如图所示，磁感应强度为B的匀强磁场有理想边界，用力将矩形线圈从有边界的磁场中匀速拉出，在其他条件不变的情况下( )
A．速度越大，拉力做功越多
B．线圈边长L1越大，拉力做功越多
C．线圈边长L2越大，拉力做功越多
D．线圈电阻越大，拉力做功越多
ABC
题目分析：因为是匀速拉出的，所以受到的拉力等于安培力，即
,所以速度越大，拉力就越大，拉力做功
，就越大，A正确，线圈边长L1越大，拉力做功越多，B正确，线圈边长L2越大，拉力做功越多，C正确，电阻越大，做功越小，D错误，
点评：做本题的关键是求出拉力做功的表达式，根据表达式分析，千万不能将两个边的作用弄反了
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| "I'm too old and it's too late," which played over and over in my mind. I was 1 and exhausted after ending my marriage and my law career at the same time. 2 my ambition to become a writer, I 3 my ability to succeed as one. Had I wasted years going 4 the wrong goals? I was at a low point 5 the voice on the radio began 6 the story of Grandma Moses. Ann Mary Moses left home at thirteen, bore ten children and worked hard to raise the five who 7 . Struggling to make a living on poor farms, she managed to 8 a bit of beauty for herself by embroidering (绣花) on cloth. At seventy-eight, her fingers became 9 awkward to hold a needle. 10 give in to aging, she went out to an empty room and began to 11 . For the first two years, these paintings were either given away or sold 12 a little money. But at the age of seventy-nine, she was "discovered" by the 13 world---and the rest is 14 . She went on to produce more than two thousand paintings, 15 her book illustrations (插图) for It was the Night before Christmas were completed in her one-hundredth year! 16 I listened to the radio, my 17 changed. If Grandma Moses 18 begin a new career and succeed after eighty, my life still had 19 after thirty. Before the program ended, I rushed to my computer to work on the novel I'd nearly 20 . It was published eight months later. | |||
| ( )1. A. encourage ( )2. A. Despite ( )3. A. believed ( )4. A. by ( )5. A. unless ( )6. A. telling ( )7. A. succeeded ( )8. A. notice ( )9. A. so ( )10. A. Other than ( )11. A. paint ( )12. A. off ( )13. A. literature ( )14. A. story ( )15. A. but ( )16. A. Until ( )17. A. mood ( )18. A. should ( )19. A. ability ( )20. A. finished | B. discouraged B. Except B. expected B. after B. because B. reminding B. left B. offer B. too B. As well as B. write B. at B. art B. fortune B. and B. Thus B. heart B. must B. hope B. abandoned | C. thrilled C. For C. proved C. against C. when C. saying C. survived C. give C. such C. Rather than C. think C. by C. education C. history C. so C. As C. interest C. might C. freedom C. published | D. inspired D. With D. doubted D. over D. while D. playing D. grew D. provide D. that D. Instead of D. change D. for D. entertainment D. fact D. therefore D. Then D. spirit D. could D. dream D. sold |