问题 选择题

如图所示,竖直放置的平行板电容器,A板接电源正极,B板接电源负极,在电容器中加一与电场方向垂直的、水平向里的匀强磁场。一批带正电的微粒从A板中点小孔C射入,射入的速度大小方向各不相同,考虑微粒所受重力,微粒在平行板A、B间运动过程中(    )

A.所有微粒的动能都将增加

B.所有微粒的机械能都将不变

C.有的微粒可以做匀速圆周运动

D.有的微粒可能做匀速直线运动

答案

答案:D

题目分析:带电微粒受到重力、洛伦兹力和电场力。若速度较大,则洛伦兹力向上且可能大于重力,此时将向上偏,洛伦兹力不做功,重力做负功,电场力做正功,则动能可能不变,可能减小,也可能增大,若速度较小,则洛伦兹力可能小于重力,微粒将向下偏,重力做正功,电场力做正功,动能一定增大,选项A错误.由于除重力以外的电场力对微粒做了功,机械能不守恒,故选项B错误。因液滴重力与电场力不可能平衡,所以粒子不可能做匀速圆周运动,选项C错误。若洛伦兹力与重力、电场力三力平衡,粒子可能做匀速直线运动,选项D正确.故选D.

阅读理解

阅读理解。

     No one in my family can understand why so many of our bikes have disappeared. My mum told me

11 bikes of my family had been stolen (被偷了).

     I think it is funny, like my family has become a small shop giving out bikes for free. The number of

missing bikes is growing, so I can't help worrying. Is there someone who is crazy about my family's

bikes? Maybe he minks our bikes are the most beautiful ones around.

     I often leave my bike somewhere and return to find that it has disappeared. Nobody knows where the

missing bike has gone as if it has grown wings and flown away.

     One day last year, I locked my bike near our flat and went upstairs to get something. Thirty minutes

later, it was gone. My mum happened (碰巧) to see it on the street recently, but it was no longer mine.

The thief bad sold it to someone else. I really hate the thieves.

     Sometimes I calm down and try to understand why the thieves act as they do. Maybe they lost their

jobs and need money to support their families. But what they do really hurts others. Nobody has the right

(权利) to hurt others for his own sake.

     Everyone in the world is connected. One person's action can have a great effect on another's life. I

wish the thieves would stop taking my family's bikes. I wish the thieves would stop hurting my family.

1. Where was the writer's bike stolen last year?

A. Near the writer's flat.

B. Outside a small shop.

C. In the writer's school.

D. On the street. 

2. What does the word"sake" mean in the passage?

A. 安全

B. 利益

C. 企图

D. 享受

3. From the passage, what's the real reason why their bikes were stolen?

A. The thieves had no money to support their families.

B. The bikes of the writer's family are beautiful.

C. The thieves lost their jobs.

D. The passage doesn't tell. us.

4. What's the passage mainly about?

A. The writer loved his bikes.

B. The writer found his missing bike.

C. The writer's family had a bike shop.

D. The writer wished the thieves would stop taking bikes.

单项选择题