题目内容
.如图17-124所示,导轨是水平的,其间距L1=0.5m,ab杆与左端的距离L2=0.8m,由导轨与ab杆所构成的回路电阻为0.2Ω.方向垂直导轨平面向下的匀强磁场的磁感应强度B=1T,滑轮下挂一重物质量为0.04kg,ab杆与导轨的摩擦不计,现使磁场以
的变化率均匀地增大,由磁场开始变化起计时,求当时间t为多少时,重物刚好离开地面?(g取10m/s2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623171876.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623109573.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623171876.gif)
5s
重物刚好离开地面时mg=FA
因为FA=BIL1
由题知B=B0+![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623187330.gif)
据欧姆定律I=E/R
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623203489.gif)
S=L1L2
t=5s
因为FA=BIL1
由题知B=B0+
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623187330.gif)
据欧姆定律I=E/R
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115623203489.gif)
S=L1L2
t=5s
![](http://thumb2018.1010pic.com/images/loading.gif)
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