带电粒子在有界磁场中的临界运动或计算有界磁场的边界问题一直是高考的热点和重点.解决该类问题的步骤:(1)根据带电粒子速度方向找出半径方向;(2)根据磁场边界和题设条件画出粒子运动的轨迹;(3)根据运动轨迹确定圆心位置,建立几何关系;(4)根据运动规律列方程.下面就常见的几种临界问题举例分析. The critical problem of the critical motion of charged particles in a bounded magnetic field or the boundary problem of computing a bounded magnetic field has always been the hot spot and focus of the college entrance examination.The steps to solve such problems are as follows: (1) finding the radial direction according to the charged particle velocity direction; (2) (3) According to the trajectory to determine the location of the center of the circle, the establishment of geometric relations; (4) According to the law of motion equations. Below are some common examples of critical issues.