如图所示，在O≤x≤L和2L≤x≤3L的区域内存在着匀强磁场，磁场的_爱下问搜题

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问题选择题

如图所示，在O≤x≤L和2L≤x≤3L的区域内存在着匀强磁场，磁场的方向垂直于xOy平面（纸面）向里，具有一定电阻的正方形线框abcd边长为2L，位于xOy平面内，线框的ab边与y轴重合．令线框从t=0时刻由静止开始沿x轴正方向做匀加速直线运动，则线框中的感应电流i（取逆时针方向的电流为正）随时间t的函数图象大致是下列图中的（　　）

A．

B．

C．

D．

答案

设线框进入磁场的时间为t₀，则由L=

1

2

a

t

20

，得t₀=

2L

a

．

在0-t₀时间内，由楞次定律判断可知感应电流方向沿顺时针方向，感应电流大小为I=

BLv

R

=

BLat

R

；故CD均错误．

设线框ab从x=L运动到x=4L的时间为t₁，则由运动学公式得t₁=

2×4L

a

-t₀=2t₀-t₀=t₀，这段时间内穿过线框的磁通量不变，没有感应电流．故B错误．

设线框ab从x=4L运动到x=5L的时间为t₂，则由运动学公式得t₂=

2×5L

a

-

2×4L

a

t₀=

5

t₀-2t₀=0.236t₀的时间内，根据楞次定律得知，感应电流方向沿顺时针方向，为负值．感应电流为I=-

BLv

R

=-

BLat

R

；故A正确．

故选A

完形填空

A lady wanted a birthday gift from her husband. For many months she had liked a beautiful   1 , and knowing her husband could   2  it, she told him that was all she wanted. On the morning of the   3 , her husband told her how   4  he was to have such a good wife, and how much he loved her. He   5  her a beautiful gift package (纸盒). She opened it and   6  a Bible.
7 , she raised her voice and said to her husband, “With all your money, you give me a Bible?” She was so   8  that she left him.
Many years passed and the lady was very   9  in business. She owned two large companies. She   10  her husband was very old, and thought perhaps she should go to   11  him. But before she could, she   12  a letter telling her that he had died, and gave all of his possessions (财产) to her. She needed to come back and   13  things.
When she arrived at his house, sadness   14  her heart. She saw the still new
15 , just as she had left it years before. With   16 , she opened it and began to turn the pages. A ring   17  from the Bible to the floor and a 18  could be seen. She picked it up and found it was the ring she wanted in those days. And on the card was the date of her birth, and the words “LUV U ALWAYS”.
19  your gift is not packaged the way you want it, it’s because it is better packaged the way it is! Always appreciate little things; they usually lead you to bigger things! The best and most beautiful things in the world cannot be seen or even touched for they must be felt with the   20 .

B．performance

A．take care of

C．disappeared

单项选择题

不属八脉交会组穴的是（）。

A.内关、公孙

B.合谷、太冲

C.列缺、照海

D.后溪、申脉

E.外关、足临泣