长为L、质量为m的均匀金属细棒以其一端O点为圆心在水平面内旋转，另一端恰好搭接在半径为L的金属圆环上滑动。设金属细棒在O点与圆环间接一电阻R，在竖直方向加一均匀磁场B。当时间t=0时，金属细棒的转动角速度为ω0。忽略摩擦力及金属细棒、导线和圆环的电阻，问：
若在任意时刻金属细棒的转动角速度为ω，求通过金属细棒的电流的大小与方向。
参考答案：
动生电动势为
，电流从圆心沿棒流向圆环，
。
Have you ever heard of Teddy bears? Teddy bears were invented by accident. In 1902, American president Theodore (Teddy) Roosevelt was hunting (打猎) in the forest with several of his friends. Almost everyone had successfully shot (射中) an animal, but the president not. Nobody wanted to make the president look , so they tied an American black bear to a . They wanted the president to shoot it． The president was very surprised to see the bear tied to a tree. He felt it was to shoot the bear, because it couldn’t run away. He to shoot it, and the story of his decision went over the country. Everybody was moved by the president’s beliefs (信念). There was a cartoon showing the president refusing to shoot a cute bear. It was this cartoon and the story behind the president’s hunting trip that gave Morris an idea to make a new ．He made a small, soft bear and showed it in his shop window. Next to the bear he made a sign that read, “Teddy’s Bear.” The new toy was a big success and it Morris with the money to create a toy company. Teddy bears have been since they were first made. A teddy bear would be the first choice when people think about children’s toys. Theodore Roosevelt’s famous on a hunting trip in 1902 brought millions of soft, stuffed bears. They are being loved by people across the world.
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