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如图所示,固定于水平桌面上的光滑金属架cdef,处在一竖直向下的匀强磁场中,磁感强度的大小为B0,金属棒ab搁在框架上,与adeb构成一个边长为l的正方形,金属棒的电阻为r,其余部分的电阻不计.从t=0的时刻起,磁场开始均匀增加,磁感强度变化率的大小为k(k= △B △t ).同时用垂直于金属棒的水平拉力F使金属棒保持静止,则,F的方向______,F的大小随时间 t变化的关系式______.
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![](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAA5ElEQVRYR2NkGGDAOMD2M+B0wKpVq9huv/jbxPD/fxwDA4MkhQ59zsDIuEhVgrkuLCzsF7JZOB3QOml5B8P//+UUWoyqnZGxszovsoI4B0xc9gyfzxmZmb2rcsK3IRvWMWm599///7fgcfTz6vwoKWId8B+f75kZGX0q8iK3kugAhur8KJRQxx0FE5fhdQC5UTPqgNEQGA2B0RAYDYHREBgNgaETAuTW96TqG7yt4tbRFhGRIQBqHTP9+8dIoDUMTxqk5AK8zXKYiaDWMYhNpANIaJYPdMdkwLtmpOZnctUPeDkAAP6tmiH1ggTLAAAAAElFTkSuQmCC)
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