题目如图1所示,一个质量为m、带正电荷q的小球用长为l的细线悬挂在匀强磁场中,拉开至最大角α后释放,令其自由摆动,磁场方向垂直纸面向里,为使其能正常摆动且周期不受磁场影响,问对B大小的取值有什么限制? The subject is shown in Figure 1, a ball of mass m with a positive charge q is suspended in a uniform magnetic field by a thin wire of length l, opened to a maximum angle α and released, allowing it to swing freely with the field direction perpendicular Paper-oriented, in order to make it normal swing and the cycle is not affected by the magnetic field, ask the size of the B What is the limit?