题干

在图中所示的电路中,电源两端电压不变。闭合开关S,滑动变阻器滑片移至点M时,变阻器接入电路的电阻为RM,电阻R2消耗的电功率为P2,电压表V1、V2的示数分别为U1和U2;滑动变阻器滑片移至点N时,变阻器接入电路的电阻为RN,电阻R2消耗的电功率为P2,电压表V1的示数为U1。已知:U1∶U2=2∶5, U1∶U1=1∶2,P2=8W,P2=2W。求:

(1)滑动变阻器接入电路的电阻RM与RN之比;
(2)滑片左右移动的过程中,电路消耗的最大电功率Pmax

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答案(点此获取答案解析)

解:(1) 当开关S闭合,滑动变阻器滑片移到M点时,电路图如图甲所示;当开关S闭合,滑动变阻器滑片移到N点时,电路图如图乙所示。

∵P=I2R,且P2

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    After my brother died in an accident, my mother was very sad. I was only 4 years old at the time, but I still understood the 1 in my mom's attitude toward safety.2, everything around us was potentially 3.

    I grew up with a lot of 4 and rules that were meant to 5 me. For example, I was not 6 to walk home from school by myself, even though everyone I knew already did. I couldn't 7 evening parties or go to summer camp,8 what if something happened to me?

    As I got older, the list of things to 9 got longer. I became a natural 10. I was concerned about things like getting cancer, losing my wallet, being caught in car accidents and earthquakes —11 big and small, real and 12.The funny part is that you'd never know it by looking at my life, because I'm constantly 13 myself to do the things that frighten or worry me. In fact, I've developed a 14 for myself: If it scares me, then I have to do it 15 once. I've done lots of things that would have 16 my mom: I've ridden a motorcycle, I've traveled alone, and I've performed stand-up comedy.

    Courage isn't a natural attribute (品质) of human beings. I believe that we have to practice being courageous. The more 17 I do things that scare me or that make me 18, the more I realize that I can do a lot more than I originally thought I could do.

    Even though I inherited (经遗传获得) my mother's cautious19, I've also come to believe that fear can be a good thing, if we 20 it. Believing that has made my world a less scary place.