问题 问答题 简答题

SF6气体气体绝缘强度高的原因?

答案

参考答案:

(1)卤族元素中氟的电负性最强,SF6分子具有很强的电负性,容易吸附电子形成负离子,阻碍放电的形成和发展。

(2)SF6分子的直径比氧、氮分子的要大,使得电子在SF6气体中的平均自由行程缩短,不易积累能量,电离电位又比氧、氮分子的大,因此减小了电子碰撞电离的可能性

(3)电子与SF6气体分子相遇时,还会因极化等过程增加能量损失,减弱其碰撞电离能力。

阅读理解

A bag is useful and the word “bag” is useful, too. Here is an interesting phrase(短语), “to let the cat out of the bag”. In meaning it is the same as “to tell the secret(秘密)”. And there is an old interesting story about it.

  Long ago, when farmers wanted to sell chickens at the market, they would usually just throw the chickens in cloth (布) bags. But cats were cheaper than chickens at that time, so many farmers would put cats into the cloth bags but not chickens. One day, a woman asked a man for a chicken. The man gave her a cloth bag in which there was a cat.      When the man opened the cloth bag, a big black cat ran out. Not a chicken! The man’s secret was out and everyone knew it.

  Now when we say someone gives away a secret, we say “he lets the cat out of the bag”. And that is the story where the interesting phrase came from.

小题1:The underlined word “it” in the passage refers to (指的是)     .

A.the useful bag

B.the phrase

C.the cat

D.the chicken小题2:The farmers put cats but not chickens in the bags to      when they sold chickens.

A.tell the secret

B.sell the cats

C.get more money

D.keep the cats quiet小题3:Which is the best for the blank (空白处) in the passage?

A.The woman was angry to see a cat.

B.The man put a cat in the bag.

C.The man asked her to see the chicken.

D.The woman wanted to see the chicken.小题4:The writer wrote the story to tell us     .

A.the woman was smart

B.how farmers sold their cats

C.the phrase is useful

D.where the phrase came from

单项选择题