题目

We do the shopping on Sundays . I often go to the supermarket with my mother on Sunday morning in a car . Father likes fruit very much. We buy some red apples for him. Mother likes yellow bananas , but they’re green. “Look! The strawberries are very nice, mum. Do you like them?” I say to Mum. “Yes, I like strawberries very much . ” Mother buys some for herself. The oranges are very nice. Mother buys some for me . I like orange very much. We need some vegetables . The vegetables in the supermarket are very good . We buy some tomatoes, carrots and broccoli . I don’t like broccoli, bu 风能将成为21世纪大规模开发的一种可再生清洁能源．风力发电机是将风能(气流的功能)转化为电能的装置，其主要部件包括风轮机、齿轮箱，发电机等．如图所示． (1)利用总电阻R＝10 Ω的线路向外输送风力发电机产生的电能．输送功率P0＝300 kW，输电电压U＝10 kW，求异线上损失的功率与输送功率的比值； (2)风轮机叶片旋转所扫过的面积为风力发电机可接受风能的面积．设空气密度为p，气流速度为v，风轮机叶片长度为r．求单位时间内流向风轮机的最大风能Pm；在风速和叶片数确定的情况下，要提高风轮机单位时间接受的风能，简述可采取的措施． (3)已知风力发电机的输出电功率P与Pm成正比．某风力发电机的风速v1　9 m/s时能够输出电功率P1＝540 kW．我国某地区风速不低于v2＝6 m/s的时间每年约为5000小时，试估算这台风力发电机在该地区的最小年发电量是多少千瓦时．