如图,间距
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3EL%3C%2Fmi%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E1%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3Em%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
的U形金属导轨,一端接有
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0.1%3C%2Fmn%3E%3Cmtext%3E%E2%80%89%3C%2Fmtext%3E%3Cmi%3E%CE%A9%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
的定值电阻
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3ER%3C%2Fmi%3E%3C%2Fmath%3E)
,固定在高
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3Eh%3C%2Fmi%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E0.8%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3Em%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
的绝缘水平桌面上。质量均为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0.1%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3Ekg%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
的匀质导体棒a和b静止在导轨上,两导体棒与导轨接触良好且始终与导轨垂直,接入电路的阻值均为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0.1%3C%2Fmn%3E%3Cmtext%3E%E2%80%89%3C%2Fmtext%3E%3Cmi%3E%CE%A9%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
,与导轨间的动摩擦因数均为0.1(设最大静摩擦力等于滑动摩擦力),导体棒
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3Ea%3C%2Fmi%3E%3C%2Fmath%3E)
距离导轨最右端
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E1.74%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3Em%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
。整个空间存在竖直向下的匀强磁场(图中未画出),磁感应强度大小为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0.1%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3ET%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
。用
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmi%3EF%3C%2Fmi%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E0.5%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3EN%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
沿导轨水平向右的恒力拉导体棒a,当导体棒a运动到导轨最右端时,导体棒b刚要滑动,撤去
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EF%3C%2Fmi%3E%3C%2Fmath%3E)
,导体棒a离开导轨后落到水平地面上。重力加速度取
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E10%3C%2Fmn%3E%3Cmi+mathvariant%3D%22normal%22%3Em%3C%2Fmi%3E%3Cmo%3E%2F%3C%2Fmo%3E%3Cmsup%3E%3Cmi%3Es%3C%2Fmi%3E%3Cmn%3E2%3C%2Fmn%3E%3C%2Fmsup%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
,不计空气阻力,不计其他电阻,下列说法正确的是( )
![](http://tikupic.21cnjy.com/2022/06/14/49/45/4945e9d730e62fd483f5d2f8b8f902cd.png)