，设r为圆形磁场的半径，R为粒子轨迹半径．由：qvB=
| mv2 |
| R |
| mv |
| qB |
由几何知识可得：R=rcot30°=
| 3 |
粒子在磁场中运动周期：T=
| 2πm |
| qB |
设圆心角为θ，粒子在磁场中运动的时间t1=
| θ |
| 2π |
| 1 |
| 6 |
当粒子速度变为
| v | ||
|
| mv |
| qB |
| R1 | ||
|
故此时粒子偏转圆心角等于90°，故粒子在磁场中运动时间t2=
| ||
| 2π |
| T |
| 4 |
故
| t2 |
| t1 |
| ||
|
| 3 |
| 2 |
即：t2=1.5t
故ACD错误，B正确
故选：B
| v | ||
|
A. 0.5t，设r为圆形磁场的半径，R为粒子轨迹半径．| mv2 |
| R |
| mv |
| qB |
| 3 |
| 2πm |
| qB |
| θ |
| 2π |
| 1 |
| 6 |
| v | ||
|
| mv |
| qB |
| R1 | ||
|
| ||
| 2π |
| T |
| 4 |
| t2 |
| t1 |
| ||
|
| 3 |
| 2 |