, "Good match, Chris." I humbly say, "Thank you," knowing I did something I believed in.

     As my senior year winds down, I'll remember all of my high school memories. But what will stick out

most is the memory that I did something I loved, despite what everyone said or thought. I accomplished

my goal ... I lived my dream.

1. What makes the writer different from the others is __________.

A. the different sports he loves        

B. the different clothes he wears and the different music he listens to

C. that he is younger than the others.    

D. the different ideas he has

2. What does the underlined sentence in Paragraph mean?

A. I practiced wrestling secretly in my spare time.    

B. I put the clothes for wrestling on the shelf.

C. I decided to quit practicing wrestling.            

D. I began not to watch wrestling on TV.

3. When the writer was a freshman, he ___________.

A. knew he couldn't be a football player.    

B. realized he was being himself.

C. became very sociable.                

D. built a wrestling ring.

4. The writer built the wrestling ring in order to ________.

A. play football there    

B. make his dream realized  

C. be a professional player    

D. have parties there.

5. What is the writer's attitude towards his experience in high school?

A. Optimistic    

B. Pessimistic    

C. Doubtful    

D. Surprised

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