| 如图所示，在x轴上方存在着垂直于纸面向里、磁感应强度为B的匀强磁场．一个不计重力的带电粒子从坐标原点O处以速度v进入磁场，粒子进入磁场时的速度方向垂直于磁场且与x轴正方向成120°角，若粒子从x轴上的P点射出磁场，已知p点与O点的距离为a，则该粒子的比荷和所带电荷的正负是（ ） A． B．v/2aB，正电荷 C． D．v/2aB，负电荷  |
答案：C
| 补全对话。 | |
| A: Hey, John! B: Yes? A: Is Tim's dictionary on the table? B: 1______. It's on the bed. A: Where are Tim's English books? Are they in the bookcase? B: No, they aren't. 2______ A: Where is his pencil case then? B: 3______. A: And where's his video tape? B: It's under the table. 4______. A: Oh, thanks. And where are his CDs? B: 5______ A: Thanks.
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v/aB，正电荷
v/aB，负电荷