题目

一台交流发电机额定输出功率P=4.0×103 kW，以400 V电压接在升压变压器上向远方输电.若输电导线的总电阻为10 Ω，允许输电线损失的功率为输出功率的10%，问:（1）升压变压器的变压比多大？（2）为了使远方用电器能得到220 V的工作电压，降压变压器的变压比多大？ 答案：解析：（1）P损=I线2R线，所以I线= A=200 A,I0= A=104 A,所以=50.（2）U2=U1=50×400 V=2×104 V,U线=I线R线=200×10 V=2 000 V，所以降压变压器的输入电压U1′=U2-U线=18 000 V，所以.答案：(1)50  (2)阅读下面短文，从短文后各题所给的四个选项（A、B、C、D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。 Travis laughed as he tore at the wrapping (包装) paper on his birthday present. He was so       ! Finally, he would have the coolest pair of name-brand basketball shoes. All the guys on his team were wearing the name-brand shoes of a popular basketball      , Chuck Hart.     Hart was criticized for his poor sportsmanship and infamous        he was a great player. In fact, Travis wasn’t thinking about Hart’s behavior; he had only expected to see Hart’s       on the side of the box. He realized that something was     as he tore away the last piece of paper. Not Hart’s. The new shoes were the name-brand of another player, Robert Ryann, who was        for his amazing work in the community.Travis’s hands     ; his heart stopped. It wasn’t that the Ryann shoes weren’t nice, but what would his friends think?They were the wrong shoes and Travis would be     by the other players. When he looked up into his dad’s eyes, however, Travis knew he     tell him. “Thanks, Dad. I was really hoping for shoes,” Travis said as he pulled the shoes out of the box.Next morning his dad drove him to school. When they      in front of his destination, Travis slowly opened the car door. Just then, his dad stopped him.“Hey, Travis, wait a minute…” his dad said     “Travis, I know those aren’t the shoes you had hoped for, but I saw the names of the two guys and made a(n)      . The guy whose name is on those shoes,” he said, pointing down at Travis’s feet, “is someone I     . Do you know how often Ryann has found himself in     ? ““ No,” Travis said.“ Never. He’s never talked back to his coach or started a fight, and he’s a team player. You could have acted like a(n)      when you didn’t get the shoes you wanted, Travis,     you were polite and made the best of it. You have honor, like the guy whose name is on these shoes. I am hoping that someday, your       will be on the coolest pair of shoes I’ll ever see.”When Travis looked down at his feet, he saw the shoes     . His dad had used his mind and heart to give the son a thoughtful      .【小题1】 A．surprised B．ashamed C．excited D．worried  【小题2】 A．team B．player C．coach D．game  【小题3】A Unless             B. If                 C. Because         D. Although【小题4】 A．skill B．performance C．behavior D．action  【小题5】 A．name B．photo C．sign D．model  【小题6】 A．strange B．wrong C．true D．funny  【小题7】 A．known B．encouraged C．adopted D．influenced  【小题8】 A．fell B．froze C．shook　 D．folded  【小题9】 A．questioned B．noticed C．teased　 D．attacked  【小题10】 A．mustn’t B．needn’t C．wouldn’t D．couldn’t  【小题11】 A．pulled up B．put up C．took up D．turned up  【小题12】 A．peacefully B．hesitantly C．delightedly D．naturally  【小题13】 A．choice B．effort C．comment D．mistake  【小题14】 A．believe B．miss C．admire D．remember  【小题15】 A．danger B．anger C．sorrow D．trouble  【小题16】 A．teammate B．adult C．kid D．student  【小题17】 A．so B．but C．and D．or  【小题18】 A．honor B．courage C．name D．belief  【小题19】 A．clearly B．carefully C．patiently D．differently  【小题20】 A．gift B．smile C．wish D．lesson