题目

如图,灯泡L1的电阻是6Ω,L2的电阻是4Ω,S1闭合、S2断开时,电流表的读数是I1=0.8A;S1断开、S2闭合时,电流表的读数为I2=0.5A. 求:(1)电源电压; (2)灯泡L3的阻值. 答案:【考点】欧姆定律的应用. 【分析】(1)当S1闭合、S2断开时,灯泡L1、L2串联,根据串联电路电阻特点求出总电阻,利用欧姆定律求出电源的电压; (2)当S1断开、S2闭合时,灯泡L1、L3串联,根据欧姆定律先求出电路中的总电阻,再利用串联电路电阻特点求出灯泡L3的阻值. 【解答】解:(1)S1闭合完形填空   DC Hilton was one of the first American to find out that there was money to be made in the middle of the night. 42 years ago be bought a small __1___on US highway 69, in Oklahoma. His___2___customers were truck drivers and___3___salesmen who drank coffee and ate cheese-burgers when they stopped to___4___their journey.     It was they who first tried to____5____Hilton to remain open all night. He thought about it for a while, and then suddenly made up his___6____. He took the door key and threw it across the road. He hasn't closed the door____7___.   Over the years his simple cafe expanded into a 24-hour roadside ____8___,with a 100-seat restaurant, a petrol station, a mini shopping market, a car ___9___for mobile homes and all-night selfhelp laundry(洗衣店).     Hilton was a ___10__in a 24-hour working trend which has now ___11___ around the world. Today not only restaurants but also hanks, supermarkets, mail-order ___12___and many other businesses are___13___to be open all night. But is this really a good thing?   A lot of ____14____has been done in America on the effect of 24-hour working and there is growing ___15____about the long term dangers of society that doesn't sleep.   Americans are said to be___16___20% less than they did 100 years ago, and 55% claim to __17___at least occasionally from overtiredness. ___18____of the worst man-made accidents happened in the last few hours before ___19___, when even the most experienced night-worker has difficult____20____awake.         (1)A.market                B.business                      C.station                D.restaurant                (2)A.main                B.common                      C.strange                D.only                (3)A.shopping                B.clothing                      C.traveling                D.cooking                (4)A.start                B.break                      C.enjoy                D.continue                (5)A.persuade                B.suggest                      C.order                D.warn                (6)A.plan                B.thought                      C.mind                D.decision                (7)A.then                B.since                      C.later                D.yet                (8)A.cafe                B.building                      C.workshop                D.hotel                (9)A.garage                B.park                       C.repair                D.rest                (10)A.pioneer                B.dealer                       C.master                D.manager                (11)A.caught    in                B.caught    out                       C.caught    on                D.caught    up                (12)A.shops                B.firms                       C.markets                D.departments                (13)A.ordered                B.forced                       C.performing                D.beginning                (14)A.research                B.effort                       C.work                D.information                (15)A.interest                B.sense                       C.concern                D.progress                (16)A.working                B.playing                       C.sleeping                D.relaxing                (17)A.recover                B.suffer                       C.return                D.prevent                (18)A.Some                B.Few                       C.None                D.All          (19)A.dust                B.midnight                       C.darkness                D.dawn                (20)A.staying                B.becoming                       C.continuing                D.insisting           
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