问题 选择题

如图是检查酒驾的电路原理图,图中酒精传感器的电阻Rm与酒精气体的浓度成反比.测量时,下列判断中正确的是

A.当驾驶员吹出的气体酒精浓度升高时,电流表A的示数减小

B.当驾驶员吹出的气体酒精浓度升高时,电压表V1的示数减小

C.当驾驶员吹出的气体酒精浓度升高时,电压表V2的示数减小

D.电池长久使用导致电压降低时,会误认为驾驶员吹出气体的酒精浓度升高

答案

答案:B

等效电路图如下图所示:

(1)由题意可知,酒精浓度越大Rm的阻值越小,电路中的总电阻越小;

根据I="U/" R 可知,电路中的电流变大,即电流表的示数变大;

根据U=IR可知,R0两端的电压变大,即V2的示数变大;

根据串联电路的总电压等于各分电压之和可知,Rm两端的电压减小,即V1的示数变小.

(2)当电源电压较低时,电路的电阻不变,

由I="U" /R 可得,电路中的电流变小,两电压表的示数都变小,

因此误认为是驾驶员吹出气体的酒精浓度降低.

问答题 简答题
单项选择题

阅读下面短文,掌握其大意,然后从21—40各题所给的四个选项(A、B、C和D)中,选出最佳选项,并在答题纸上将该选项标号涂黑。

During the war,my husband was stationed at an army camp in a desert in California.I went to live there in order to be 21  him. I hated the place .I had never  22  been so unhappy . My husband was ordered out on a long-term duty,and I was left in a tiny shack(棚屋) alone. The heat was  23  -almost 125 0F even in the shade of a cactus(仙人掌)。 24  a soul to talk to . The wind blew non-stop ,and all the food I ate ,and the evry air I breathed,were  25   with sand , sand , sand !

I was so sorry for myself that I wrote to my parents. I told them I was  26   and coming back home.I said I couldn’t stand it one minute longer. I  27  be in prison! My father answered my  28  with just two lines-two lines that will always sing in my  29  - two lines that completely changed my life :

Two men looked out from prison bars

One saw the mud ,the other saw the stars

I read those two lines  30   I was ashamed of myself. I made up my mind I would find out what was good in my present  31  ; I would look for the stars

I made friends with the natives,and their  32  amazed me. They gave me presents of their favorite artworks which they had  33  to sell to toueists . I studied the delightful forms of the cactus .I watched for the desert sunsets,and  34  for seashells that had been left there millions of years ago when the desert had been an ocean  35 

What brought about this  36   change in me ? The desert hadn’t changed , 37   I had .I had changed my  38   And by doing so ,I changed an unhappy experience into the most amazing  39  of my life . I was excited by this new world that I had dicovered I had looked out of my self-creatded prison and  40   the stars

28()

A.request

B. call

C. question

D.letter