问题 计算题

如图甲所示,在两平行金属板的中线OO′某处放置一个粒子源,粒子沿OO1方向连续不断地放出速度的带正电的粒子。在直线MN的右侧分布有范围足够大的匀强磁场,磁感应强度B=0.01T,方向垂直纸面向里,MN与中线OO′垂直。两平行金属板间的电压U随时间变化的U—t图线如图乙所示。已知带电粒子的荷质比,粒子的重力和粒子之间的作用力均可忽略不计,若时刻粒子源放出的粒子恰能从平行金属板边缘离开电场(设在每个粒子通过电场区域的时间内,可以把板间的电场看作是恒定的)。求:

(1)时刻粒子源放出的粒子离开电场时的速度大小和方向。

(2)从粒子源放出的粒子在磁场中运动的最短时间和最长时间。

答案

(1)θ=45°(2) 

题目分析:(1)设板间距为dt=0.1s时刻释放的粒子在板间做类平抛运动

在沿电场方向上   ① 

粒子离开电场时,沿电场方向的分速度

粒子离开电场时的速度   ③

粒子在电场中的偏转角为θ    ④

由①②③④得

  θ=45°

说明:用联立求出正确结果,参照上述评分标准给分。

(2)带电粒子在匀强磁场中做匀速圆周运动的周期

不同时刻释放的粒子在电场中的偏转角θ不同,进入磁场后在磁场中运动的时间不同,θ大的在磁场中的偏转角大,运动时间长。

t=0时刻释放的粒子,在电场中的偏转角为0,在磁场中运动的时间最短:

  

t=0.1s时刻释放的粒子,在电场中的偏转角最大为45°,在磁场中运动的时间最长:

 

点评:本题关键是画出粒子进入磁场后的各种可能的运动轨迹,根据洛伦兹力提供向心力列式后得出半径,然后求出磁偏转的距离表达式,并得出回旋角度的范围,从而得到磁偏转的范围.

阅读理解

Have you ever seen a horse with toes(脚趾)?Millions of years ago, horses had many toes. They had four toes on each front foot. They had three toes on each back foot. The horses were no bigger than the cats.These small horses lived in the forest. The toes helped the horses run on the soft, wet ground.

At first, it was very hot in the forest. But the weather changed. It became very cold. Many trees could not live in cold weather. The trees died and fell. Open fields took the place of forests. The sun made the ground dry and hard.

Horsed began to change, too. They began to get bigger. This took a long time. On the dry, hard land, horses needed only their middle toes for running. Their middle toes became hard. After a long time, horses had only one hard toe on each foot. Now we call this hard toe a hoof.

小题1:How many toes did a horse have millions of years ago?

A.14

B.12

C.16

D.13小题2:What does the underlined word “hoof” mean in Chinese?

A.尾巴

B.马鬃

C.腿

D.蹄子小题3:What was the weather like in the forest at first?

A.Very warm

B.Very hot

C.Very cold

D.Very cool小题4:Which is WRONG according to the passage?

A.The horses were much bigger than the cats millions of years ago.

B.The trees died and feel because the weather changed.

C.On the dry, hard land, horses needed only their middle toes for running.

D.The horses have fewer toes than before.小题5:The main idea of the passage is that_______________.

A.horses had three toes on each back foot.

B.horses lived in the hot forest

C.horses changed a lot over the years

D.horses didn’t like the cold weather

问答题 简答题