问题 选择题

如图,两条平行虚线之间存在匀强磁场,虚线间的距离为l,磁场方向垂直纸面向里.abcd是位于纸面内的梯形线圈,ad与bc间的距离也为l.t=0时刻,bc边与磁场区域边界重合.现令线圈以恒定的速度v沿垂直于磁场区域边界的方向穿过磁场区域.取沿a→b→c→d→a的感应电流方向为正,则在线圈穿越磁场区域的过程中,感应电流i随时间t变化的图线是(  )

A.

B.

C.

D.

答案

线框进入磁场过程中,bc边切割磁感线,根据右手定则可知,电流方向为逆时针,即为负方向,当bc边离开磁场时,由右手定则可知,感应电流沿顺时针方向,电流为正的,故ACD错误;

线圈接入磁场时,bc边切割磁感线,根据感应电动势大小公式:E=BLV可得,有效切割长度越来越短,感应电动势减小,故感应电流越来小;当bc边离开磁场时,根据感应电动势大小公式:E=BLV可得,切割长度越来越短,感应电动势减小,故感应电流越来越小,故B正确.

故选:B.

阅读理解

I’ve loved my mother’s desk since I was just tall enough to see above the top of it as mother sat doing letters. Standing by her chair, looking at the ink bottle, pens and white paper, I decided that the act of writing must be the most wonderful thing in the world.

Years later, during her final illness, mother kept different things for my sister and brother. “But the desk,” she said again, “is for Elizabeth.”

I never saw her angry, never saw her cry. I knew she loved me; she showed it in action. But as a young girl, I wanted heart-to-heart talks between mother and daughter.

They never happened. And a gulf opened between us. I was “too emotional(易动感情的)”. But she lived “on the surface”.

As years passed and I had my own family. I loved my mother and thanked her for our happy family. I wrote to her in careful words and asked her to let me know in any way she chose that she did forgive me.

I posted the letter and waited for her answer, none came.

My hope turned to disappointment, then little interest and, finally, peace. It seemed that nothing happened. I couldn’t be sure that the letter had even got to Mother. I only knew that I had written it, and I could stop trying to make her into someone she was not.

Now the present of her desk told me that she was pleased that writing was my chosen work though she’d never been able to. I cleaned the desk carefully and found some papers inside — a photo of my father and a one-page letter, folded(折叠) and refolded many times.

Give me an answer, my letter asks, in any way you choose, Mother, you always chose the act that speaks louder than words.

小题1:The writer began to love her mother’s desk _______.

A.after Mother died

B.before she became a writer

C.when she was a child

D.when mother gave it to her小题2:The passage shows that _______.

A.Mother was cold on the surface but kind in her heart to her daughter

B.Mother was too serious about everything her daughter had done

C.Mother cared much about her daughter in words

D.Mother wrote to her daughter in careful words.小题3: The world “gulf” in the passage means _______.

A.deep understanding between the old and the young.

B.different ideas between the mother and the daughter.

C.free talks between mother and daughter.

D.part of the sea going far in land.小题4:What did Mother do with her daughter’s letter asking for forgiveness?

A.She had never received the letter.

B.For years, she often talked about the letter.

C.She didn’t forgive her daughter at all in all her life.

D.She read the letter again and again till she died.小题5: What’s the best title of the passage?

A.My letter to Mother

B.Mother and Children

C.My Mother’s Desk

D.Talks between Mother and me.

单项选择题