问题 选择题

边长为h的正方形金属导线框,从图所示的初始位置由静止开始下落,通过一匀强磁场区域,磁场方向垂直于线框平面,磁场区宽度等于H,H>h.从线框开始下落到完全穿过磁场区的整个过程中(  )

A.线框运动的方向始终是向下的

B.线框速度的大小不一定总是在增加

C.线框中总是有感应电流存在

D.线框受到的磁场力的合力的方向有时向上,有时向下

答案

因为H>h,故可以分为三个过程:①从下边开始进入磁场到全部进入磁场;②从全部开始进入磁场到下边开始离开磁场;③下边开始离开磁场到全部离开磁场.

A、线框运动方向始终是向下的,故A正确.

B、线框在进入和穿出磁场的过程中有感应电流,且受到安培力的合力方向始终向上,由于线框完全进入磁场的过程做匀加速运动,可能会使线框离开磁场时线框所受的安培力大于线框的重力,从而使线框的速度减小.故B正确.

C、线框完全进入磁场到下边开始离开磁场的过程,磁通量不变,无感应电流,故C错误.

D、线框在进入和穿出磁场的过程中,由楞次定律和左手定则得有感应电流,且受到安培力的合力方向始终向上,故D错误.

故选AB.

选择题
写作题

第二节:读写任务(共1小题,满分25分)

阅读下面的短文,然后按照要求写一篇150词左右的英语短文。

A father had three sons, but the brothers were always quarreling. Their father tried to make them good fiends, and called all his sons to him. He showed them three sticks and said, “I want to talk to you. Take these sticks and tie them together. Then try to break the bundle of sticks.” The oldest son tried with all his strength but he could not break it. Then the other two did the same, but neither of them could break it. “Now,” said the father, “untie the bundle and each of you take one stick and try to break it.” Either of them could break the stick easily. Then the father said, “My sons, when the sticks are bound together, it is very strong, and you cannot break it. But when they are united, you can break each stick easily. When you work together and help one another, you can become as strong as the bundle. But if you only quarrel and do not stand together, you will be broken as easily as these sticks.”

[写作内容]

1)   以约30个词概括短文的要点;

2)   以约120词就“团结”为主题写一篇短文,至少包括以下内容要点:

a) 用具体实例说明团结的重要性;

b) 从中得到了什么启示。

[写作要求]

(1)  可以使用实例或其它论述方法支持你的论点,也可以参照阅读材料的内容,但不得直接引用原文中的句子;    (2)  标题自定。

[评分标准]           概括准确、语言规范、内容合适,篇章连贯。