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如图甲,电磁继电器和热敏电阻R1等组成了温度控制箱电路,R1处于温箱内,电源电压U=6V,继电器线圈的电阻可不计,图乙为热敏电阻的R1﹣t图象,且已知在50~150℃范围内,热敏电阻的阻值随温度的变化规律是:R1•t=K(常数);当线圈中的电流达到40mA时,继电器的衔铁被吸合,下触点闭合,此时箱内温度为80℃,当线圈中的电流为30mA时,电磁继电器的衔铁释放,上触电闭合,此时箱内温度为60℃,这样就可以时控制箱内温度在60℃~80℃范围内变化.图中的“交流电源”是控制箱加热器的电源,它的额定功率是1500W.

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    City people usually think they are a lot smarter than country people. They often laugh at simple country ways. But people do not laugh at country music. It is one of the most popular kinds of music in the United States today.

    Perhaps it is so popular because it is about simple but strong human feelings and events—love, sadness, good times and bad times, it tells real life stories and sounds the way people really talk. As life becomes more and more complicated(复杂的), it is good to hear music about ordinary people.

    Country music, sometimes called country western, comes from two kinds of music. One is the traditional music of the people in the Appalachian Mountains in the eastern United States. The other is traditional cowboy music from the West. The singers usually play the guitars, and in the 1920s they started using electric guitars.

    At first city people said country music was low-class. It was popular mostly in the South. But during World War Ⅱ, thousands of Southerners went to the Northeast and Midwest to work in the factories. They took their music with them. Soldiers from the rest of the country went to army camps in the South. They learned to like country music. Slowly it became popular all over the country.

    Today country music is popular everywhere in the United States and Canada, in small towns and in New York City, among black and white, and among educated and uneducated people. About 1,200 radio stations broadcast country music twenty-four hours a day. People sing it in their languages. The music that started with cowboys and poor farmers is now popular all over the world.