问题 选择题
如图所示,两根相距为l的平行直导轨ab、cd、b、d间连有一固定电阻R,导轨电阻可忽略不计;MN为放在ab和cd上的一导体杆,与ab垂直,其电阻也为R;整个装置处于匀强磁场中,磁感应强度的大小为B,磁场方向垂直于导轨所在平面(指向图中纸面内);现对MN施力使它沿导轨方向以速度v(如图)做匀速运动;令U表示MN两端电压的大小,则(  )

A.U=vBl,流过固定电阻R的感应电流由b到d

B.U=vBl,流过固定电阻R的感应电流由d到b

C.U=vBl,流过固定电阻R的感应电流由b到d

D.U=vBl,流过固定电阻R的感应电流由d到b

答案

答案:A

单项选择题

The most frightening words in the English language are, "Our computer is down. " You hear it more and more when you are on business. The other day I was at the airport waiting for a ticket to Washington and the girl in the ticket office said, "I’m sorry, I can’t sell you a ticket. Our computer is down. "

"If your computer is down, just write me out a ticket. "

"I can’t write you out a ticket. The computer is the only one allowed to do so. "

I looked down on the computer and every passenger was just standing there drinking coffee and staring at the black screen. Then I asked her, "What do all you people do"

"We give the computer the information about your trip, and then it tells us whether you can fly with us or not. "

"So when it goes down, you go down with it. "

"That’s good, sir. "

"How long will the computer be down" I wanted to know.

"I have no idea. Sometimes it’s down for 10 minutes, sometimes for two hours. There’s no way we can find out without asking the computer, and since it’s down it won’t answer us. "

After the girl told me they had no backup computer, I said. "Let’s forget the computer. What about your planes They’re still flying, aren’t they"

"I couldn’t tell without asking the computer. "

"Maybe I could just go to the gate and ask the pilot if he’s flying to Washington, " I suggested.

"I wouldn’t know what gate to send you to. Even if the pilot was going to Washington, he couldn’t take you if you didn’t have a ticket. "

"Is there any other airline flying to Washington within the next few hours"

"I wouldn’t know, " she said, pointing at the dark screen. "Only ’IT’ knows. ’It’ can’t tell me. "

By this time there were quite a few people standing in lines. The word soon spread to other travelers that the computer was down. Some people went white, some people started to cry and still others kicked their luggage.

The best title for the article is ()

A. When the Computer Is Down

B. The Most Frightening Words

C. The Computer of the Airport

D. Asking the Computer

判断题