两个带电粒子以同一速度、同一位置进入匀强磁场，在磁场中它们的运动轨迹如图8－2－20所示．粒子a的运动轨迹半径为r1，粒子b的运动轨迹半径为r2，且r2＝2r1，q1、q2分别是粒子a、b所带的电荷量，则
A．a带负电、b带正电，比荷之比为
∶
＝2∶1
B．a带负电、b带正电，比荷之比为∶＝1∶2
C．a带正电、b带负电，比荷之比为∶＝2∶1
D．a带正电、b带负电，比荷之比为∶＝1∶1
答案：C
题目分析：由粒子的运动轨迹及左手定则可判断a带正电、b带负电，根据
，可得
，所以
∶
＝r2：r1=2∶1。选项C正确。
I drove a school bus as a part-time job. Thirty-nine children all had stories to tell me about their summers. Charlie though __16__me. From the first day on Charlie was __17__. If a fight broke out I didn’t have to turn my __18__to know who had started it. If a girl was crying chances were that Charlie had __19__ her hair. No matter how I spoke to him gently or __20__ he wouldn’t say a word. He’d just __21__ me with those big gray eyes of his. One day I found a small tin heart (小锡心)on the bus was gone. “Does anyone know __22__happened to the little heart here” I asked. Some children cried “Charlie __23__it Search him” When I asked him to come forward his gaze( 注视) __24__ my eyes. I stuck my hand into one of his pockets. Then I felt it—the __25__ outline(轮廓)of the small tin heart. Charlie stared at me for a long time. There were no __26__ in those big gray eyes no plea( 恳求) for mercy. He seemed to be waiting for what he’d come to __27__ from the world. I was about to pull the tin heart out of Charlie’s pocket __28__I stopped myself. Let him keep it, I said “It must have fallen off before I got here.” Without a word Charlie __29__ to his seat. When he got off at his stop, he didn’t so much as glance at me. Several days later just that__30__ Charlie moved away… A dozen years after retirement we met in the street. To my __31__, he hugged me. After he let go, he pulled something from his pocket and held it up for me to see. An old key chain… bent out of __32__, the lettering faded (消褪). You __33__probably guess what it was—the little tin heart. “You were the only one who kept trying.” He explained. We __34__ again and went our separate ways. I am so __35__ I’d done a good job.
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