如图甲所示,两平行导轨是由倾角为
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的两倾斜导轨与水平导轨用极短的圆弧导轨平滑连接而成的,并处于磁感应强度大小为
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、方向竖直向上的匀强磁场中,两导轨间距为
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,上端用阻值为
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的电阻连接。一质量为
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的金属杆
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在
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时由静止开始在沿倾斜导轨向下的拉力(图中未画出)作用下沿导轨下滑,在
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时刻金属杆
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滑至
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处时撤去拉力,金属杆
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在水平导轨上继续运动,其速率
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随时间
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的变化规律如图乙所示,在
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时刻刚好停在水平导轨的
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处。若全过程中电阻
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上产生的总热量为
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,金属杆
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始终垂直于导轨并与导轨保持良好接触,导轨和杆
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的电阻以及一切摩擦均不计。求: