问题 选择题

如图所示,电源电压保持不变.滑动变阻器R0的滑片P在移动过程中,电压表的示数变化范围是0~4伏,电流表的示数变化范围是0.5~1安,求:电源电压、电阻R的阻值和变阻器R0的最大阻值及电源电压(  )

 

A.8Ω 4Ω 8V

B.4Ω 8Ω 8V

C.8Ω 8Ω 4V

D.8Ω 8Ω 8V

答案

答案:D

已知:U1=0V,I1=1A;U2=4V,I2=0.5A;

求:R,R0,U

解:由图可知,滑动变阻器R0与定值电阻R串联,电压表并联在滑动变阻器两端,

当电流表示数是0.5A时,滑动变阻器两端的电压是4V,则电源电压U=0.5A×R+4V-----①,

当电流表示数是1A时,滑动变阻器两端的电压是0V,则电源电压U=1A×R------------②;

由①②可得:R=8Ω;U=8V;

当滑片P在a端时,滑动变阻器接入电路的阻值最大,并且滑动变阻器两端电压及通过的电流为U2=4V,I2=0.5A;

滑动变阻器的最大阻值为R0=U2/I2=4V/0.5A=8Ω.

单项选择题

When Dave was eighteen, he bought a secondhand car for 200 so that he could travel to and from work more (1) than by bus. It worked quite well for a few years, but then it got so old, and it was costing him (2) much in repairs that he decided that he had better (3) it.

He asked among his friends to see if anyone was particularly (4) to buy a cheap car, but they all knew that it was falling to pieces, so (5) of them had any desire to buy it. Dave’s friend Sam saw that he was (6) when they met one evening, and said, "What’s (7) , Dave"

Dave told him, and Sam answered, "Well, what about advertising it in the paper You may (8) more for it that way than the cost of the advertisement!" Thinking that Sam’s (9) was sensible, he put an advertisement in an evening paper, which read "For sale: small car, (10) very little petrol, only two owners. Bargain at 50. " For two days after the advertisement first appeared, there was no (11) . But then on Saturday evening he had an enquiry. A man rang up and said he would like to (12) him about the car. "All right, " Dave said, feeling happy. He asked the man whether ten o’clock the next morning would be (13) or not. "Fine, " the man said, "and I’ll (14) my wife. We intend to go for a ride in it to (15) it. "

The next morning, at a quarter to ten, Dave parked the car in the square outside his front door, (16) to wait there for the people who had (17) his advertisement. Even Dave had to (18) that the car really looked like a wreck. Then, soon after he had got the car as clean (19) it could be, a police car stopped just behind him and a policeman got out. He looked at Dave’s car and then said, "Have you reported this (20) to us yet, sir\

11().

A.doubt 

B.help 

C.trouble 

D.answer

填空题