问题 计算题

如图(a)所示,为某同学设想的粒子速度选择装置,由水平转轴及两个薄盘N1、N2构成,两盘面平行且与转轴垂直,相距为L,盘上各开一狭缝,两狭缝夹角θ可调(如图(b));右为水平放置的长为d的感光板,板的正上方有一匀强磁场,方向垂直纸面向外,磁感应强度为B一小束速度不同、带正电的粒子沿水平方向射人N1,能通过N2的粒子经O点垂直进入磁场O到感光板的距离为d/2,粒子电荷量为q,质量为m,不计重力

(1)若两狭缝平行且盘静止(如图(c)),某一粒子进入磁场后,竖直向下打在感光板中心点M上,求该粒子在磁场中运动的时间t;

(2)若两狭缝夹角为θ0,盘匀速转动,转动方向如图(b)要使穿过N1、N2的粒子均打到感光板P1P2连线上,试分析盘转动角速度ω的取值范围(设通过N1的所有粒子在盘旋转一圈的时间内都能到达N2)。

答案

解:(1)粒子运动半径为

由牛顿第二定律

匀速圆周运动周期

粒子在磁场中运动时间

(2)如图所示,设粒子运动临界半径分别为R1和R2

设粒子临界速度分别为v1和v2

由②⑤⑥式,得

若粒子通过两转盘,由题设可知

联立⑦⑧⑨,得

对应转盘的转速分别为

 ⑩

粒子要打在感光板上,需满足条件

阅读理解

阅读理解。

     Computer games are just fun, I have been playing them for years, and I have become really good at it.

Some of my classmates think that I'm quite special. What's so special about that?

     One day I was having lunch at school. Suddenly a girl I didn't know came and sat next to me "What do

you do for fun?" She asked. "I am a gamer," I said in a low voice. "A gamer!" She couldn't believe what I

was saying and she burst into laughter, with all the apple pie out of her mouth.

     Most people think gaming is suitable for boys, but not for girls. Maybe I should cut my hair short and

change my actions so that I look like a boy.

     Actually, most boys don't feel there is anything wrong with my hobby. I have met many boy gamers-

some online and some face-to-face. None of them think badly about me being a girl gamer. I have made

some good friends with them.

     Playing computer games is also a kind of relaxing. I don't know why people feel strange about girls

who play games. Girls who play games do it for the same reason as boys do: it's something fun to do in

your spare time.

1. The writer can play computer games quite well because _____.

A. she is very smart

B. she is very special

C. she plays it a lot

D. she studies very hard

2. What do some of the writer's classmates think of her?

A. Helpful.

B. Friendly.

C. Intelligent.

D. Different.

3. It seems that _____ are more surprised at the writer's hobby than _____.

A. boys, girls

B. teachers, students

C. girls, boys

D. students, teachers

4. The writer has a _____ attitude towards girls playing computer games.

A. bad

B. strange

C. negative

D. positive

5. The best title for the passage is _____.

A. Computer games

B. Boy gamer

C. Girl gamers

D. Boys and Girls

单项选择题