(18分)如图,空间存在匀强电场和匀强磁场,电场方向为y轴正方向,磁场方向垂直于

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问题:

(18分)如图,空间存在匀强电场和匀强磁场,电场方向为y轴正方向,磁场方向垂直于xy平面(纸面)向外,电场和磁场都可以随意加上或撤除,重新加上的电场或磁场与撤除前的一样.一带正电荷的粒子从P(x=0,y=h)点以一定的速度平行于x轴正向入射.这时若只有磁场,粒子将做半径为R0的圆周运动;若同时存在电场和磁场,粒子恰好做直线运动.现在,只加电场,当粒子从P点运动到x=R0平面(图中虚线所示)时,立即撤除电场同时加上磁场,粒子继续运动,其轨迹与x轴交于M点.不计重力.求:

⑴粒子到达x=R0平面时速度方向与x轴的夹角以及粒子到x轴的距离;

⑵M点的横坐标xM.

考点:带电粒子在复合场中的运动
题型:计算题

无差异曲线的特点有( )。

A.投资者的无差异曲线形成密布整个平面又互不相交的曲线簇

B.是由左至右向上弯曲的曲线

C.无差异曲线的位置越高,其上的投资组合带来的满意程度就越高

D.落在同一条无差异曲线上的组合有相同的满意程度

题型:计算题
16
的相反数是(  )
A.4B.-4C.±4D.-16
题型:计算题

以下属于EVDO反向信道的是()。

A.导频信道

B.控制信道

C.MAC信道

D.同步信道

E.ACK子信道

F.数据信道

题型:计算题

If English means endless new words,difficult gramrnar and sometimes strange pronunciation,you are wrong.Haven’t you noticed that you have become smarter since you started to learn a language?

According to a new study by a British university,learning a second language can lead to an increase in your brain power,Researchers found that learning other languages changes grey matter.This is the area of the brain which processes information.It is similar to the way that exercise builds muscles.

The study also found the effect is greater,the younger people learn a second language.

A team led by Dr Andrea Mechelli,from University College London,took a group of Britons who only spoke English.They were compared with a goup of “early bilinguals.’’who had learnt a second language before the age of five,as well as a number of later learners.

Scans showed that grey matter density(密度)in the brain was greater in bilinguals than in people without a second language.But the longer a person waited before mastering a new language,the smaller the difference.

“Our findings suggest that the structure of the brain is changed by the experience of learning a second language,”said the scientists.

It means that the change itself increases the ability to learn.

Professor Dylan Vaughan Jones of the University of Wales has researched the link between bilingualism and maths skills. ‘‘Having two languages gives you two windows on the world and makes the brain more flexible,”he said.“You are actually going beyond language and have a better understanding of different ideas.”

The findings were matched in a study of native Italian speakers who had learned English as a second language between the ages of 2 and 34. Reading,writing,and comprehension were all tested.The results showed,that the younger they started to learn,the better.“Studying a language means you get an entrance to another world,’’explained the scientists.

小题1:The main subject talked about in this passage is         

A.language can help brain power

B.man's ability of learning a second language

C.science on learning a second language

D.language learning and maths study小题2:In the second paragraph,the writer mentions “exercise” in order to     

A.say language is also a kind of physical labor

B.make people believe language learning helps grey matter work well

C.show the importance of using the language when you learn the language

D.prove that one needs more practice when he(she)is learning a language小题3:The underlined word‘‘bilinguals”in Paragraph 4 probably means       

A.people who can speak two languages

B.second language learners

C.researchers on language learning

D.active language learners小题4:We may know from the scientific findings that        

A.the experience of learning a second language has bad effect on people’s brain

B.there is no difference between a later second language learner and one who doesn’t know a second language

C.the earlier you start to learn a second language,the higher the grey matter density is

D.the ability of learning a second language is changing all the time小题5:In the last two paragraphs,the author wants to tell us that      

A.learning a second language is the same as studying maths

B.Italian is the best choice for you as a second language

C.you’d better choose the ages between 2 and 34 to learn a second language

D.early learning of a second language helps you a great deal in studying other subjects

题型:计算题

下列反映肝细胞损害最敏感的检查指标是

A.AFP
B.ALT
C.AST
D.γ-GT
E.ALP

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