问题 填空题

如图所示电路,电源电压不变,R1、R2的阻值分别为10Ω和20Ω.闭合S1、断开S2时,电流表的示数为0.3A;当同时闭合S1、S2时,电压表的示数为______V,电路消耗的总功率为______W.

答案

闭合S1、断开S2时,为R1的基本电路;则电源电压:U=I1R1=0.3A×10Ω=3V;

当同时闭合S1、S2时,为R1、R2的并联电路,电压表示数等于电源电压,即3V;此时电路的总电阻:

1
R
=
1
R1
+
1
R2

1
R
=
1
10Ω
+
1
20Ω

R=

20
3
Ω

干路中的电流:I=

U
R
=
3V
20
3
Ω
=0.45A

电路消耗的总功率:P=UI=3V×0.45A=1.35W

故答案为 3,1.35

阅读理解

Jack went to a barber's shop and had his hair cut, but when he came out, he was not happy with the result(结果). When his friend Bob saw him, he laughed and said, "What has happened to your hair, Jack?"

Jack said, "I tried a new barber's shop today, because I wasn't quite satisfied with my old one, but this one seems even worse."

Bob agreed(同意). "Yes, I think you're right, Jack. Now I'll tell you what to do when you go into a barber's shop next time: look at all the barber's hair, find out whose hair looks worst, and then go straight to him."

"Why shall I go to him?" Jack asked. "But that would be foolish!"

"Oh, no, it wouldn't," answered Bob. "Who cut that man's hair? Just think it. He couldn't cut it himself, could he? Another of the barbers cut it. So you know he can't be the worst barber."

小题1:When Jack went out of the barber's shop, he was not happy because                      ____.

A.nobody had cut his hair

B.the barber hadn't cut his hair well

C.the barber had cut his hair carefully

D.he was not satisfied with his old barber小题2:"I tried a new barber's shop today." means ____.

A.this barber's shop was a new one

B.this was the only barber's shop in this town

C.Jack often went to this barber's shop

D.Jack had not been to this barber's shop before小题3:After Jack had his hair cut, he thought ____.

A.the new barber's shop was the best one

B.the old barber's shop wasn't so good as the new one

C.the new barber's shop was worse than the old one

D.the old barber's shop was the worst one小题4:Bob told Jack to find out which barber's hair looked the worst and then go straight to him. Why?

A.Because he was certainly the best barber.

B.Because he was free all the time.

C.Because he was the worst barber.

D.Because he wasn't the worst barber.小题5:From the story we know that ____.

A.it's foolish to have one's hair cut at a barber's shop

B.barbers cut each other's hair

C.barbers never have their hair cut

D.a barber always cuts his hair by himself

单项选择题