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．
| 选择正确答案填空 | ||||
| Tom 1 a very good student, and he 2 doing homework, because he liked 3 other things in his free time. Usually he didn't do 4 homework, and when he did it, he always made 5 mistakes. Then one day, his maths teacher looked 6 Tom's homework and found that he had done 7 right. He was very happy and very surprised. He asked Tom to his desk and 8 to him, "You have done all your homework right this time, Tom. 9 your father help you?" Usually Tom's father did help him with his homework, but this time he didn't, because he was not at home. Tom answered, "No, sir. He was busy last night, so I had to do it all 10 myself." | ||||
|