问题 填空题

电子绕核运动可等效为一环形电流,设氢原子中的电子以速率v在半径为r的轨道上运动,用e表示电子的电荷量,则其等效电流的大小为______.

答案

由题,电子圆周运动的速率为v,半径为r,则电子运动的周期T=

2πr
v

根据电流的定义式得到,等效电流为I=

e
T
=
ev
2πr

故答案为:

ev
2πr

阅读理解

阅读理解。

     Some wonderfully creative uses of X-rays have been in the world of art.But the most impressive one

is to uncover what has been covered up.Details unfolded by an X-ray of a painting can ' t be seen any

other way.For example, in the past, paintings were often repaired if small pieces of paint fell off the

canvas (帆布). These repairs cannot be seen just by looking at the painting.It takes an X-ray to discover where the repairs have been made.

    An X-ray of a painting is one of the best ways to tell if it was created by a famous painter or if it is a

forgery.If there is a question as to whether an old master has painted a certain painting or not, X-rays

are made and compared to those of paintings that are known to be original.If the brushstrokes (笔迹) ,

for example, are shown to be totally different than in other work done by a certain artist, then the painting

is proved to be a fake copy.

     Since modern paints are usually made from different materials than paints of hundreds of years ago,

the image they leave on an X-ray film looks different.This is another way X-rays can be used to spot a

forgery.A modem artist will try to make a forgery look hundreds of years old by painting dirty vamish on

it or by using artificial means to get the varnish to look cracked.To the naked eye the forgery may look

old, but when a careful study of the X-ray image is made, the deception is obvious.

     It was common many years ago for an artist to finish a painting, decide it wasn't any good, and paint

a completely different picture on top of it.Or artists would paint over others' work -it was their version

of recycling.Taking an X-ray is the only way to find out if there is a picture under the picture you see with

your eyes.Sometimes museums will be more interested in the painting beneath than the one on top, and

will restore the older one by having the top layer painstakingly removed.

1.What is the main purpose of the article?  

A. To provide information to the police who are looking for stolen art.

B. To persuade art dealers to check for fake copies before selling art.

C. To explain how modern technology can be applied in unique ways.

D. To help artists restore damaged artwork to its original appearance.

2. According to the passage, X-rays are used to ______. 

A. make repairs on paintings in the past

B. tell if an artwork is original or not

C. change the materials of a forgery

D. remove the recyclable paints

3. In Paragraph 3, the word "deception" probably means ______.

A. comparison  

B. drawback    

C. trickery          

D. judgement

4.Which is the best title for the passage?

A. X-ray Detectives              

B. Creative Uses of Technology

C. Forgery Studies                

D. X-ray Painting Technique