实际的电源都有一定的电阻，如干电池，我们需要用它的电压U 和

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问题计算题

实际的电源都有一定的电阻，如干电池，我们需要用它的电压U 和电阻r两个物理量来描述它。实际计算过程中，可以把它看成是由一个电压为U、电阻为0的理想电源与一个电阻值为r的电阻串联而成，如图甲所示：

在图乙中R₁= 14Ω，R₂= 9Ω。当只闭合S₁时，电流表读数I₁=0.2A ；当只闭合S₂时，电流表读数I₂=0.3A，把电源按图甲中的等效方法处理。求电源的电压U 和电阻r。

答案

解：

由题意可得：

U=I₁（R₁+r）=I₂（R₂+r）

代入数据得：0.2A×（14Ω+r）=0.3A×（9Ω+r）

解得：r=1Ω

将r=1Ω代入U=I₁（R₁+r）

可得：U=3V。

阅读理解

One day, a man found the cocoon (茧) of a butterfly and he brought it home.

A few days later, the man saw a small hole in the cocoon. He sat and watched for several hours as a butterfly struggled to force its body through that little hole. Then it stopped.

So the man decided to help the butterfly. He took a knife and cut a bigger hole in the cocoon. The butterfly came out of the cocoon but it had a weak body and small, thin wings. The butterfly didn’t start to fly.

In fact, the butterfly spent the rest of its life crawling(爬行) around with a weak body and thin wings. It was never able to fly.

The man acted with kindness but he didn’t understand that he had done something wrong. When a butterfly crawls out of the cocoon, it must struggle. The hard work of getting out of the cocoon forces the fluid(液体) from the butterfly’s body into wings. It helps the butterfly be ready to fly. If the butterfly never has to force itself out of the cocoon, its wings never get the fluid and it can never fly.

Sometimes struggles are exactly what we need in our lives. If we lived our lives without any problems, we would never learn or grow. We would not be as strong as we could have been and we would never fly.

So, the next time you are faced with a challenge or a problem, remember the butterfly.

Struggle a little – then fly!

小题1:Why did the man cut a bigger hole in the cocoon?

A．To catch the butterfly easily.

B．To play with the butterfly.

C．To help the butterfly come out.

D．To make the butterfly stronger.小题2:What happened after the butterfly got out of the cocoon?

A．It died soon.

B．It had a strong body and wings.

C．It could fly after a few days.

D．It couldn’t fly during the rest of its life.小题3:Why couldn’t the butterfly fly? Because ___________ .

A．it got out of its cocoon without struggle