问题 问答题 简答题

晶体的对称性的特征

答案

参考答案:

①微观对称:由于晶体内部都具有格子构造,而格子构造本身就是质点在三维空间周期重复的体现。因此,所有的晶体都具有晶体内部结构的对称,即微观的对称。

②晶体的对称受格子构造性质的限制:也就是说只有符合格子构造特征的对称才能在晶体上体现。因此,晶体的对称是有限的,它遵循“晶体对称定律”。

③晶体的对称不仅体现内部结构和几何外形上,同时也体现在物理性质(如光学、力学、热血、电学性质等)上,也就是说晶体的对称不仅包含着几何意义,也包含着物理意义。

阅读理解

阅读理解

     Sun Mei was 14 when she began working iri a small restaurant last year. Usually she works for 13 to 15 hours a day.

She welcomes people, serves tea, washes dishes and cleans the floor. She seldom has a day off. At the end of the day,

"I feel so tired," said Sun, "My family is poor. There is no money for me to go to school. "

     There are 250 million children in the world who are working in hard, sometimes dangerous conditions. India has the

biggest number of child workers in the world 44 million.

     Children do all kinds of work. Most of them work in the fields. They pick vegetables, clear grass and move rocks.

Some work in factories making clothes, glass, matches and many other things. Others sell things in the markets.

     Why do children work? Many children work to make money for their families to live on. Another reason is that

schools are too far away or there are no schools in the area at all.

     In China, there is a law called "Prohibition of Child Labour" 《<禁止使用童工规定 》). It says any company

mustn't hire children under 16 and anyone who introduces a child to work will be fined 5, 000 yuan.

1. What does "she seldom has a day off" mean?

A. Shc couldn't finish her work.

B. She never has a rest.

C. She hardly has a free day.

D. She seldom gets full pay.

2. Which country has the most child workers in the world?

A. China.

B. India.

C. Japan.

D. Korea.

3. According to the writer, why do children work?

A. Because they want to.

B. Because there're no places to study in.

C. Because they hate going to school.

D. Because they want to pay less money.

4. Which of the following things do child workers often not do?

A. Pick vegetables and clear grass.

B. Make clothes and matches.

C. Go to school.

D. Move rocks.

5. Which can be the best title for the passage?

A. Too Young to Work.

B. Sun Mei's Hard Life.

C. Children in China.

D. Child Workers in the World.

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