问题 选择题

电炉通电后,电炉丝热得发红,而跟电炉连接的铜导线却不那么热,这是因为     ( )

A.通过铜导线的电流小,所以它消耗的电能也较少

B.电炉丝和铜导线消耗的电能相同,但铜导线散热快,所以就不那么热

C.通过电炉丝的电流大,所以它消耗的电能较多

D.铜导线电阻较小,所以它消耗的电能较少

答案

答案:D

题目分析:电炉在使用时,电炉丝和导线串联,I电炉丝=I导线,通电时间t相同,

∵Q=I2Rt,R电炉丝>R导线

∴电流产生的热量:

Q电炉丝>Q导线

从而出现电炉丝热得发红,而与电炉丝相连的导线却不怎么发热的现象;

由上述分析可知选项D正确.

所以选D.

点评:由焦耳定律知道,电流通过导体产生的热量跟电流的平方、导体电阻大小和通电时间成正比.电炉丝和连接的导线串联在电路中(通过的电流相等),通电时间是相同的,而电炉丝的电阻比导线的电阻大,据焦耳定律分析判断.

阅读理解

At midnight Mr. King coughed again. He got up and took some medicine. Before he lay down again, he smoked. And he couldn’t go to sleep any longer. He found the medicine book and began to read it. He was afraid he had lung cancer. He seemed to see death was waiting for him. Tears(眼泪) ran down his face. He didn’t see his wife was standing by him.

“What’s wrong with you, dear?” asked the woman.

“Nothing,” the old man answered and covered his face with his hands.

The old woman opened the book and understood at once. She said, “You must stop smoking right now.”

“I began to smoke when I was nine, you know. How can’t I do that?”

“But I think health is more important.”

Mr. King coughed again and his wife said, “Go to see a doctor tomorrow. He will be able to help you, I think.”

The doctor told Mr. King to have an Xray examination of his lungs. And then he looked at the Xray carefully for a long time.

“Tell me the truth, doctor.”said Mr.King, “Do you see any shadows(阴影)in my lungs?”

“No, I don’t see anything.”

“Yes?” the old man said happily. “Really?”

“Yes. Your lungs are turning black. How can I see any shadows?”

(   )6. Mr.King got up to      .

A. smoke again

B. talk with his wife

C. read the medicine book

D. take some medicine

(   )7. Mr. King coughed because      .

A. he couldn’t go to sleep

B. he was afraid to die

C. he smoked too much

D. he was too old

(   )8.    , so tears ran down his face.

A. Mr. King thought he had lung cancer

B. Mr. King couldn’t understand the book

C. Mr. King was too sad to go to sleep

D. The doctors refused to help Mr. King

(   ) 9.The doctor told Mr. King to have an Xray examination because      .

A. he was sure the man would die soon

B. he wanted the man to stop smoking right now

C. he wanted to know if there was something wrong with his lungs

D. he was sure the man would soon be all right

(   )10. Which of the following is true?

A. There was nothing serious with Mr. King.

B. Smoking too much caused Mr. King’s lungs to turn black.

C. The doctor found some shadows in Mr. King’s lungs.

D. Mr. King decided to stop smoking at once.

单项选择题