27. ____ plants and animals deep in the ocean, scientists may find a cure for some of our most serious human diseases.
A. To study B. Studying C. Study D. By studying
26. At the shopping centre, he didn’t know what _____ and _____ with an empty bag.
A. to buy; leave B. to be bought; left C. to buy; left D. was to buy; leave
25. A few people were killed in the fire, but _____ were saved.
A. the most B. most of them C. most of whom D. most
24. – Why not go and have dinner in a restaurant?
-- _____. It’s too expensive.
A. I agree B. I’m afraid not C. I don’t think so D. I’m sure
23. The plan was in _____ just because people were unwilling to cooperate.
A. ruins B. pieces C. danger D. trouble
22. --Can I have something to eat, Mum? I feel hungry.
-- You ____ be hungry. You’ve just had some bread and ham.
A. mayn’t B. won’t C. mustn’t D. can’t
第一节 单项填空(共15小题,每小题1分,满分15分)
从A、B、C、D四个选项中,选出可以填入空白处的最佳选项,并在答题卡上将该项涂黑。
21. It’s quite obvious that the aging population in China will cause _____ heavy pressure
on _____ whole society in the future.
A. a; / B. /; / C. a; the D. the; a
20.(上海市2009届高三上期期末浦东区)如图(a)所示电路中,电源电动势E=12V,内阻r=2Ω,R1=4Ω,R2=6Ω,R3=3Ω。
(1)若在C、D间连一个理想电压表,其读数是多少?
(2)若在C、D间连一个理想电流表,其读数是多少?
(3)图(a)中虚线框内的电路可等效为一个电源,即图(a)可等效为图(b),其等效电动势E′等于CD间未接入用电器时CD间的电压;若用导线直接将CD两点连接起来,通过该导线的电流等于等效电源的短路电流。则等效电源的内电阻r′是多少?
(4)若在C、D间连一个“6V,3W”的小灯泡,则小灯泡的实际功率是多少?
解析:.
(1)若在C、D间连一个理想电压表,根据闭合电路欧姆定律,有
理想电压表读数为
UV=I1R2=6V
(2)若在C、D间连一个理想电流表,这时电阻R2与R3并联,并联电阻大小
根据闭合电路欧姆定律,有
理想电流表读数为
(3)依题意,该等效电源的电动势,
短路电流,所以其等效内阻
(4)小灯泡的电阻
将小灯泡连在C、D之间,相当于接在等效电源E′两端,则流过小灯泡的电流大小为
小灯泡的实际功率为
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