问题 计算题

如图是某研究性学习小组自制的电子秤原理图,它利用电压表的示数来指示物体的质量。托盘、弹簧上端和滑动变阻器滑片固定在一起,托盘和弹簧的质量不计,OA间有可收缩的导线,当盘中没有放物体时,电压表的示数为0。已知电阻R0=5Ω,滑动变阻器最大阻值为15Ω,电源电压U=3V,电压表的量程为0~3V。现将1kg的物体放在托盘中,滑片刚好指在距R上端l/3处(不计摩擦,弹簧始终在弹性限度内),请计算回答:

(1)将1kg的物体放在托盘中时,电压表的示数为多少?

(2)该电子秤能测量的最大质量是多少?此质量数应标在电压表多少伏的位置上?

答案

解:(1)当滑片位于R上端l/3处时,R=R0+ R/3 = 5Ω+15Ω/3 =10Ω,此时电路中的电流I=U/R =3V/10Ω=0.3A,电压表的示数为UR=IR=0.3A × 5Ω =1.5V。

       (2)因为弹簧的伸长(或压缩)与弹簧所受拉力(或压力)成正比,又1kg物体放在托盘中时,滑片指在1/3处,故滑片指到R最下端时,就是该电子秤所能测量的最大质量,应为3kg, 此时R'= R0 + R =5 Ω+15 Ω = 20Ω 电路中的电流I'=U/R'=3V/20Ω =0.15A,电压表的示数为U'=I'R=0.15A × 15Ω= 2.25V,故电子秤所能测量的最大质量数3kg,应标在电压表示数的2.25V处。

阅读理解

Over the last six years I have learned what patience is. Growing up I did not have this virtue(优点)and it is a very important virtue to have. Now I can see that if you are patient you will almost always get what you want if you are supposed to have it. I gained patience when I lost my freedom. I knew that I would eventually get it back in time.

I was locked up in prison for a period of six years because at 19 years old I was playing with a handgun and I accidentally shot and killed my friend. The first couple of years was the hardest.I was always stressed out about everything.Time was dragging by because I was always paying attention to it.While I was locked up,Tom who I talked to helped put this virtue in me.He told me that it was possible that he would never make it home but that being patient and believing that one day he would was what had made him feel better.After a while I realized why he said a lot of things to me.Once I stopped paying attention to the days and just kept in mind that I would get freedom,it seemed that my time passed quickly.

Now I am home and can see that patience paid off for me. I now use this virtue in my daily activities. I know that sometimes life gets hard and that it will pay in the end to stay patient and not to get discouraged. So if you are having a hard time in life,be patient and it will eventually work out one way or another.

小题1:We can learn from the passage that the writer ______.

A.got patience in prison

B.always wants to know what patience is

C.began to teach others how to be patient

D.is a successful man now through failure小题2:The writer was put in prison because he ______.

A.sold handguns

B.stole a handgun

C.played handguns with friends

D.killed his friend accidentally with a handgun小题3:During the first few years when he was in prison the writer ______.

A.made a couple of good friends

B.thought time passed too slowly

C.felt his life was full of unexpected things

D.was very nervous and frightened小题4:In the writer's opinion people will get a good result at last if they ______.

A.try their best to help others

B.turn to others for help

C.keep patient and don't lose heart

D.set different goals and keep them up

填空题