问题 问答题

如图所示,质量为m,电荷量为q的带电粒子,以初速度v沿垂直磁场方向射入磁感应强度为B的匀强磁场,在磁场中做匀速圆周运动。不计带电粒子所受重力。

(1)求粒子做匀速圆周运动的半径R和周期T;

(2)为使该粒子做匀速直线运动,还需要同时存在一个与磁场方向垂直的匀强电场,求电场强度E的大小。

答案

参考答案:

(1)洛伦兹力提供向心力,有

带电粒子做匀速圆周运动的半径

匀速圆周运动的周期

(2)粒子受电场力F=qE,洛伦兹力f=qvB。粒子做匀速直线运动,则场强E的大小

qE=qvB

E=vB

单项选择题
阅读理解

Removing the sleeve(封套), you will find a book that is entirely white, except for the names of its author and subject in elegant black type on the cover. It is the perfect design for the biography of a man who insisted that even the insides of his products be perfectly constructed, and that his factory wails flash in the whitest white.

The cover was the only part of the book Steve Jobs wanted to control, writes Isaacson in his introduction. Though Mr. Jobs pushed the biographer of Albert Einstein and Benjamin Franklin to write in his own way, generously allowing the writer more than 40 interviews, this book offers quite a different view of Mr. Jobs, who won much praise from his fans after his death on October 5th at the age of 56.

As a biographer of Albert Einstein and Benjamin Franklin, Mr. Isaacson knows how to celebrate long-dead genius, but he claimed that "Steve Jobs" would not be entirely praiseful words. The picture he paints, particularly in the first half of this book, is not nice. Mr. Jobs emerges as a controlling and often cold-blooded character. A child of the 1960s counter-culture (反主流文化), he hated, materialism and lived in simply furnished houses ( in part because he was too particular about furniture). But when Apple went public in 1980, he refused to give any share to Daniel Kottke, a Iongtime supporter and soul mate from college. "He has to abandon the people he is close to," observes Andy Hertzfeld, an early Apple engineer.

Mr. Jobs was undoubtedly possessing an extraordinary ability to attract others and inspire a kind of faith that could not be questioned. But also he could be cold and cruel. If he disapproved of an employee's work, he often shamed him. "This is who I am," he once said after being challenged,"and you can't expect me to be someone I'm not. " This disgusting personality wasn't always helpful,but it served a purpose, writes Mr. Isaacson, many would "end their chain of horror stories by saying that he got them to do things they never dreamed possible. "

Mr. Isaacson treats "Steve Jobs" as the biography of record, which means that it is a strange book to read so soon after its subject's death.   

小题1:The biography for Jobs is believed to have the perfect design because __

A.it follows Jobs' style

B.its cover is entirely white

C.black and white are his favorite .

D.it is designed by a famous biographer小题2:The picture of Jobs that Isaacson paints in his book is __

①cruel    ②humorous   ③particular  ④generous

A.①③

B.②③

C.①④

D.③④小题3:It can be safely concluded that        

A.Jobs is highly spoken of in the book

B.Isaacson doesn't think Jobs a good man

C.Jobs didn't care about the design of the book

D.all descriptions of Jobs are not nice in the book小题4:This passage can be classified as          

A.a personal diary

B.a book review

C.a news report

D.a TV interview