问题 选择题

如图所示,平行板电容器的两极板A、B接于电池两极,一带正电的小球悬挂在电容器内部,闭合S,电容器充电,这时悬线偏离竖直方向的夹角为θ,则下列说法正确的是

A.保持S闭合,将A板向B板靠近,则θ增大

B.断开S,将A板向B板靠近,则θ不变

C.保持S闭合,悬线烧断,则小球在电场中将作自由落体运动    

D.断开S,悬线烧断,则小球在电场中将作变加速曲线运动

答案

AB

考点:

专题:电容器专题.

分析:小球受重力、电场力和绳子的拉力处于平衡状态,当电场力变大时θ增大,电场力变小时θ减小,因此解决本题关键是判断小球所受电场力的变化情况;注意电容器的两种状态的不同,电键闭合其电压不变,电键断开电容器所带电量保持不变.

解答:解:若保持S闭合,则电容器两极板之间的电压保持不变,因此根据E=

可知,当将A板向B板靠近时,电场强度增大,则电场力增大,θ将增大,若悬线烧断,小球受重力恒定的电场力作用,所以小球沿悬线延长线做匀加速直线运动运动,故A正确,C错误.

若断开S,电容器带电量保持不变,由C=和C=以及E=

可得:E=,由于Q、S不变,只是d变化,所以电场强度不变,故电场力不变,则θ不变,若悬线烧断,小球受重力恒定的电场力作用,所以小球沿悬线延长线做匀加速直线运动运动,故B正确,D错误.

故选AB.

点评:对于电容器的讨论注意电容器的两种状态,同时熟练掌握公式:C= 、C= 、E= 

之间的推导,尤其是在电容器电量保持不变时,要正确根据这三个公式推导电场强度的表达式,从而正确判断电场强度的变化.

阅读理解

Chinese female film stars, like Gong Li and Zhang Ziyi, due to the charm of their films, have set a good image for Chinese women in the world. Compared with Chinese women, the image of Chinese men is not so good in the eyes of Western women, the Shanghai Evening Post reported.

In March 2006, Zhang Jiehai, a doctorate(博士学位) holder at the Sociology Institute of the Shanghai Academy of Social Sciences, led a research team to investigate how Western women look at Chinese men. The survey shows that many Western women think Chinese men are mostly known for their care for the family. Chinese men, as the Western women say, are willing to spend money for women. As to their shortcomings, Western women think that Chinese men do not have good manners and that some Chinese men have the habit of spitting everywhere, keeping nails and hair long, picking their noses and ears in public places, and not cleaning their teeth thoroughly. These bad habits make many Western women think that Chinese men are unwilling to take care of their personal image.

The survey was carried out in two ways, by questionnaires and interviews. All the respondents(调查对象) agreed that Chinese men show much tender loving care for the family and that many Chinese men do know how to respect women. They open the door for women and carry their handbags for them. For some American women, the bad image of Chinese men in the world is largely because of Hollywood films. In many Hollywood films, Chinese men usually do not have a good image. But this has been changed by Hong Kong film star Chow Yun-Fat. After the Chinese film Crouching Tiger, Hidden Dragon was shown in cinemas in the United States in 2002, many Westerners began to change their impression of Chinese men.

小题1:Which statement best expresses the main idea of the first paragraph?

A.Chinese female film stars enjoy as great fame as Chinese male film stars in the world.

B.Chinese male film stars do not perform well in their films in the eyes of Western women.

C.Chinese female film stars don’t like Western male film stars.

D.Chinese female film stars have set a good image for Chinese women.小题2:Which is NOT mentioned as the shortcoming of Chinese men in Paragraph 2?

A.Spitting everywhere.

B.Speaking too loudly in public places.

C.Not cleaning their teeth thoroughly.

D.Picking their noses and ears in public places.小题3:In some American women’s eyes, Chinese men’s images are bad because of ______.

A.Chinese female film stars

B.Chinese male film stars

C.Hollywood films

D.Film stars in Hong Kong小题4:What does the passage mainly deal with?

A.Chinese men in the eyes of Western women.

B.Foreign film stars and Chinese film stars.

C.Chinese women in the eyes of Western men.

D.Chinese film stars in Hollywood films.

多项选择题