题目内容
如图所示,水平桌面上放着一对平行金属导轨,左端与一电源相连,中间还串有一开关K.导轨上放着一根金属棒ab,空间存在着匀强磁场.已知两导轨间距为d,电源电动势为E,导轨电阻及电源内阻均不计,ab棒的电阻为R,质量为m,棒与导轨间摩擦不计.闭合开关K,ab棒向右运动并从桌边水平飞出,已知桌面离地高度为h,金属棒落地点的水平位移为s.下面的结论中正确是
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201112262383.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201112262383.gif)
A.磁场力对ab棒的冲量大小![]() |
B.磁场力对ab棒所做的功![]() |
C.开始时ab棒离导轨右端的距离![]() |
D.ab棒在导轨上运动时间![]() |
AB
设ab棒做平抛运动的初速为v0,运动时间为t’,则
h=gt’2/2,s=v0t’.
解出t’=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113191073.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113351082.gif)
根据动量定理,磁场力对ab棒的冲量
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113351228.gif)
根据动能定理,磁场力对ab棒所做的功
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113511243.gif)
有人会认为
C.D两选项也正确,理由是:由于冲量I=F·t,而磁场力F=Bid=Bed/R,因此t=I/F=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113821233.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241201113821170.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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