问题 计算题

水平放置的两根间距d=0.1m的平行光滑直导轨,左端接有电阻R=9Ω,以及电键S和电压表.大小为B=5T的匀强磁场垂直导轨平面向下,如图所示。垂直导轨搁置一根电阻r=1Ω的金属棒ab,棒与导轨良好接触.现使金属棒以速度v=10m/s匀速向右移动,试求:

(1)电键S闭合前、后电压表的示数;

(2)闭合电键S,外力移动棒的机械功率.

答案

(1)5V、4.5V    (2)2.5W

题目分析:(1)金属棒切割磁感线产生的电动势为

这就是电键闭合前电压表的示数

由闭合电路欧姆定律可知电键闭合后电压表的示数为

(2)S闭合后外力的功率应等于电路中的电功率

点评:要区分清楚内外电压,掌握它们之间的联系.有时画等效电路帮助分析。

听力题

A. 根据句意和提示写出单词,完成句子。

小题1:—I can’t stand it any more. The computer is so slow.

—Perhaps you need to         (重启) it. 

小题2:Many of the Australians like to take some ________(户外) exercises.

小题3:I believe you will enjoy your ________(成功)if you don’t give up.  

小题4:His father promised to buy him a _____ (笔记本) but he didn’t keep it.

小题5:The story happened on a cold windy     (十一月)afternoon.

小题6:Thank you. Your explanation is so ___ (清楚) that I am sure I’ve completely got it.

小题7:Mr Zhang bought a car yesterday. It is the      (最新的) of all the cars in our school.

小题8:Mrs Wang came to see me          (不久)after Mr Zhang left my house.

小题9:The girl couldn’t wait to share her          (happy) with her family.

小题10:All the farmers in my country are enjoying a cheap    ▲     (medicine) care now thanks to the party’s great policy(政策).

小题11:He said he was terribly sorry for the few      (mistake) he had made the day before.

小题12:The king decided to build a big palace          (special) for his new daughter.

小题13:All of us must do the washing and cooking by          (we) so that we can live better in the future.

小题14:In his speech he said that all his Chinese friends are worth        (make) and he would come back to see them in a short time.

小题15:Xiangshui Senior High School will celebrate its        (sixty) birthday this coming October.

单项选择题