问题 选择题

如图甲所示,Q1、Q2为两个被固定的点电荷,其中Q1带负电,a、b两点在它们连线的延长线上.现有一带负电的粒子以一定的初速度沿直线从a点开始经b点向远处运动(粒子只受电场力作用),粒子经过a、b两点时的速度分别为va、vb,其速度图象如图乙所示.以下说法中正确的是(  )

A.Q2一定带负电

B.Q2的电量一定大于Q1的电量

C.b点的电场强度一定为零

D.整个运动过程中,粒子的电势能先减小后增大

答案

A、从速度图象上看,可见a到b做加速度减小的减速运动,在b点时粒子运动的加速度为零,则电场力为零,所以该点场强为零.Q1对负电荷的电场力向右,则Q2对负电荷的电场力向左,所以Q2带正电.故A错误,C正确.

    B、b点场强为零,可见两点电荷在b点对负电荷的电场力相等,根据F=k

q1q2
r2
,b到Q1的距离大于到Q2的距离,所以Q1的电量大于Q2的电量.故B错误.

    C、整个过程动能先减小后增大,根据能量守恒电势能先增大后减小.故D错误.

故选C.

阅读理解

Just how popular was Mini? Very. Every year her classmates voted(推举) her the favorite student. The friends she had were much more than those of anyone else.

One day Mini was caught yakkety-yakking(大声说笑) and ordered to stay back as punishment(惩罚). Staying back meant doing something “useful”, like helping in the library after school.

Now Mini’s school had a rule. Once a month, if you had something else to do the day you were punished, you could ask a friend to stand in for you.

Unluckily, her aunt was visiting Mini that day. That afternoon was the only chance Mini had of seeing her for another five years.

Mini needed someone to stand in for her. She asked Betty, her “best” friend. “Oh, no, Mini, I can’t,” Betty said. “I must help my brother do his homework.”

“Tony, will you stay back for me?” Mini asked another friend. “I wish you’d told me earlier. I have to go to a relative’s house with my father,” he said.

Now, Mini was beginning to doubt(怀疑) if even one of her friends would agree to stay. But she tried again. She asked three more friends and each one refused.

So, after school Mini stayed to help the librarian. And, when she found Mini crying after everyone had left, it was she who surprised Mini by allowing her to leave to meet her aunt. “I may have many friends,” Mini said to herself, “but in reality I have none.”

小题1:What does staying back mean in Mini’s school?

A.It means you have to do more homework.

B.It means you must study harder.

C.It means you have to do something “useful”.

D.It means you should ask a friend for help.小题2:How many friends did Mini ask for help? 

A.Two.

B.Three.

C.Four.

D.Five.小题3:Who helped Mini in the end?

A.The teacher.

B.Tony.

C.The librarian.

D.Nobody.小题4:What is the meaning of the underlined phrase “in reality”?

A.最后

B.事实上

C.终于

D.首先小题5: Which of the following can describe Mini’s feeling?

A.Happy.

B.Excited.

C.Sad.

D.Bored.

单项选择题