问题 选择题

在如图所示的电路中,定值电阻,滑动变阻器的最大阻值为40,电源两端的电压恒为20V。闭合开关S,滑动变阻器的滑片P从端移到端的过程中,定值电阻消耗的电功率的变化范围是

A.0~10W

B.2.5W~10W

C.5W~10W

D.10W~20W

答案

答案:B

:(1)滑片在a点时,I="U" Ra+R ="20" 40+40 A=0.25A,

定值电阻消耗的电功率P1=I2R=0.252×40W=2.5W.

(2)滑片在b点时,P2="U2" R ="202" 40 W=10W.

所以定值电阻消耗的电功率的变化范围是2.5W--10W.

故选B.

完形填空

As she waited at the edge of the ice for her music to start, Peggy took a quick look at her father standing nearby with a group of parents and teachers. He smiled at her. Then she ___1___ out at the audience, ___2___ to see her mother. These two, Alvert and Doris Fleming, had ___3___ all the way from California more than 2,000 miles away, to see their ___4___ compete in this sports meet in Cleveland, Ohio.

The music ___5___ and Peggy moved onto the ice, letting the music ___6___ her along into her turns, and she began skating with much ___7___ in herself. The cold fear she always had in the ___8___ seconds before skating onto the ice was ___9___. She was feeling the movement of the ___10___ and letting it carry her. She skated easily, ___11___ did some jumps, a final turn and her performance was ___12___.

The crowd loved it and cheered ___13___ she skated off the ice. “Nice job,” said one of the other ___14___. It was the remark that ___15___ came after a free-skating performance. But what would the ___16___ say? Standing beside her father, Peggy ___17___ for the scoring to be finished. On all sides were other young skaters, some waiting ___18___ alone, others with a parent. Shortly before 10 o’clock the results were ___19___. The new United States Women’s Skating Champion was Peggy forming of Padena, ___20___!

1. A. looked    B. watched     C. found  D. stepped

2. A. tried       B. hoped       C. hoping       D. looking forward

3. A. flown     B. driven C. hoped D. prepared

4. A. friend     B. children     C. son     D. daughter

5. A. started    B. played C. develop      D. sang

6. A. allow     B. set out C. carry   D. support

7. A. thought  B. belief  C. success       D. design

8. A. last B. following   C. recent D. past

9. A. lost B. present       C. string  D. gone

10. A. ice       B. fear    C. music  D. audience

11. A. so  B. or       C. before D. then

12. A. satisfied       B. finished      C. unsatisfactory    D. welcome

13. A. because B. until   C. as     D. before

14. A. skaters  B. parents       C. judges D. audiences

15. A. seldom B. always       C. again  D. hardly

16. A. players B. audience     C. parents       D. judges

17. A. looked  B. wished       C. waited D. asked

18. A. comfortably           B. anxiously       C. hurriedly    D. happily

19. A. cried out      B. let out C. declared     D. announced

20. A. California    B. Cleveland   C. Ohio   D. England

选择题