问题 填空题

随着生活水平的提高,汽车已经进入百姓家庭,它给我们的生活提供了便利,促进了社会的发展。汽车应用了许多物理知识,请你解答

(1)汽油机是汽车的“心脏”。一台四冲程汽油机工作时曲轴转速是1800r/min,此汽油机每秒钟完成  个做功冲程,其中  冲程将机械能转化为内能;

(2)当汽油在发动机内燃烧不充分时会冒“黑烟”,这时发动机的效率将变

(3)汽车发动机在工作时:发动机的温度会升得很高,通常在发动机外装有水套,汽车发动机用水作为冷却剂的主要原因是                

答案

15  压缩  低   水的比热容大吸热升温慢(且沸点低)

题目分析::(1)已知四冲程汽油机曲轴转速是1800 r/min,所以此汽油机每秒钟转30圈,可完成15个工作循环,60个冲程,对外做15次功;在压缩冲程中活塞向上运动,压缩燃料混合物对其做功,所以将机械能转化为内能;

(2)汽油在发动机内燃烧不充分时消耗能量将会增加,故机械效率降低;

(3)因为水这种物质的比热容较大,在吸收相同的热量时,相同质量升高的温度相对较小,常用来作为冷却剂使用;

点评:此类题目考查知识了热机、内燃机的四个冲程、机车功率和比热容的相关问题,要结合相关知识点进行分析解答,是中考的难点.

单项选择题
单项选择题

A good modern newspaper is an extraordinary piece of reading. It is remarkable first for what it contains the range of news from local crime to international politics, from sports to business to fashion to science, and the range of comment and special features (特定) as well, from editorial page to feature articles and interviews to criticism of books, art, theatre and music. A newspaper is even more remarkable for the way one reads it never completely, never straight through, but always by jumping from here to there, in and out glancing at one piece, reading another article all the way through, reading just a few paragraphs of the next.
A good modern newspaper offers a variety to attract many different readers, but far more than any one reader is interested in. What brings this variety together in one place is its topicality (时事性), its immediate relation to what is happening in your world and your locality now. But immediacy and the speed of production that goes with it mean also that much of what appears in a newspaper has no more than transient (短暂的) value.
For all these reasons, no two people really read the same paper: what each person does is to put together out of the pages of that day’s paper, his own selection and sequence, his own news paper. For all these reasons, reading newspapers efficiently, which means getting what you want from them without missing things you need but without wasting time, demands skill and self-awareness as you modify and apply the techniques of reading.

A good newspaper offers a variety to readers because ______.

A. it tries to serve different readers
B. it has to cover things that happen in a certain locality
C. readers are difficult to please
D. readers like to read different newspapers