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问题 多选题

如图所示的电路中，电池的电动势E=5V，内电阻r=10Ω，固定电阻R=90Ω，R0是可变电阻．在R0 由零增加到 400Ω的过程中，以下说法中正确的是（　　）

A．当可变电阻R0=0Ω时，整个电路消耗的电功率最大

B．当可变电阻R0=0Ω时，固定电阻R消耗的电功率最小

C．当可变电阻R0=100Ω时，可变电阻R0消耗的电功率最大

D．当可变电阻R0=400Ω时，电源的效率（电源的输出功率与电源释放的总功率的比值）最大

答案

A、整个电路消耗的电功率P=EI，当可变电阻R0=0Ω时，电路中电流最大，而电源的电动势E不变，则知整个电路消耗的电功率最大．故A正确．

B、当可变电阻R0=0Ω时，电路中电流最大，根据公式P=I2R分析得知电阻R消耗的功率最大，故B错误．

C、将R看成电源的内电阻，可变电阻R0消耗的电功率即为等效电源的输出功率，根据结论：当电源的内电阻和外电阻相等的时候，电源的输出功率最大，可知当可变电阻R0=R+r=100Ω时，可变电阻R0消耗的电功率最大，故C正确．

D、电源的效率η=

UI
EI
=
R0+R
R0+R+r
，则当可变电阻R0=400Ω时，路端电压U最大，则电源的效率最大，故D正确．

故选ACD

单项选择题

正常小儿前囟关闭的时间在

A．6～8个月
B．8～10个月
C．10～12个月
D．12～18个月
E．18～24个月
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单项选择题

I first went to hear a live rock concert when I was eight years old. My brother and his friends were all fans of a heavy metal group called Black Wednesday. When they discovered that Black Wednesday were going to perform at our local theatre, they all bought (21) for the performance. However, at the last minute, once of the friends couldn’t go, so my brother offered me the ticket. I was really excited!

I remember the buzz (嘈杂声) of excitement inside the theatre as we all found our seats. After a few minutes, the lights went down and everybody became quiet. I could barely make out the stage in the (22) . We waited. Then there was a roar from the crowd, like an explosion, as the first members of the band stepped onto the stage. My brother leaned over and shouted something in my ear, but I couldn’t hear what he was saying. The first song was already starting and the music was as (23) as a jet engine. I could (24) the drum beats and bass notes in my stomach.

I can’t recall any of the songs that the band played. I just (25) that I really enjoyed the show and didn’t want it to (26) . But in the end, after three encores (加演), the show finished. We left the (27) and walked unsteadily out onto the pavement. I felt a little dizzy, as if I had just woken from a long sleep. My ears were still (28) with the beat of the last song.

After the (29) , I became a Black Wednesday fan too for a few years before getting into other kinds of music. Once in a while, though, I listen to one of their songs and (30) I’m back at that first show.

29().

A. competition

B. performance

C. interview

D. celebration
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