题目

如图所示，足够长的平行光滑金属导轨水平放置，宽度L=0.4m一端连接R=1 的电阻。导线所在空间存在竖直向下的匀强磁场，磁感应强度B=1T。导体棒MN放在导轨上，其长度恰好等于导轨间距，与导轨接触良好，导轨和导体棒的电阻均可忽略不计。在平行于导轨的拉力F作用下，导体棒沿导轨向右匀速运动，速度v=5m/s。求：（1）感应电动势E和感应电流I；（2）在0.1s时间内，拉力的冲量 的大小；（3）若将MN换为电阻r=1 的导体棒，其他条件不变，求导体棒两端   1. If you want to buy a camera, you can go to                 . A. the corner of Main Street                       B. East Street   C. Fort Street Mall                              D. First Street 2. If a typewriter is $200 originally at Apple, but you can only pay       for it now. A. $140             B. $160          C. $180            D. $400  3. You can go to Jackson’s to buy                   .                    A. radios            B. calculators     C. telephones        D. swimwear  4. Mr. Green is going back home after work at 7:10 p.m., he may choose some __________      from these shops.    A. briefcases and rings                           B. pens and paper   C. telephones and earrings                        D. watches and computers 5. Our school needs sofas and something for teachers to make notes, we should go to        . A. Jackson’s and Smile’s                        B. Smile’s and Apple C. Apple and Jackson’s                          D. Smile’s or Jackson’s