问题 多选题

如图所示电路中,电源电压保持不变,R1和R2是定值电阻,滑动变阻器的最大阻值是R3.当滑动变阻器的滑片P由一端向另一端移动时,电压表的示数在

1
2
U和
1
3
U之间变化,则(  )

A.R1与R2的阻值之比是1:2

B.R1与R2的阻值之比是1:1

C.R2与R3的阻值之比是1:1

D.R2与R3的电功率之比是2:3

答案

由图可知,当滑片P在左端时,滑动变阻器连入的电阻为0,电路中电流最大,R1两端的电压最大,所以当滑片P在左端时,电压表的示数为

1
2
U,

此时R1和R2串联,

∵U=U1+U2

∴U2=U1=

1
2
U,

∴R1=R2,即R1:R2=1:1,故A错、B正确;

当滑片P在右端时,三电阻串联,

∵电压表的示数为

1
3
U,即U1=
1
3
U,R1=R2

∴U2=

1
3
U,

∵U=U1+U2+U3

∴U3=

1
3
U,

∴R2=R3,即R2:R3=1:1,故C正确;

P2:P3=I2R2:I2R3=R2:R3=1:1,故D错.

故选BC.

阅读理解

    Mr. Brown lived in a house less than two miles from his office, so he was able to drive home every day for lunch. Every time he drove home at noon, he found many cars were parked outside his house and there was no room for his own car. He had to drive somewhere else to park his car. Then he had to walk back home. This made him very angry.

He had put up a board, which said, “ No parking” in the garden facing the road, but nobody noticed it. People seemed to obey(遵守) only a police notice with letters on a blue board:

POLICE NOTICE

NO PARKING

Mrs. Brown suggested that he could steal a police notice. He was afraid to do so. She then suggested that he could make one just like a police notice. He said he was not the police and couldn’t use the word “police”. Several days later, Mr. Brown made a blue board with white letters:

POLITE NOTICE

NO PARKING

“ Oh!” Mrs. Brown said, “You told me you weren’t going to use the word  “police”, but why do you use it now?” “Really?” he asked, “Look again.” She started to laugh. “You are really clever.”

小题1:How many notice boards did Mr. Brown make altogether?

A.Only one

B.Two

C.Three

D.None小题2:Mr. Brown made a notice board which ________________ in the end.

A.was just the same as a police notice

B.was different in colour from a police notice

C.looked just like a police notice

D.said “ POLICE NOTICE, NO PARKING”小题3:We can infer(推断) that ____________ after he put up the blue board.

A.more people will park their cars outside his house

B.more policemen will come to park their cars at noon

C.fewer people will visit him at noon later on

D.fewer people will park their cars outside his house

单项选择题