如下图所示，条形磁铁放在桌子上，一根通电直导线由S极的上端平移到N极的上端的过程中，导线保持与磁铁垂直，导线的通电方向如图，在这个过程中磁铁保持静止，则磁铁受到的摩擦力（ ）
A．为零.
B．方向由左变为向右
C．方向保持不变.
D．方向由右变为向左.
答案：B
题目分析：如图，
导体棒在S极附近时受力向左上方，根据牛顿第三定律知磁铁受反作用力向右下方，对磁铁受力分析，由平衡条件知磁铁受静摩擦力向左；同理，当直导线运动到N极附近时受力向右上方，磁铁受摩擦力向右，故选项B正确。
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| Fifteen years spent in the field of education has provided me with many treasured moments. One of the most unforgettable __1__ when I was teaching the __2__ grade 10 years ago. In May, I decided to plan something __3__ for the children: a Mother's Day tea. We had put our heads together to __4__ ideas of how to __5__ our mothers. We practiced singing songs, memorized poems and wrote cards. We decided to hold our tea on Friday before Mother's Day. I was surprised but very __6__ to learn that every mother was planning to attend.I __7__ invited my own mother. Finally, the big day arrived.Each child __8__ up at our classroom door, expecting the arrival of his or her mom.__9__ it got closer to starting time, I looked around and my eyes quickly found Jimmy. His mother hadn't shown up and he was looking __10__. I took my mother by the hand and walked over to Jimmy. "Jimmy," I said, "I have a bit of problem here and I was __11__ if you could keep my mother __12__ while I'm busy." My mom and Jimmy sat at a table. Jimmy __13__ my mom her treats, presented her with the gift I had made, just as we had __14__ the day before. Whenever I looked over, my mother and Jimmy were in deep __15__. Now 10 years later, I work with students of all ages, educating them about the environment. Last year, I took a senior class on a field trip, and there was Jimmy. I had the students complete an outline of the day's __16__ and an evaluation of our trip. Then I collected the students' booklets and checked them to see if __17__ was completed. When I came __18__ Jimmy's page, he had written "Remember our Mother's Day tea we had in the second grade, Mrs. Marra? I do! Thanks for all you did for me, and thank your mother, too." I told him I really enjoyed what he had written.He looked rather embarrassed and __19__ his own thanks and walked away. Suddenly he ran back and gave me a big hug. I ended my workday with a hug from a teenage boy who had __20__ stopped hugging teachers years ago. | ||||
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