题目

Nancy would like to take part in a fancy-dress party (化妆舞会). She had to go to the party with her friends because her husband has been to another city to go on a business trip. Later that night, she was busy in dressing up herself as a ghost, including putting on masks and some special costumes. After finishing it, she went downstairs to look at herself in the mirror. Suddenly, there was a knock on the front door. She knew that it must be the baker (面包师). She had told him to come straight in if ever she failed to open the door and to leave the bread on the kitchen table. Not wanti如图所示为一种磁性加热装置，其关键部分由n根间距相等的平行金属条两端焊接在两个等大的金属圆环上，成鼠笼状。每根金属条的长度为l，电阻为R，金属环的直径为D、电阻不计。图中虚线所示的空间范围内存在着磁感强度为B的匀强磁场，磁场的宽度恰好等于“鼠笼”金属条的间距，当金属环以角速度ω绕过两圆环的圆心的轴OO′旋转时，始终有一根金属条在垂直切割磁感线。“鼠笼”的转动由一台电动机带动，这套设备的效率为η，求电动机输出的机械功率。