问题 问答题

如图所示,竖直长方形区域ABCD长4m,高3m,其中存在水平正交的匀强磁场和匀强电场,磁场垂直纸面向外,B=1T。匀强电场平行纸面,电场线如图中实线。一带电微粒以速度v = 5m/s从长方形区域的左下角A点沿AC方向射入其中,恰能做直线运动。

(1)判断匀强电场的方向,求出场强的大小。

(2)若在粒子射至对角线的某点时将磁场撤去,粒子恰好从长方形区域的D点射出,求撤去磁场时粒子离A点的距离。

答案

(1)3v/m (2)0.1m

(1)物体做直线运动,故只能是匀速直线运动。                    

由图可知:此矩形的对角线和水平面的夹角为370

因为F="0" ,故重力、电场力、洛仑兹力只能在如图的情景下平衡。   (3分)

由左手定则,粒子带负电荷得:电场方向向左。  (2分)                                         

电场力F=qE=Gtan370=0.75G

洛仑兹力f=qvB=G/cos370 = 1.25G

两式比较E="3v/m          " (3分)                                      

(2)撤去磁场后,粒子做类平抛运动,轨迹如图。(3分)

a = = 1.25g (3分)                

FD = AD sin370 =" 2.4m        "

FD=at2

t=s    (3分)                                                

EF = vt=0.8m

AE = ADcos370EF = 3.2-0.8m =" 0.1m       " (3分)

阅读理解

Today it is found that school students hardly pay much attention to sports. Is it because they aren’t interested in sports? No. They often say they have other more important things to do.

What are these important things? Examinations! Students have to spend much time on all kinds of examinations at school. So many of them almost become bookworms (书呆子). Books stop them from doing sports. Because of the pressure from their parents and teachers, the students have to work harder and spend most of their time on books. As for (就……而言) the students, they want to get good results so that they can improve their studies. So it is necessary(必要) for them to give all of their free time to their studies and stop their school sports

In fact, education can’t go without physical (身体的) exercises, because a quick mind hardly goes along with a weak body. If you don’t have a strong body, you can never get anything, let alone (更不用说) a great success in your life.

1.       are the greatest problems, so the school students don’t pay much attention to sports now.

A. No interest     B. Exercises     C. Education     D. Examinations         

2. The underlined word “pressure” means “        ”.

A. happy feeling   B. good care  C. nervous (紧张) feeling  D. deep love

3. If you don’t have a         , you can never get anything.

A. good face      B. weak body   C. strong body      D. good father

4. The writer thinks the students should          .

A. work hard all day and all night

B have all kinds of sports after studying

C. do not have sports

D. spend the free time on the studies and give up (放弃)the sports.

5. which sentence is true?

A. If students want to get better results in studies, they must not do sports.

B. If students spend all their time on sports, they will improve studies.

C. If you have a weak body, you will have a hard mind.

D. If you want to be successful in your life, you must have a strong body.

单项选择题