题干

科技小组的同学们设计了一个多档位的电热器模型,电路实物连接示意图如图所示,电路中电源两端电压保持不变,滑动变阻器的最大阻值为R3.将滑动变阻器的滑片P置于中点M,且只闭合开关S1时,电压表V1的示数为U,电流表的示数I1为0.8A;将滑动变阻器的滑片P置于B端,断开开关S1,闭合开关S2时,电压表V1的示数为
3
4
U,电流表的示数为I2,R2消耗的电功率P2为2.7W.
(1)求电流表的示数I2
(2)将滑动变阻器的滑片P置于A端,闭合开关S1和开关S2时,电压表V2的示数为3U,求此时R1消耗的电功率P1
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解:滑片P置于变阻器的中点M,且只闭合开关S1时,等效电路如图(甲)所示;将滑片P置于变阻器的B端,且只闭合开关

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    It was a Christmas night.I was an unlucky nurse who had to work on such a beautiful festival.When I was complaining about it,three people appeared at my desk﹣﹣a tired woman and two children

    "Are you all sick?"I asked doubtfully because they seemed all right.

    "Yes,"the woman answered weakly and lowered her head.

    But when they started to present their problems,things because unclear.One child had a fever,but his temperature was OK.The other child had an earache,but she could not tell me which ear hurt.It seemed that the mother was pretending to cough.

    Something was wrong.But I only explained that it might take a while before a doctor could meet them."Take your time,please,"said the mother.I checked their charts﹣﹣﹣no address.Suddenly I knew,they were homeless while the hospital was warm.

    The family huddled(卷缩)together under the Christmas tree,smiling and talking with each other sweetly.Quietly,I went back the nurses'station and told them what happened in the waiting room.It was just like God sending us a gift on Christmas Day.The nurses'station suddenly came back to life.All the nurses went into action for"a Christmas emergency".

    We took out our meals for our Christmas"patients".We also put together oranges and apples as presents.We tried to exceed the needs of a family who needed only a warm place on Christmas night.Later,the little girl kissed me and said,"Thanks for being our angel."