题目

正方形闭合线框abcd，每边长为L，总电阻为R，总质量为m．将其置于磁感强度为B的水平匀强磁场上方h处由静止自由下落，如图所示．线框平面保持在竖直平面内，且cd边始终与磁场边界平行．当cd边刚进入磁场时，   （1）求线框中产生的感应电动势大小;    （2）若此时线框加速度恰好为零，求线框下落的高度h．It was Christmas Eve, 1881. I was 15 and feeling likethe world was for me I didn't have enough money to myself the gift.Dad came back in and there was ice in his beard "You've been by Widow Jensen's house recently? he asked. Widow Jensen two miles down the road. Her died a year ago and left her with three children "I rode by just today, n Dad continued. " Little Jakey was out looking for wood. They must be out of , Matt. "We filled our sled (雷橇) high wood. Then we went to the store and Dad bought some food, and a smaller pair of shoes. I knew we didn't have much money, so was Dad buying them so many things?We got to the Jensen's house "We brought you a few things, madam," Dad said Then Dad Widow Jensen the shoes. She bit（咬） her lower lip (嘴唇) to keep it from trembling and then tears filled her eyes and started running down her face.My throat seemed to be filled with something, and there were tears in my eyes too. Just then my dream gift seemed very . Dad had given me the look on Widow Jensen's face and smiles of her children He had given me the best Christmas of my life.【1】A. out B. off C. over D. away【2】A. catch B. buy C. choose D. control【3】A. lived B. took C. spent D. drove【4】A. father B. friend C. husband