题干

某一用直流电动机提升重物的装置,如图所示,重物质量m=50kg,电源的电压为U=110V,不计电源内阻及各处摩擦,当电动机以v=0.90m/s的恒定速度向上提升重物时,电阻中的电流I=5A,由此可知电动机的线圈的电阻R为多少?

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答案(点此获取答案解析)

解:电动机消耗的总能量一部分用于提升重物的机械能,一部分消耗在电动机线圈电阻发热上.根据能量守恒定律得

     P电=P机+P热,

其中P电=IU,P机=mgv,P热=I2R,

所以有IU=mgv+I2R,

代入解得  R= <

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    When I was a kid, I always used to wonder how in the world my father worked outside in the winter without a coat. It could be minus 20 degrees centigrade and there'd be Dad, removing snow, or perhaps chopping(砍) some wood — his coat thrown aside — wearing a shirt, a cap, and a pair of gloves.

    “Aren't you cold, Dad?” I'd ask. “No,” Dad would reply. “I'm not cold—working too hard to be cold.”

    Many times I wondered whether my father was an extremely tough man, or whether he was foolish.

    One time when I was quite young, perhaps five or so, I went ice fishing with Dad. It was a bright, clear day—and bitterly cold.

    After we'd been out on the ice for a little while, my feet started getting cold.

    “Daddy, my feet are cold.” I said.

    “Yeah, it's cold out here today,” he replied.

    “Tell you what,” he said. “Walk around. Make some circles in the snow. See how many different patterns you can make. That will get your feet warm.”

    I was just a little girl at the time but I remember thinking, “How in the world will walking around in the snow make my feet warm? Dad must be out of mind.”

    But he was my father, after all. I made circles in the snow. I made squares. Pretty soon I was having so much fun making patterns in the snow. I forgot about my feet being cold.

    Now, all these years later, I know, too, from personal experience how my father was able to take his coat off and work outside in the winter wearing just a shirt, a cap and gloves. Because I do it, too. “Aren't you cold?” my husband asked one winter day. “No,” I replied. “I'm not cold—working too hard to be cold.”

    I hope my husband has decided I'm both tough and smart. But I guess quite a bit of the time he thinks I'm foolish.

    Wherever Dad is on that great big farm in the sky—I'm sure he can't help but smile whenever I take my coat off while I'm working outside in the winter.