带电粒子在匀强有界磁场中做圆周运动的问题比较常见,它难在确定临界点.本文提出一个好方法——动态逼近画轨迹圆. 例1 如图1,足够长的矩形区域ABCD内有一磁感应强度为B,方向垂直纸面向里的匀强磁场.现从矩形区域AD边的中点O处,垂直于磁场射入一速度方向与AD边夹角为30°,大小为v_0的带正电粒子,已知粒子质量为m,电荷量为q,AD边长为l,重力影响不计,试求粒子能从AB边上射出磁场的速度v_0的大小范围. The problem of circular motion of charged particles in a uniformly bounded magnetic field is relatively common, and it is difficult to determine the critical point. This paper presents a good method - dynamic approximation of the tracing circle. Example 1 As shown in Figure 1, a sufficiently long rectangular area ABCD There is a magnetic induction intensity of B. The vertical magnetic paper is facing the inside of the uniform magnetic field. Now from the midpoint O of the AD side of the rectangular region, perpendicular to the magnetic field, an angle between the velocity and the AD side is 30°, and the size is v_0. Positive particles, known particle mass m, charge q, AD side length l, gravity effects do not count, try to particle from the AB side of the magnetic field velocity v_0 size range.