问题 选择题

如图所示,用三条细线悬挂的金属圆环,金属环粗细均匀、单位长度的质量为2.5g,三条细线呈对称分布,稳定时金属环面水平,在金属环正下方放有一个圆柱形磁铁,磁铁的中轴线OO′垂直于金属环面且通过其圆心O,测得金属环所在处磁感应强度大小为0.5T,与竖直方向成30°角.现在要使金属环各部分所受安培力的合力竖直向上且恰好等于其自身的重力,则在金属环中通过的电流大小至少为(  )(取g=10m/s2

A.0.01A

B.0.2A

C.0.05A

D.0.1A

答案

设圆环的半径为r,则圆环的质量m=2πrm,

磁场的水平分量为Bsin30°,环受到的安培力为:

F=Bsin30°×I×2r×2=4BIrsin30°,由于环所受向上的安培力等于环的重力,

则2πrmg=4BIrsin30°,解得:I≈0.2A;

故选B.

单项选择题
阅读理解

(2013·高考湖南卷,C)It’s such a happy­looking library,painted yellow,decorated with palm­tree stickers and sheltered from the Florida sun by its own roof.About the size of a microwave oven,it’s pedestrian­friendly,too,waiting for book lovers next to a sidewalk in Palm Beach County Estates,along the northern boundary of Palm Beach Gardens.

It’s a library built with love.

A year ago,shortly after Janey Henriksen saw a Brian Williams report about the Little Free Library organization,a Wisconsin­based nonprofit that aims to promote literacy and build a sense of community in a neighborhood by making books freely available,she announced to her family of four,“That’s what we’re going to do for our spring break!”

Son Austin,now a 10th­grader,didn’t see the point of building a library that resembles a mailbox.But Janey insisted,and husband Peter unwillingly got to work.The 51­year­old owner of a ship supply company modified a small wooden house that he’d built years earlier for daughter Abbie’s toy horses,and made a door of glass.

After adding the library’s final touches (装点),the family hung a signboard on the front,instructing users to“take a book,return a book,”and making the Henriksen library,now one of several hundred like it nationwide and among more than 2,500 in the world,the only Little Free Library in Palm Beach County.

They stocked it with 20 or so books they’d already read,a mix of science fiction,reference titles,novels and kids’ favorites.“I told them,keep in mind that you might not see it again,”said Janey,a stay­at­home mom.

Since then,the collection keeps replenishing (补充) itself,thanks to ongoing donations from borrowers.The library now gets an average of five visits a day.

The project’s best payoff,says Peter,are the thank­you notes left behind.“We had no idea in the beginning that it would be so popular.”

小题1:In what way is the library “pedestrian­friendly”?

A.It owns a yellow roof.

B.It stands near a sidewalk.

C.It protects book lovers from the sun.

D.It uses palm­tree stickers as decorations.小题2:Janey got the idea to build a library from________.

A.a visit to Brian Williams

B.a spring break with her family

C.a book sent by one of her neighbors

D.a report on a Wisconsin­based organization小题3:The library was built________.

A.by a ship supply company

B.on the basis of toy horses

C.like a mailbox

D.with glass小题4:What can we infer about the signboard?

A.It was made by a user of the library.

B.It marked a final touch to the library.

C.It aimed at making the library last long.

D.It indicated the library was a family property.小题5:The passage tells us that the users________.

A.donate books to the library

B.get paid to collect books for the library

C.receive thank­you notes for using the library

D.visit the library over 5 times on average daily