问题 计算题

在题图1所示的装置中,粒子源A产生的初速为零、比荷为的正离子沿轴线进入一系列共轴且长度依次增加的金属圆筒,奇数和偶数筒分别连接在周期为T、最大值为U0的矩形波电源两端,电源波形如图2示,离子在每个圆筒内做匀速直线运动的时间等于交变电源的半个周期,在相邻两筒之间受电场力作用被加速(加速时间不计)。离子离开最后一个圆筒后垂直于边OE进入磁感应强度为B的匀强磁场,最后从OF边出射。(不计离子所受重力)

(1)求离子在第一个金属筒内的速率。

(2)求离子在第n个筒内的速率及第n个筒的长度。

(3)若有N个金属筒,求离子在磁场中做圆周运动的半径。

(4)若比荷为的离子垂直于OF边出射,要使比荷为的离子也能垂直于OF边出射,求电源电压最大值的改变量以及磁感应强度的改变量。

答案

解:(1)设离子在第n个筒内的速度为

在每个筒内受到电场力做功为

根据动能定理

解得

(2)设第n个筒的长度为

(3)设粒子在磁场中做圆周运动的半径为R,有

(4)需要使离子加速后也获得的速度,设比荷为离子的加速电压为

的离子在磁场中做圆周运动的半径为,要使的离子在磁场中同样在S点射出,有

选择题
阅读理解

Mark Twain has been called the inventor of the American novel.And he surely deserves additional praise:the man who popularized the clever literary attack on racism.

I say clever because anti­slavery fiction had been the important part of the literature in the years before the Civil War.H.B.Stowe’s Uncle Tom’s Cabin is only the most famous example.These early stories dealt directly with slavery.With minor exceptions,Twain planted his attacks on slavery and prejudice into tales that were on the surface about something else entirely.He drew his readers into the argument by drawing them into the story.

Again and again,in the postwar years,Twain seemed forced to deal with the challenge of race.Consider the most controversial,at least today,of Twain’s novels,Adventures of Huckleberry Finn.Only a few books have been kicked off the shelves as often as Huckleberry Finn,Twain’s most widely read tale.Once upon a time,people hated the book because it struck them as rude.Twain himself wrote that those who banned the book considered the novel“trash and suitable only for the slums(贫民窟).”More recently the book has been attacked because of the character Jim,the escaped slave,and many occurrences of the word nigger.(The term Nigger Jim,for which the novel is often severely criticized,never appears in it.)

But the attacks were and are silly—and miss the point.The novel is strongly anti­slavery.Jim’s search through the slave states for the family from whom he has been forcibly parted is heroic.As J.Chadwick has pointed out,the character of Jim was a first in American fiction—a recognition that the slave had two personalities,“the voice of survival within a white slave culture and the voice of the individual:Jim,the father and the man.”

There is much more.Twain’s mystery novel Pudd’nhead Wilson stood as a challenge to the racial beliefs of even many of the liberals of his day.Written at a time when the accepted wisdom held Negroes to be inferior (低等的) to whites,especially in intelligence,Twain’s tale centered in part around two babies switched at birth.A slave gave birth to her master’s baby and,for fear that the child should be sold South,switched him for the master’s baby by his wife.The slave’s light­skinned child was taken to be white and grew up with both the attitudes and the education of the slave­holding class.The master’s wife’s baby was taken for black and grew up with the attitudes and intonations of the slave.

The point was difficult to miss:nurture (养育),not nature,was the key to social status.The features of the black man that provided the stuff of prejudice—manner of speech,for example—were,to Twain,indicative of nothing other than the conditioning that slavery forced on its victims.

Twain’s racial tone was not perfect.One is left uneasy,for example,by the lengthy passage in his autobiography (自传) about how much he loved what were called“nigger shows”in his youth—mostly with white men performing in black­face—and his delight in getting his mother to laugh at them.Yet there is no reason to think Twain saw the shows as representing reality.His frequent attacks on slavery and prejudice suggest his keen awareness that they did not.

Was Twain a racist? Asking the questioning the 21st century is as wise as asking the same of Lincoln.If we read the words and attitudes of the past through the“wisdom”of the considered moral judgments of the present,we will find nothing but error.Lincoln,who believed the black man the inferior of the white,fought and won a war to free him.And Twain,raised in a slave state,briefly a soldier,and inventor of Jim,may have done more to anger the nation over racial injustice and awaken its collective conscience than any other novelist in the past century.

小题1:How do Twain’s novels on slavery differ from Stowe’s?

A.Twain was more willing to deal with racism.

B.Twain’s attack on racism was much less open.

C.Twain’s themes seemed to agree with plots.

D.Twain was openly concerned with racism.小题2:Recent criticism of Adventures of Huckleberry Finn arose partly from its________.

A.target readers at the bottom

B.anti­slavery attitude

C.rather impolite language

D.frequent use of“nigger”小题3:What best proves Twain’s anti­slavery stand according to the author?

A.Jim’s search for his family was described in detail.

B.The slave’s voice was first heard in American novels.

C.Jim grew up into a man and a father in the white culture.

D.Twain suspected that the slaves were less intelligent.小题4:The story of two babies switched mainly indicates that________.

A.slaves were forced to give up their babies to their masters

B.slaves’ babies could pick up slave­holders’ way of speaking

C.blacks’social position was shaped by how they were brought up

D.blacks were born with certain features of prejudice小题5:What does the underlined word “they” in Paragraph 7 refer to?

A.The attacks.

B.Slavery and prejudice.

C.White men.

D.The shows.小题6:What does the author mainly argue for?

A.Twain had done more than his contemporary writers to attack racism.

B.Twain was an admirable figure comparable to Abraham Lincoln.

C.Twain’s works had been banned on unreasonable grounds.

D.Twain’s works should be read from a historical point of view.