如图，一束电子（电量为e）以速度v0垂直射入磁感应强度为B，宽为d的匀强磁场中，穿出磁场的速度方向与电子原来的入射方向的夹角为30°，求：
（1）电子的质量是多少？
（2）穿过磁场的时间是多少？
（3）若改变初速度，使电子刚好不能从A边射出，则此时速度v是多少？
解：（1）设电子在磁场中运动轨道半径为r，电子的质量是m，由几何关系得：r=dlsin30° =2d ①
电子在磁场中运动Bev0=
，r=
②
由①②得：m=
（2）电子运动轨迹圆心角θ=30°
周期T=
穿过磁场的时间t=
=
=
=
（3）电子刚好不能从A边射出电子轨道半径为r'=d
由Bev=
，得：V=
=
| 阅读短文，回答问题。 | |
But we decide (决定) to have a swim first. After swimming, we have lunch under the trees. Then we lie (躺) in the sun for a while. In the afternoon, it begins to get cloudy. At five o'clock, we have to go back because it starts to rain. 1. What do we do on a sunny day? We have ___________ outside. 2. What do we decide to do first? We decide to ___________ a ___________ first. 3. What's the weather like in the afternoon? It is ___________. 4. Why do we have to go back home? Because it begins to ___________. 5. When do we go home? We go home at ___________ o'clock. |