在竖直方向的匀强磁场中，水平放置一个面积不变的单匝金属圆线圈，规定线圈中感应电流的正方向如图甲所示，取线圈中磁场B的方向向上为正，当磁场中的磁感应强度B随时间t如图乙变化时，以下四图中正确表示线圈中感应电流变化的是
答案：A
解答：解：在0-
内，根据法拉第电磁感应定律，E=
=
=
．根据楞次定律，感应电动势的方向与图示箭头方向相反，为负值；在-T内，根据法拉第电磁感应定律，
E′===
．根据楞次定律，感应电动势的方向与图示方向相反，为负值．且电流为0-内的2倍，故A正确
故选A
点评：解决本题的关键掌握法拉第电磁感应定律E=n=n，会根据楞次定律判断感应电动势的方向．
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