小红在商店买了两个做实验用的小灯泡，其中甲灯的电阻是30Ω，乙灯的电阻是20Ω．实验中，两灯串联时，总电阻为______Ω；若两灯并联接入某电路时均能发光，则______灯较亮（选填“甲”或“乙”）．灯泡在使用一段时间后，它的实际功率将______（选填“变大”“变小”或“不变”）．
（1）两灯泡串联，总电阻R=R甲+R乙=30Ω+20Ω=50Ω；
（2）∵R甲＞R乙，两灯并联时，灯泡两端电压U相等，
∴由电功率公式P=
可知，P甲＜P乙，乙灯较亮；U2 R
（3）使用一段时间后，灯丝材料、长度不变，但横截面积变小，
灯丝电阻R变大，电压U不变，由P=
可知，灯的实际功率变小．U2 R
故答案为：50；乙；变小．
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| Jack and Tom worked for a builder. When this story happened, they were working on a very large building. Their job was to pick up all the 1 , put it in wheel-barrows and take it to the rubbish place. It was not difficult work, but it was a little 2 .They had to walk under the place where men were working. Sometimes these men 3 their tools. Then the tools fell down from the top of the building to the ground. One morning, Jack was pushing his wheel-barrow to-wards the rubbish place when a voice shouted, "Look out!" But Jack did not "look out". He 4 up. And as he did so, a saw fell down from the top of the building. As the saw passed Jack's head, it cut off one of his 5 . Immediately he put his hand to his ear and cried out," I've lost an ear. Help! Help!" Tom ran up to help his 6 ."Look for my ear," Jack told Tom. "It's on the ground somewhere.“ While Jack held a handkerchief to his 7 to stop the bleeding, Tom looked everywhere for the missing ear. At last he found an ear on the ground. He picked it up and carried it to Jack. "Here you are," he said." I've found 8 ." Jack looked at it carefully. "No, that's not my ear," he said." Mine had a cigarette behind it." | |||
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