题目内容
(09·浙江·17)如图所示,在磁感应强度大小为B、方向竖直向上的匀强磁场中,有一质量为
、阻值为
的闭合矩形金属线框
用绝缘轻质细杆悬挂在
点,并可绕
点摆动。金属线框从右侧某一位置静止开始释放,在摆动到左侧最高点的过程中,细杆和金属线框平面始终处于同一平面,且垂直纸面。则线框中感应电流的方向是 ( )

A.
B.
C.先是
,后是
D.先是
,后是






A.

B.

C.先是


D.先是


B
由楞次定律,一开始磁通量减小,后来磁通量增大,由“增反”“减同”可知电流方向是
。


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