题干

市场上有一种用两节干电池供电的小型电风扇.某物理研究小组想粗测电风扇正常运转时电能转化为机械能的效率.于是找到了一台该种电风扇并从铭牌上读出了额定电压U,但是其它字迹看不清楚.该研究小组成员打开电风扇底座上的电源盖并取出了两节电池,并从电风扇装电池处两端各接出一条引线,手边提供的器材有电流表,电压表,滑动变阻器,备用电池若干节,电键,若干导线,固定电池的电池盒.

(a)该小组进行了如下的部分实验步骤,请写出第三步实验过程

①按事先设计的电路连接各元件,并使滑片处于使电路电阻最大处

②合上开关,使滑片从最左端开始缓慢移动,使电压表和电流表都有明显的读数,但电风扇并未转动,读出此时的电压表和电流表读数U0、Ⅰ0.

③继续向右调节滑动变阻器,使____,

④用测的量表示出电动机正常运转时的效率的表达式____.

(b)按上述实验过程,本实验的电路图应选择____(填“甲”或“乙”)

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\u7535\u538b\u8868\u8bfb\u6570\u4e3aU\uff0c\u8bfb\u51fa\u7535\u6d41\u8868\u8bfb\u6570\u4e3aI,I0U\u2212I

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    Music is magic! Music speaks louder than words and it is a “language” that the whole world can understand. A piece of music can produce a response in the heart and mind. Like feeling an electrical current or receiving a personal radio signal, music has a spiritual effect on a person. Different kinds of music influence people in different ways.

    I have listened to music all my life. When I was twelve years old, the Beatles came to America and my whole world opened up. Maybe young people today cannot understand the influence of the Beatles when they exploded across America. Their influence changed the way we dressed, looked, acted and spoke... even our culture. The Beatles arrived in America from the UK just under three months after the assassination(暗杀) of President John Kennedy, which had put America into a great depression. And the freshness and lively spirit of the Beatles was exactly what the country needed to refresh itself.

    Music links the heart of the hearer with that of the composer. This means that it mixes the spirit of the composer with your spirit when you listen to it. And the music can take your spirit out of your body and transport you into another world. Music has a great way of touching people. Music can make you laugh, cry or shout. It's also a great source of inspiration.

    Try this one day and notice what happens: make yourself a cup of tea, sit on your sofa and play one of your favorite songs. Close your eyes, and soon you'll find yourself creating vivid mental images—matching the music that you are listening to.