问题 选择题

等量异种点电荷的连线和中垂线如图所示,现将一个带负电的试探电荷先从图中点沿直线移动到点,再从点沿直线移动到点,则试探电荷在此过程中(   )

A.所受电场力的方向不变

B.所受电场力的大小恒定

C.电势能一定减小

D.电势能先不变后减小

答案

AD

题目分析:等量异种电荷的中垂线是一条零电势的等势线,所以从a到b电场力做功为零,从b到c时,电场线从正电荷指向负电荷,所以负电荷受力向上,电场力做正功,所以电势能减少,所以电势能先不变后减小,D对。根据等量异种电荷的电场线分布,从a到b电场越来越大,所以B错。等量异种电荷的中垂线是一条零电势的等势线,说明电场垂直与ab,向下,所以电场方向整个过程不变,即电场力方向不变,所以A对

点评:本题考查了等量异种电荷的电场分布特点,包括电场线与等势线的关系,本题还涉及了电场力做功与电势能的变化关系。

问答题
单项选择题

In choosing a friend, one should be very careful. A good friend can help you study. You can have fun together and make each other happy. Sometimes you will meet fair weather friends. They will be with you as long as you have money or luck, but when you are down, they will run away. How do I know when I have found a good friend I look for certain qualities of character, especially understanding, honesty and reliability.
Above all else, I look for understanding in a friend. A good friend tries to understand how another person is feeling. He is not quick to judge. Instead, he tries to learn from others. He puts himself in the other person’s place, and he tries to think of ways to be helpful. He is also a good listener.
At the same time, however, a good friend is honest. He does not look for faults in others. He notices their good points. In short, a friend will try to understand me and accept me.
Another quality of a friend is reliability. I can always depend on a good friend. If he tells me he will meet me somewhere at a certain time, I can be sure that he will be there. If I need a favor, he will do his best to help me. If I am in trouble, he will not run away from me.
There is a fourth quality that makes a friend special. A special friend is someone with whom we can have fun. We should enjoy our lives, and we would enjoy our friendship. That is why I especially like friends who are fun to be with. A good friend likes the same things I like. We share experience and learn from each other. A good friend has a good sense of humor, too. He likes to laugh with me. That is how we share in the joy of being friends. And I know that he is looking for the same quality in me.
When I meet someone who is reliable, honest, and understanding, I know I’ve found a friend!

Which of the following qualities the writer thinks is the most important in choosing a friend

A.Understanding.

B.Honesty.

C.Reliability.

D.A sense of humor.