题目(2012,福建理综卷第22题)如图1甲所示,在圆柱形区域内存在一方向竖直向下、磁感应强度大小为B的匀强磁场,在此区域内,沿水平面固定一半径为r的圆环形光滑细玻璃管,环心O在区域中心.一质量为m、带电量为q(q>0)的小球,在管内沿逆时针方向(从上向下看)做圆周运动.已知磁感应强度大小B随时间t的变化关系如图1乙所示,其中T_0=(2πm)/(_qB).设小球在运动过程中电量保持不变,对原磁场的影响可忽略. (2012, Fujian Science Volume 22) as shown in Figure 1 A, there is a cylindrical region in the direction of a vertical downward direction, the magnetic flux density of B uniform magnetic field, in this region, along the horizontal plane fixed A circular, smooth, thin glass tube of radius r centered on the center of the circle O. A mass of mass m with a small charge of q (q> 0) rotates counterclockwise in the tube ) To do circular motion.It is known that the magnitude of the magnetic induction intensity B changes with time t as shown in Figure 1 B, where T_0 = (2πm) / (_ qB). Let the ball keep the same electricity during the movement, The effect is negligible.