如图所示，有两根和水平方向成α角的光滑平行金属轨道，上端接有可变电阻R，下端足够长，空间有垂直于轨道平面的匀强磁场，磁感应强度为B．一根质量为m的金属杆从轨道上由静止滑下，经过足够长的时间后，金属杆的速度会趋近于一个最大速度vm，则（ ）
A．如果B增大，vm将变大
B．如果α变大，vm将变大
C．如果R变大，vm将变大
D．如果m变大，vm将变大
金属杆受重力、支持力、安培力，开始时重力沿斜面的分力大于安培力，所以金属杆做加速运动．
随着速度的增加，安培力在增大，所以金属杆加速度逐渐减小，当加速度减小到零，速度最大．
当加速度为零时，速度最大．有mgsinα=BIL，
I=BLvm R
vm=mgsinα?R B2L2
A、如果B增大，vm将变小，故A错误．
B、如果α变大，vm将变大，故B正确．
C、如果R变大，vm将变大，故C正确．
D、如果m变大，vm将变大，故D正确．
故选BCD．
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| Mark lived in a village far away. One day he became very ill and everyone thought he would l soon. They sent for a doctor. Two days 2 the doctor came and looked over the sick man. 3 asked for a pen and some paper to write down the name of the medicine. But there was no pen 4 paper in the village, because no one could write. The doctor 5 up a piece of burnt wood from the fire and wrote the name of the medicine (药) on the 6 0f the house. "Get this medicine for him," he said, " and he will soon get 7 ." Mark's family and friends did not know 8 to do. They could not read the strange words. Then a young man 9 an idea. He took off the door of the house, put it on his carriage (四轮马车) and drove to the nearest 10 . He bought the medicine there, and Mark was soon well again. | ||||
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