问题 问答题

一个密度ρ="9" g/cm3、横截面积S="10" mm2的金属环,处于径向对称、方向发散的磁场中,如图15-2-25所示,环上各点的磁感应强度为B="0.70" T,与环面夹角α=60°.若在环中通以顺时针方向(俯视)电流I="2" A,并保持Δt="0.2" s,试分析环将做什么运动,上升的最大高度是多少.(不计空气阻力)

图15-2-25

答案

先做匀加速直线运动,后做竖直上抛运动  H="2.26" m

设环的半径为r,则通电时环所受安培力的竖直分量

Fy=BI·2πr·cos60°=BIπr

环的质量m=2πrSρ

则环的加速度a=-g="29" m/s2.

上升高度H=aΔt2+="2.26" m.

单项选择题
阅读理解

阅读理解

     Color is very important in our daily life. It can influence our choice of purchase when we go shopping

and can also affect the way we look and feel. A person can be naturally attracted to one particular color

while finding another color repulsive. The influence of color on human emotions and feelings is very

powerful and designers, advertisers, artists and even doctors use it in their professions for different

purposes.

     Every day, without even thinking about it, we use color to communicate our feelings. For example, the

color we choose to wear is one way of expressing ourselves. It says something about the person, how he

or she feels, and how he or she wants others to feel about the wearer. In other words, color talks.

     A person who wants to appear stylish and mature will usually wear dark colors or black. A person

who wishes to appear youngish, pure and innocent dresses in white, like the choice of a bride's wedding

gown.   Also clothed in white are doctors, dentists, nurses and hospital in-patients as the color expresses

comfort, cleanliness and professionalism. Brown, blue or grey clothes are the usual colors worn by office

workers so as not to draw unnecessary attention to them. They are, in a way, conveying the message that

they want to be noticed for their work, not their appearance. People who wear bright, strong colors like

to be attractive and these colors are particularly popular with actresses, singers and party goers.

    In the past, common people were not allowed to wear certain colors. In Thailand, for example, only

kings, queens and members of the royal family could wear purple while in ancient China, yellow was the

color reserved for the emperor and the empress. Nowadays, people can wear whatever color they like

or as they think appropriate.

1. The word "it" in the first paragraph refers to ________.

A. color  

B. influence  

C. human emotion  

D. personal feeling

2. How does color affect a product?

A. It describes the product.

B. It helps the product designer to express himself.

C. It may put the purchaser in a spending mood.

D. It is able to influence the consumers' choice of purchase.

3. If a girl chooses to wear a red dress to a party, it implies that she wants to ________.

A. look pretty      

B. appear innocent      

C. be noticed      

D. be an actress

4. Brides wear white because it is a color for _______.

A. comfort          

B. professionalism    

C. purity          

D. youth