问题 问答题

如图所示,静止在匀强磁场中的,

63
Li俘获一个速度为v0=7.7×104m/s的中子而发生核反应,
63
Li+
10
n→
31
H+
42
He,若已知He的速度为v2=2.0×104m/s,其方向跟中子反应前的速度方向相同,求:

(1)

31
H的速度是多大?

(2)在图中画出粒子

31
H和
42
He的运动轨迹,并求它们的轨道半径之比.

(3)当粒子

42
He旋转了3周时,粒子
31
H旋转几周?

答案

(1)36Li核俘获 01n的过程,系统动量守恒,则mnv0=mHv1+mHev2,即v1=

mnv0-mHev2
mH

代入数据mn=1 u,mHe=4 u,mH=3 u,得v1=-1.0×103 m/s,负号表示跟v0的方向相反.

(2)运动轨迹如图所示 13H和 24He在磁场中半径之比为rHrHe=

mHv1
Bq1
mHev2
Bq2
=3:40

(3)H和 He的周期之比为THTHe=

nH
Bq1
nHe
Bq2
=3:2

所以它们的转动周期之比为nH:nHe=THe:TH=2:3 当 α粒子转3周时,氘核转动2周.

答:(1)13H的速度是-1.0×103 m/s.

(2)它们的轨道半径之比:3:40.

(3)粒子13H旋转2周.

单项选择题
阅读理解

阅读理解。

      Ann loves the underground (地铁) in London. She always prefers to travel on the underground

because it is fast. She doesn't find it difficult now, as she has got used to (逐渐习惯) it. Ann knows

the underground map very well. Each line has its own color. For example, the Central Line (主干线)

is red, the Piccadilly Line (皮凯德里干线) is blue, and so on. Londoners always call the underground

"the tube" (meaning "underground") When you see the long tunnels (隧道) under ground, it isn't hard

to understand why. Yesterday Ann went home on the underground at about five o'clock in the afternoon.

She got a surprise. Since everyone was going home from work, the train was full.

      This was the "rush-hour", and it was a new experience (经历) for Ann. There were a lot of people,

but trains came every minute. Everyone waited and didn't push. As the train was full, Ann had to stand,

but she didn't mind. In the train she saw a sign. It said: "Keep away from the Rush Hour." "I will, next

time." Ann thought.

1. Travelling in London _______.

A.Ann prefers the underground to buses

B. Ann prefers buses to the underground

C. Ann prefers bike to the underground

D. Ann prefers to walk under ground

2. It's easy for Ann to take London's underground because _______.

A.she lives near the underground in London.

B. she has a good knowledge of the underground map

C. each line has its own color.

D. there are signs everywhere in London

3. Why do Londoners call the underground "the tube"?

A. Because Londoners like to use the tubes?

B. Because London's underground has too many lines.

C. Because Londoners think the underground looks like the tube

D. Because London's underground is straight and long

4. The"rush hour" here means _______.

A. 冲刺时间

B. 忙碌时间

C. 高峰时间

D. 休闲时间

5. Ann decides _______.

A. that she will take a bus instead next time

B. that she will no longer take London's underground

C. to take the underground at this time next time

D. not to take the underground at this time next time