题目

Because of the limitation of space, I had to        a lot of excellent materials, which I intend to include in another book. A．put away         B．leave out          C．cancel out        D．call over 答案：B一带电粒子在磁感应强度为B的匀强磁场中做匀速圆周运动，如果它又顺利进入另一磁感应强度为2B的匀强磁场，则 [     ]A．粒子的速率加倍，周期减半 B．粒子的速率不变，轨道半径减小 C．粒子的速率减半，轨道半径减为原来的1/4 D．粒子的速率不变，周期减半