问题 选择题

一带负电荷的质点,在电场力作用下沿曲线 abc 从 a 运动到 c ,已知质点的速率是递减的。关于 b 点电场强度 E 的方向,如图中可能正确的是(虚线是曲线在 b 点的切线)(  )

答案

答案:D

题目分析:电荷做曲线运动,电场力与速度方向不在同一直线上,应指向轨迹弯曲的内侧,不可能沿轨迹的切线方向,则场强也不可能沿轨迹的切线方向.故A错误.负电荷所受的电场力方向与场强方向相反,图中电场力方向与速度方向的夹角为锐角,电场力做正功,电荷的速率增大,与题不符.故B错误.图中场强方向指向轨迹的内侧,则电场力指向轨迹的外侧,电荷的轨迹应向上弯曲,不可能沿如图的轨迹运动.故C错误.图中场强方向指向轨迹的外侧,则电场力指向轨迹的内侧,而且电场力方向与电荷的速度方向成钝角,电场力做负功,电荷的速率减小,符合题意.故D正确.

故选D

点评:根据物体做曲线运动的条件和受力特点分析电荷受的电场力方向,再由负电荷所受的电场力方向与场强方向相反进行选择.

阅读理解

阅读理解。

     I have been employed by a charity organization for less than a year now and I have the privilege of

finding foster homes for abandoned children. It has always been my dream to work with adopted children

because I myself was an adopted child.

     I was born in California in September of 1976 and was adopted in Ohio in March of 1977. I have no

memory of being told that I was adopted, however, I have always known. I have been told that I was

wanted and I was special.

     Even though my parents provided me with lots of love, I was always annoyed by the questions about

my origin and my feeling of being unwanted. I constantly scanned every crowd I was in for another human

that looked like me or laughed like me.

     I began searching for my birthparents in 1999 and I imagined every possible scene of my birth family.

     I went to a local agency for support and five days later I met my birth mother. The story unfolded. She

flew me to Los Angeles the next day and I spent the weekend connecting with another person who looked

just like me. We have had a worthwhile relationship over the past years although she could not give me any

information about my birth father.

     In June of last year a member of my birth father's family contacted me. Now, he and I are just beginning

to get to know what it is like to be a birth child, Without the support of my parents who raised me I would

have never experienced the feeling of being truly wanted accepted. For that, I am forever grateful.

1. What's the author's attitude towards working with adopted children?

A. He feels it is worth a try.

B. He is eager to work with them.

C. He is strongly against it.

D. He thinks it is painful. .

2. How does the author feel about his birth family?

A. He feels that he can't forgive his birth parents.

B. He understands his birth parents.

C. He misses his birth parents very much.

D. He is afraid of being hurt by them again.

3. Why does the author writes this passage?

A. To complain about being abandoned by his birth parents.

B. To show how deep his desire to work with adopted children is.

C. To encourage other people to find their birth parents.

D. To show his love to his birth parents and his foster parents

单项选择题