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【单选题】
如图所示,足够长的金属导轨竖直放置,金属棒ab、cd均通过棒两端的环套在金属导轨上.虚线上方有垂直纸面向里的匀强磁场,虚线下方有竖直的匀强磁场,两匀强磁场的磁感应强度大小均为B.ab、cd棒与导轨间动摩擦因数均为μ,两棒总电阻为R,导轨电阻不计.开始两棒静止在图示位置,当cd棒无初速释放时,对ab棒施加竖直向上的力F,沿导轨向上做匀加速运动.则下列说法中错误的是( )A.
ab棒中的电流方向由b到a
B.
cd棒先加速运动后匀速运动
C.
cd棒所受摩擦力的最大值大于cd棒的重力
D.
力F做的功等于两棒产生的电热、摩擦生热与增加的机械能之和
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