问题 问答题

如图所示,边长为L的正方形区域abcd内存在着匀强电场,一带电粒子,带电量大小为q,质量为m,以初速度v0从a点沿ab方向垂直进入电场,恰好从c点射出,不计重力.试求:

(1)电场强度大小E;

(2)a、c两点间的电势差Uac

(3)从a到c过程中粒子的电势能变化量△Ep

(4)粒子从c点射出时的动能Ekc

答案

(1)粒子做类平抛运动:在垂直于电场方向  L=V0t   

在平行于电场方向 L=

1
2
at2=
qEt2
2m

解得     E=

2m
v20
qL

(2)Uac=-EL=-

2m
v20
q

(3)∵从a到c电场力做的功:Wac=qEL=2m

v20

∴△Ep=-2m

v20
即电势能减少了2m
v20

(4)从a到c由动能定理有:Wac=Ekc-

1
2
m
v20

解得Ekc=

5
2
m
v20

答:(1)电场强度大小E为

2m
v20
qL

(2)a、c两点间的电势差为-

2m
v20
q

(3)从a到c过程中粒子的电势能变化量为-2m

v20

(4)粒子从c点射出时的动能为

5
2
m
v20

阅读理解

阅读理解

      A few years ago when my daughter was about two years old,I went through a difficult time.My

marriage was in trouble,I was pregnant (怀孕的) and not particularly well and I didn't have much money.

Looking back I think I was also a bit depressed,but I had to get on with taking care of my daughter the

best I could.

      One day I thought I would take my daughter to the park for a nice day as she liked there,and I thought it was a nice thing to do.I decided to walk because the bus was expensive,so it took us a long time to get

there.When we finally got there,immediately my daughter began to cry.She was cold,she was thirsty,and

she wanted to go home.To be honest,I had not prepared properly.I didn't even have any money to buy a

drink or to take the bus back home.I was so frustrated and cross with myself that my dream of a nice day

failed.

       I started walking back home (it was nearly 2 miles),and my daughter would not stop crying.I felt so

miserable and cross with myself and my life that I started crying too.

      At that point a car stopped and a gentleman offered us a lift.He took us home,and seemed really

concerned-he even offered me all his change!The kindness of this stranger to a stranger like me really

made me feel better.I never saw him again and I don't know who he is but he really helped me out that

day,so I just want to say thank you.

1.The writer writes the passage in order to________.

A.tell us how she got through her difficult time

B.tell us what kind of life she and her daughter once lived

C.tell us a journey she and her daughter had without money

D.express her sincere thanks for the driver who once helped her

2.Why did the writer go through a difficult time when her daughter was about 2 years old?

A.Because her daughter was too young and noisy and she didn't know how to care for her.

B.Because she loved her daughter too much and took care of her as best as she could.

C.Because she was worried about her own health.

D.Because there is something wrong with her marriage,health and money.

3.What finally caused the writer to cry?

A.Her own poor situation.

B.Her feeling sorry to her daughter.

C.The lack of money.

D.The failure of her dream of a nice day.

4.According to the passage,we can infer that________.

A.the writer could afford a bus ticket at that time

B.the writer will never forget the kindness given by the driver

C.the writer received the driver's change and bought drinks for her daughter

D.the driver knew the writer and her daughter

单项选择题