问题 选择题

地面附近空间存在水平方向的匀强电场和匀强磁场,已知磁场方向垂直纸面向里,电场方向水平向右,一个带电油滴能沿一条与竖直方向成θ的直线MN运动(MN在竖直平面内),如图所示,则以下判断正确的是

A.可能是带正电的油滴,从M点运动到N点

B.可能是带正电的油滴,从N点运动到M点

C.可能是带负电的油滴,从M点运动到N点

D.可能是带负电的油滴,从N点运动到M点

答案

答案:D

题目分析:对带电油滴进行受力分析,受到竖直向下的重力,水平方向的电场力和垂直于直线MN的洛伦兹力,由于带电粒子做直线运动,所以洛伦兹力只能垂直于直线MN向上,油滴带负电,由左手定则可知,油滴是从N点运动到M点。

故选D

点评:注意带电粒子在重力场、电场、磁场的复合场中,只要是做直线运动,一定是匀速直线运动(v与B不平行)。若速度是变的,洛伦兹力会变,合力就是变的,合力与速度不在一条直线上,带电体就会做曲线运动。

阅读理解

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

     A man was driving at 70 miles in a 40milezone, when a police car came behind him with the light on.

__1__

     As the policeman came to him, he said, "Hi, officer, I guess you caught me a little bit over the limit.I was in a rush to get home, to be with my wife and kids, you know my younger son wasn't feeling too well when I left home this morning."

     The policeman said, "Well, I guess so, " and started writing something in his notebook.__2__ A few

moments later, the policeman came to his window and handed him a paper, and returned to his car without saying a single word.The man started to wonder, how much this ticket was going to be as he began to look at the paper.__3__

     "I had a daughter who was killed by a speeding car at the age of six, by a speeding driver like you.He

got a fine, a few months in prison, and was free, free to hug two other daughters.__4__ I have tried to

forgive that man a thousand times, and I thought I had.Maybe I really did forgive him, but I have to do it

again, even right now.__5__"

     The man was very surprised and could not move for the next few minutes.When finally he did, he

drove slowly, even a few miles under the speed limit, praying for forgiveness.

A. So pray for you, and be careful when you drive again, your daughter is all you have left.

B. I only had one, and now I have to wait I go to heaven before I can hug her again.

C. Right away the man started thinking of excuse to give.

D. He was surprised it was not a ticket as he began to look at.

E. So pray for me, and be careful when you drive again, my son is all I have left.

F. As minutes went by, he hadn't asked for his driver's license.

G. He was surprised it was not a ticket as he began to read.

填空题