问题 问答题

如图,在直角坐标系xoy平面内,x轴上方有磁感应强度为B、方向垂直xoy平面指向纸里的匀强磁场,x轴下方有场强为E、方向沿y轴负方向的匀强电场,x轴正方向上有一点P到原点O的距离PO=L.一质量为m、电量为e的电子,由y轴负方向上距原点O为y处由静止释放,要使电子进入磁场运动且恰可通过P点(不计重力),则

(1)y应满足的条件是什么?

(2)电子由释放至运动到P点所用的时间t为多大?

 

答案

所以:t=

填空题
阅读理解

第三部分阅读理解(共15小题,每小题2分,满分30分)

A

Our first winter in Canada was the happiest we have ever experienced. We were living in the countryside then and had just arrived from Hong Kong that autumn. One evening in mid December snow began to fall. When we looked out of the window and saw that it was snowing, we all cheered. It was the first snow we had ever seen and we were excited. We could hardly wait  for the snow to stop so that we could go out lo play. Next morning when we woke up,it was still snowing. We switched on the radio and learned that more than ten inches of snow had fallen during the night and that since the roads were blocked, there would he no school that day. We jumped up and down for joy and beg mother to let us go outside to play in the snow, She said we could go out after breakfast if we dressed warmly and promised to come back as soon as we felt cold. When we went out, the snow had almost stopped. The roads, paths, gardens, and playgrounds were all hidden under a white blanket of soft snow. It was the most beautiful sight we had ever seen. Just then we saw some neighbours, children coming out to play. We ran over to join them. Together we built a snowman and a snow house. After that, we took turns sliding down a hill on a large piece of carboard. We were having so much fun when supper time came. Mother almost had to force us to go in and eat.

56. The children in the story had come from Hong Kong______ before the event described in the story happened

A. a year       B. the summer     C. a few weeks   D. the winter

57. The children were jumping with joy when they saw the snow because        .

A. they would go to school the following day

B. it was the first time they had seen snow

C. their mother would buy them warm clothes

D. they liked cold winter

58. When the children asked their mother lo let them go out to play, she ____.

A. asked them to go to school          B. refused to let them go outside

C. told them to switch on the radio .     D. told them to wait until after breakfast