题干

图所示电路中,电源两端电压保持不变,滑动变阻器的最大阻值为R。当开关S、S1闭合,S2断开,滑动变阻器的滑片P移到A端时,电阻R2的电功率为P2,电流表的示数为I1;当开关S、S2闭合,S1断开时(滑片P位置不变),电阻R2的电功率为P2',电流表的示数为I2,且P2:P2'=16:25;当开关S闭合,S1和S2都断开,滑动变阻器的滑片P移到某点C(图中未画出)时,滑动变阻器接入电路的电阻为RC,电压表V1的示数为U1,电压表V2的示数为U2,已知U1:U2=1:2,这时R2的电功率为9W,RC的电功率为3W。

(1)求I1与I2的比值;
(2)求RA与RC的比值;
(3)若通过闭合或断开开关及移动滑动变阻器的滑片P,使电路的电功率达到最大,求此时的电功率Pm

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答案(点此获取答案解析)

解:当开关S、S1闭合,S2断开,滑动变阻器的滑片P移到A端时,等效电路如答图甲所示;
当开关S、S2闭合,S1断开时,等效电路如答图乙所示;
当开关S闭合,S1和S2都断开,滑动变阻器的滑片P移到某点C时,等效电路如答图丙所示;

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阅读下面短文,掌握其大意,然后从各题所给的四个选项(A、B、C和D)中,选出最佳选项。

    I often read of incidents of misunderstanding or conflict. I'm left1.Why do these people create mistrust and problems, especially with those from other2?

    I was growing up in Kuala Lumpur in the early 1960s,3children from different races and religions played and studied4in harmony. At that time my family lived a stone's5from Ismail's. And no one was bothered that Ismail was a Malay Muslim and I was an Indian Hindu-we just6our differences. Perhaps, our elders had not filled our heads with unnecessary advice, well7or otherwise.

    We were nine when we became friends. During the school holidays, we'd8the countryside on our bicycles, hoping to9the unexpected. At times Ismail would accompany my family as we made a rare shopping trip to town. We would be glad of his10.

    When I was twelve, my family moved to Johor. Ismail's family later returned to their village, and I11touch with him.

    One spring afternoon in 1983, I stopped a taxi in Kuala Lumpur. I12my destination. The driver acknowledged my13but did not move off. Instead, he looked14at me. “Raddar?”he said, using my childhood nickname(绰号). I was astonished at being so15addressed(称呼). Unexpectedly! It was Ismail! Even after two16we still recognized each other. Grasping his shoulder, I felt a true affection, something17to describe.

    If we can allow our children to be18without prejudice, they'll build friendships with people, regardless of race or religion, who will be19their side through thick and thin. On such friendships are societies build and20we can truly be, as William Shakespeare once wrote, “we happy few, we band of brothers”.